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Precedence and order of evaluation

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The precedence and associativity of C operators affect the grouping and evaluation of operands in expressions. An operator's precedence is meaningful only if other operators with higher or lower precedence are present. Expressions with higher-precedence operators are evaluated first. Precedence can also be described by the word "binding." Operators with a higher precedence are said to have tighter binding.

The following table summarizes the precedence and associativity (the order in which the operands are evaluated) of C operators, listing them in order of precedence from highest to lowest. Where several operators appear together, they have equal precedence and are evaluated according to their associativity. The operators in the table are described in the sections beginning with Postfix Operators . The rest of this section gives general information about precedence and associativity.

Precedence and associativity of C operators

1 Operators are listed in descending order of precedence. If several operators appear on the same line or in a group, they have equal precedence.

2 All simple and compound-assignment operators have equal precedence.

An expression can contain several operators with equal precedence. When several such operators appear at the same level in an expression, evaluation proceeds according to the associativity of the operator, either from right to left or from left to right. The direction of evaluation does not affect the results of expressions that include more than one multiplication ( * ), addition ( + ), or binary-bitwise ( & , | , or ^ ) operator at the same level. Order of operations is not defined by the language. The compiler is free to evaluate such expressions in any order, if the compiler can guarantee a consistent result.

Only the sequential-evaluation ( , ), logical-AND ( && ), logical-OR ( || ), conditional-expression ( ? : ), and function-call operators constitute sequence points, and therefore guarantee a particular order of evaluation for their operands. The function-call operator is the set of parentheses following the function identifier. The sequential-evaluation operator ( , ) is guaranteed to evaluate its operands from left to right. (The comma operator in a function call is not the same as the sequential-evaluation operator and does not provide any such guarantee.) For more information, see Sequence points .

Logical operators also guarantee evaluation of their operands from left to right. However, they evaluate the smallest number of operands needed to determine the result of the expression. This is called "short-circuit" evaluation. Thus, some operands of the expression may not be evaluated. For example, in the expression

x && y++

the second operand, y++ , is evaluated only if x is true (nonzero). Thus, y is not incremented if x is false (0).

The following list shows how the compiler automatically binds several sample expressions:

In the first expression, the bitwise-AND operator ( & ) has higher precedence than the logical-OR operator ( || ), so a & b forms the first operand of the logical-OR operation.

In the second expression, the logical-OR operator ( || ) has higher precedence than the simple-assignment operator ( = ), so b || c is grouped as the right-hand operand in the assignment. Note that the value assigned to a is either 0 or 1.

The third expression shows a correctly formed expression that may produce an unexpected result. The logical-AND operator ( && ) has higher precedence than the logical-OR operator ( || ), so q && r is grouped as an operand. Since the logical operators guarantee evaluation of operands from left to right, q && r is evaluated before s-- . However, if q && r evaluates to a nonzero value, s-- is not evaluated, and s is not decremented. If not decrementing s would cause a problem in your program, s-- should appear as the first operand of the expression, or s should be decremented in a separate operation.

The following expression is illegal and produces a diagnostic message at compile time:

In this expression, the equality operator ( == ) has the highest precedence, so p == 0 is grouped as an operand. The conditional-expression operator ( ? : ) has the next-highest precedence. Its first operand is p == 0 , and its second operand is p += 1 . However, the last operand of the conditional-expression operator is considered to be p rather than p += 2 , since this occurrence of p binds more closely to the conditional-expression operator than it does to the compound-assignment operator. A syntax error occurs because += 2 does not have a left-hand operand. You should use parentheses to prevent errors of this kind and produce more readable code. For example, you could use parentheses as shown below to correct and clarify the preceding example:

( p == 0 ) ? ( p += 1 ) : ( p += 2 )

C operators

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Order of evaluation

Order of evaluation of the operands of any C++ operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.

There is no concept of left-to-right or right-to-left evaluation in C++, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression a + b + c is parsed as (a + b) + c due to left-to-right associativity of operator+, but the subexpression c may be evaluated first (or last, or at the same time as a or b ) at run time.

[ edit ] Sequenced-before rules (since C++11)

[ edit ] definitions, [ edit ] evaluations.

There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):

  • value computation : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, e.g. if the expression returns a number, or some other value)
  • side effect : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

[ edit ] Ordering

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types are involved with suitable std:: memory_order ).

  • If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
  • evaluations of A and B are indeterminably-sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

[ edit ] Rules

1) Each value computation and side effect of a full expression (an expression that is not part of another expression, typically one that ends with a semicolon) is sequenced before each value computation and side effect of the next full expression.

2) The value computations (but not the side-effects) of the operands to any operator are sequenced before the value computation of the result of the operator (but not its side-effects).

3) When calling a function (whether or not the function is inline, and whether or not explicit function call syntax is used), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function.

4) The value computation of the built-in postincrement and postdecrement operators is sequenced before its side-effect.

5) The side effect of the built-in preincrement and predecrement operators is sequenced before its value computation (implicit rule due to definition as compound assignment)

6) Every value computation and side effect of the first (left) argument of the built-in logical AND operator && and the built-in logical OR operator || is sequenced before every value computation and side effect of the second (right) argument.

7) Every value computation and side effect associated with the first expression in the logical :? operator is sequenced before every value computation and side effect associated with the second or third expression.

8) The side effect (modification of the left argument) of the built-in assignment operator and of all built-in compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments, and is sequenced before the value computation of the assignment expression (the value is the reference it returns)

9) Every value computation and side effect of the first (left) argument of the built-in comma operator is sequenced before every value computation and side effect of the second (right) argument.

10) In list-initialization, every value computation and side effect of a given initializer clause is sequenced before every value computation and side effect associated with any initializer clause that follows it in the comma-separated list of the initializer list.

11) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (CPU instructions that constitute different function calls cannot be interleaved, even if the functions are inlined)

12) The call to the allocation function (operator new) is indeterminately sequenced with respect to the evaluation of the constructor arguments in a new-expression

[ edit ] Undefined behavior

1) If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the behavior is undefined.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior (but i = ++i + 1; is well-defined) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) If a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object, the behavior is undefined.

cout << i << i ++ ; // undefined behavior a [ i ] = i ++ ; // undefined bevahior

[ edit ] Sequence point rules (until C++11)

Evaluation of an expression might produce side effects, which are: accessing an object designated by a volatile lvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations.

A sequence point is a point in the execution sequence where all side effects from the previous evaluations in the sequence are complete, and no side effects of the subsequent evaluations started.

1) There is a sequence point at the end of each full expression (typically, at the semicolon)

2) When calling a function (whether or not the function is inline and whether or not function call syntax was used), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body

3) There is a sequence point after the copying of a returned value of a function and before the execution of any expressions outside the function.

4) Once the execution of a function begins, no expressions from the calling function are evaluated until execution of the called function has completed (functions cannot be interleaved)

5) In the evaluation of each of the following four expressions, using the built-in (non-overloaded) operators, there is a sequence point after the evaluation of the expression a .

a && b a || b a ? b : c a , b

1) Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior i = ++ i + 1 ; // undefined behavior (well-defined in C++11) ++ ++ i ; // undefined behavior (well-defined in C++11) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) Between the previous and next sequence point , the prior value of a scalar object that is modified by the evaluation of the expression, shall be accessed only to determine the value to be stored.

[ edit ] See also

  • Operator precedence which defines how expressions are built from their source code representation.
  • Order of evaluation in the C programming language.
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c assignment evaluation order

Last Update: 16 August 2021

Stricter Expression Evaluation Order in C++17

c assignment evaluation order

Table of Contents

C++ has many dark corners and many caveats that can cause you to scratch your head in confusion. One of the issues we had until C++17 was the evaluation order of expressions. In this blog post, I’ll show you the new rules that we got in C++17 that made this complicated term much simpler and practical.

Here are the main points for today:

  • What’s the case with make_unique vs unique_ptr<T>(new T) in a function call.
  • What are the new rules for C++17?
  • Are all bugs fixed and now well defined?

Let’s go.

This article is based on an excerpt from my book: "C++17 in Detail". Get the Ebook here at @Leanpub , or the print version @Amazon . And join almost 3000 readers! Also, have a look at the Anniversary Promo at the end of the article :)

Stricter Expression Evaluation Order  

Until C++17, the language hasn’t specified any evaluation order for function parameters. Period.

For example, that’s why in C++14 make_unique is not just syntactic sugar, but it guarantees memory safety:

Consider the following examples:

And with make_unique :

Considering the first case, in C++14, we only know that new T is guaranteed to happen before the unique_ptr construction, but that’s all. For example, new T might be called first, then otherFunction() , and then the constructor for unique_ptr is invoked.

For such evaluation order, when otherFunction() throws, then new T generates a leak (as the unique pointer is not yet created).

When you use make_unique , as in the second case, the leak is not possible as you wrap memory allocation and creation of unique pointer in one call.

C++17 addresses the issue shown in the first case. Now, the evaluation order of function arguments is “practical” and predictable. In our example, the compiler won’t be allowed to call otherFunction() before the expression unique_ptr<T>(new T) is fully evaluated.

In other words, in C++17 can still call otherFunction() before the memory allocation happens, but it cannot interleave sub expressions.

Read on for more details below.

The Changes  

In an expression:

The order of evaluation of a , b , c is still unspecified in C++17, but any parameter is fully evaluated before the next one is started. It’s especially crucial for complex expressions like this:

if the compiler chooses to evaluate x first, then it must evaluate a(x) before processing b , c(y) or y .

This guarantee fixes the problem with make_unique vs unique_ptr<T>(new T()) . A given function argument must be fully evaluated before other arguments are evaluated.

An Example  

Consider the following case:

You probably expect that using C++14 computeInt() happens after addFloat . Unfortunately, that might not be the case. For instance, here’s an output from GCC 4.7.3:

See the code and compare: @Compiler Explorer - GCC 4.7 and the same code @Compiler Explorer - GCC 8 .

The chaining of functions is already specified to work from left to right (thus addInt() happens after addFloat() ), but the order of evaluation of the inner expressions can differ. To be precise:

The expressions are indeterminately sequenced with respect to each other.

With C++17, function chaining will work as expected when they contain inner expressions, i.e., they are evaluated from left to right:

In the expression:

expA is evaluated before calling b() .

Compiling the previous example with a conformant C++17 compiler, yields the following result:

Another result of this change is that when using operator overloading, the order of evaluation is determined by the order associated with the corresponding built-in operator.

For example:

The above code contains operator overloading and expands to the following function notation:

Before C++17, a() , b() and c() could be evaluated in any order. Now, in C++17, a() will be evaluated first, then b() and then c() .

Rules  

Here are more rules described in the paper P0145R3 :

The following expressions are evaluated in the order a, then b: a.b a->b a->*b a(b1, b2, b3) // b1, b2, b3 - in any order b @= a // '@' means any operator a[b] a << b a >> b

If you’re not sure how your code might be evaluated, then it’s better to make it simple and split it into several clear statements. You can find some guides in the Core C++ Guidelines, for example ES.44 and ES.44 .

And here’s also a critical quote about argument interleaving; this is prohibited since C++17:\

From N4868, October 2020, Draft

[intro.execution], point 11:

When calling a function (whether or not the function is inline), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function. For each function invocation F, for every evaluation A that occurs within F and every evaluation B that does not occur within F but is evaluated on the same thread and as part of the same signal handler (if any), either A is sequenced before B or B is sequenced before A.49

And there’s also a handy and concise summary added in the note below:

In other words, function executions do not interleave with each other.

The famous example  

In the 4th Edition of The C++ Programming Language , Stroustrup , you can find the following example:

Play at @Compiler Explorer

And what’s surprising is that before C++17, this code was unspecified, and you could get different results.

Since C++17, you’ll see only one correct final value of s :

Does it mean all errors are fixed?  

I got into a discussion recently with Patrice Roy, and thanks to his knowledge, I understood that the changes in C++17 are not the solution to all of our issues.

Have a look at the following contrived code:

We said that we won’t leak from new T , but we could invent the following “deadly” code:

While the evaluation of arguments cannot be interleaved, the compiler can select the following order:

  • new int { 10 }
  • unique_ptr creation

And now, if new T throws, then new int is left as a memory leak (since the body of the function won’t be executed).

But… here’s the catch :)

The code I presented is really contrived and violates many rules of modern C++. So in practice, it’s hard to come up with code that will easily fail due to evaluation order after C++17. This might be the case with somehow wrongly passing resource ownership or functions with side effects.

Summary  

Evaluation order is one of the primary “features” of the language, and before C++17, it could cause some unexpected effects. This was especially tricky for code that was supposed to run on many platforms and compilers. Fortunately, with C++17 the order is more practical, and thus it saves us from many mistakes.

You can also look at the proposal that went into the Standard: P0145R3 .

Back to you  

  • Has the evaluation order caused some bugs/errors/unexpected behavior in your code?
  • Do you try to make your expressions simple?

Let us know in the comments below the article.

Special Promo  

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  • How to Parallelise CSV Reader - C++17 in Practice
  • C++ Lambda Story in Spanish!
  • Software Architecture with C++, Book Review
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  • std::filesystem in C++17 In Detail

Order of evaluation

Order of evaluation of the operands of almost all C++ operators (including the order of evaluation of function arguments in a function-call expression and the order of evaluation of the subexpressions within any expression) is unspecified. The compiler can evaluate operands in any order, and may choose another order when the same expression is evaluated again.

There are exceptions to this rule which are noted below.

Except where noted below, there is no concept of left-to-right or right-to-left evaluation in C++. This is not to be confused with left-to-right and right-to-left associativity of operators: the expression f1() + f2() + f3() is parsed as (f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to f3 may be evaluated first, last, or between f1() or f2() at run time.

Sequenced-before rules (since C++11)

Definitions, evaluations.

There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):

  • value computation : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, e.g. if the expression returns a number, or some other value)
  • side effect : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread.

  • If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
  • evaluations of A and B are indeterminately sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.
  • unevaluated operand
  • constant expression
  • an entire initializer , including any comma-separated constituent expressions
  • the destructor call generated at the end of the lifetime of a non-temporary object
  • an expression that is not part of another full-expression (such as the entire expression statement , controlling expression of a for / while loop, conditional expression of if / switch , the expression in a return statement, etc),

Undefined behavior

1) If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the behavior is undefined .

i = ++ i + 2 ;       // undefined behavior until C++11 i = i ++ + 2 ;       // undefined behavior until C++17 f ( i = - 2 , i = - 2 ) ; // undefined behavior until C++17 f ( ++ i, ++ i ) ;       // undefined behavior until C++17, unspecified after C++17 i = ++ i + i ++ ;     // undefined behavior

2) If a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object, the behavior is undefined .

cout << i << i ++ ; // undefined behavior until C++17 a [ i ] = i ++ ;       // undefined behavior until C++17 n = ++ i + i ;       // undefined behavior

Sequence point rules (until C++11)

Evaluation of an expression might produce side effects, which are: accessing an object designated by a volatile lvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations.

A sequence point is a point in the execution sequence where all side effects from the previous evaluations in the sequence are complete, and no side effects of the subsequent evaluations started.

1) There is a sequence point at the end of each full expression (typically, at the semicolon).

2) When calling a function (whether or not the function is inline and whether or not function call syntax was used), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body.

3) There is a sequence point after the copying of a returned value of a function and before the execution of any expressions outside the function.

4) Once the execution of a function begins, no expressions from the calling function are evaluated until execution of the called function has completed (functions cannot be interleaved).

5) In the evaluation of each of the following four expressions, using the built-in (non-overloaded) operators, there is a sequence point after the evaluation of the expression a .

a && b a || b a ? b : c a , b

1) Between the previous and next sequence point a scalar object must have its stored value modified at most once by the evaluation of an expression, otherwise the behavior is undefined .

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior i = ++ i + 1 ; // undefined behavior (well-defined in C++11) ++ ++ i ; // undefined behavior (well-defined in C++11) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) Between the previous and next sequence point, the prior value of a scalar object that is modified by the evaluation of the expression, must be accessed only to determine the value to be stored. If it is accessed in any other way, the behavior is undefined .

cout << i << i ++ ; // undefined behavior a [ i ] = i ++ ; // undefined behavior

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

  • Operator precedence which defines how expressions are built from their source code representation.
  • C++11 standard (ISO/IEC 14882:2011):
  • 1.9 Program execution [intro.execution]
  • 5.2.6 Increment and decrement [expr.post.incr]
  • 5.3.4 New [expr.new]
  • 5.14 Logical AND operator [expr.log.and]
  • 5.15 Logical OR operator [expr.log.or]
  • 5.16 Conditional operator [expr.cond]
  • 5.17 Assignment and compound assignment operators [expr.ass]
  • 5.18 Comma operator [expr.comma]
  • 8.5.4 List-initialization [dcl.init.list]
  • Pages with unreviewed CWG DR marker
  • C++ Data Types
  • C++ Input/Output
  • C++ Pointers
  • C++ Interview Questions
  • C++ Programs
  • C++ Cheatsheet
  • C++ Projects
  • C++ Exception Handling
  • C++ Memory Management
  • Solve Coding Problems
  • Class Template Argument Deduction in C++17
  • Character Literal in C++ 17: Unicode and UTF-8 Prefix
  • C++17 - <charconv> Header
  • Inline Variables in C++ 17
  • __has_include in C++17
  • Temporary Materialization in C++ 17
  • C++17 - <memory_resource> Header
  • Noexcept Specifier in C++17
  • How to Parse an Array of Objects in C++ Using RapidJson?
  • Parameter Passing Techniques in C++
  • Constants in C++
  • How to Lock Window Resize C++ sfml?
  • C++ Program For Iterative Quick Sort
  • Bitwise Operators in C++
  • Lambda Capture of *this in C++17
  • Fold Expressions in C++ 17
  • File System Library in C++17
  • Types of Fold Expressions in C++ 17
  • C++ 23 - Header[Testing Demo Copy]

Order of Evaluation in C++17

In C++ programming, the order of evaluation of expressions can have a significant impact on the behavior and correctness of the code. C++17 introduced changes to the order of evaluation rules, providing clearer guidelines and improving consistency across different compilers. In this article, we will explore the order of evaluation in C++17 and understand its implications.

Need of Order of Evaluation Rules in C++17

  • In a program, when the same memory location has to be modified by multiple operations within the program and the operations are unsequenced, this can lead to undefined behavior.
  • In a program, if one operation modifies a memory location and the same memory location is used in an evaluation, but these actions are unsequenced, this can also lead to undefined behavior.

Due to these possible undefined behaviors, C++ 17 defined rules for Order of Evaluation.

Order of Evaluation Rules in C++17

C++17 introduced the following rules for the order of evaluation:

  • Left-to-right Evaluation: C++17 guarantees that subexpressions within an expression are evaluated from left to right. This means that the leftmost subexpression is evaluated first, followed by the next subexpression on the right, and so on.
  • Sequenced Before Relationship: If two subexpressions within an expression are not directly related by a common sequence point, C++17 guarantees that they will be evaluated in an order that is either left-to-right or according to the dependencies imposed by the expressions themselves.
  • Function Call Sequencing : When you call a function, evaluations related to the arguments of the called function are evaluated before the evaluation of called function.
  • Indeterminately Sequenced Expressions: C++17 allows for expressions that are indeterminately sequenced. This means that the order of evaluation between these expressions is unspecified, and the compiler has the freedom to choose the order.
  • Post-Increment/Decrement Sequencing : When using x++ or x-- , the value is first used, then increased/decreased.
  • Pre-Increment/Decrement Sequencing : For ++x or --x , the value is increased/decreased before being used.
  • Logical AND/OR Sequencing : In && and || , the left part is evaluated before the right.
  • Conditional Operator Sequencing : In the ? : operator, the left part is evaluated before the right parts.
  • Assignment Operator Sequencing : For = and += , the right value is evaluated first, then the left value is updated.
  • Comma Operator Sequencing : In the , operator, the left part is evaluated before the right part.
  • List-Initialization Sequencing : When initializing multiple values, calculations and side effect for each value is done before moving to the next value.
  • Indeterminate Function Call Sequencing : Usually, function calls are done in a specific order, but if they are indeterminately sequenced, the compiler can decide the sequence of evaluation.
  • Allocation and Constructor Sequencing : When we create objects using new keyword , the evaluation of constructor arguments is done before the memory is allocated to the object.
  • Return and Destructor Sequencing : When a function returns, operations on the temporary data is sequenced before, then the local variables are destroyed.
  • When calling a function, the name of the function specifying the function call is evaluated before the arguments passed in the function.
  • When we overloads an operator and use it in a program, if follows the same order of evaluation as the built in operator that is overloaded.
  • When we access an array element using square brackets like arr[I], the evaluation and side effects related to the expression arr are sequenced before the evaluation of expression inside the square brackets.
  • When we use shift operators in a program like num << shift_value , the evaluations and side effects related to expression num is sequenced before evaluation of expression shift_value.
  • In simple assignment variable = value and compound assignments like variable @= value , the evaluation and side effect related to expression value is sequenced before the evaluation and side effect related to variable .
  • The evaluation of comma separated expressions within a parenthesis are evaluated indeterminately.

Example: C++ Program to Illustrate the Order of Evaluation

Explanation.

In the above example, we have an expression a++ + getValue(), first the value of a is used in the operation then the value of a is incremented and getValue() returns a value. The order of evaluation determines whether a++ or getValue() is evaluated first. In C++17, the order of evaluation is left-to-right, so a++ is evaluated before getValue().

The order of evaluation in C++17 is crucial for writing correct and predictable code. With the introduced rules of left-to-right evaluation, sequenced before relationship, and indeterminately sequenced expressions, C++17 provides clearer guidelines for the order of evaluation. By understanding and applying the rules of order of evaluation in C++17, we can write robust and reliable code that works consistently across different compilers and platforms. Take advantage of the improved guidelines provided by C++17.

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Order of evaluation

Order of evaluation of the operands of any C++ operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.

There is no concept of left-to-right or right-to-left evaluation in C++, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression f1() + f2() + f3() is parsed as (f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to f3 may be evaluated first, last, or between f1() or f2() at run time.

[ edit ] Sequenced-before rules (since C++11)

[ edit ] definitions, [ edit ] evaluations.

There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):

  • value computation : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, e.g. if the expression returns a number, or some other value)
  • side effect : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

[ edit ] Ordering

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types are involved with suitable std::memory_order ).

  • If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
  • evaluations of A and B are indeterminably-sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

[ edit ] Rules

1) Each value computation and side effect of a full expression (an expression that is not part of another expression, typically one that ends with a semicolon) is sequenced before each value computation and side effect of the next full expression.

2) The value computations (but not the side-effects) of the operands to any operator are sequenced before the value computation of the result of the operator (but not its side-effects).

3) When calling a function (whether or not the function is inline, and whether or not explicit function call syntax is used), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function.

4) The value computation of the built-in postincrement and postdecrement operators is sequenced before its side-effect.

5) The side effect of the built-in preincrement and predecrement operators is sequenced before its value computation (implicit rule due to definition as compound assignment)

6) Every value computation and side effect of the first (left) argument of the built-in logical AND operator && and the built-in logical OR operator || is sequenced before every value computation and side effect of the second (right) argument.

7) Every value computation and side effect associated with the first expression in the logical ?: operator is sequenced before every value computation and side effect associated with the second or third expression.

8) The side effect (modification of the left argument) of the built-in assignment operator and of all built-in compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments, and is sequenced before the value computation of the assignment expression (that is, before returning the reference to the modified object)

9) Every value computation and side effect of the first (left) argument of the built-in comma operator , is sequenced before every value computation and side effect of the second (right) argument.

10) In list-initialization , every value computation and side effect of a given initializer clause is sequenced before every value computation and side effect associated with any initializer clause that follows it in the comma-separated list of the initializer list.

11) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (the program must behave as if the CPU instructions that constitute different function calls were not interleaved, even if the functions were inlined)

12) The call to the allocation function (operator new) is indeterminately sequenced with respect to the evaluation of the constructor arguments in a new-expression

[ edit ] Undefined behavior

1) If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the behavior is undefined.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior (but i = ++i + 1; is well-defined) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) If a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object, the behavior is undefined.

cout << i << i ++ ; // undefined behavior a [ i ] = i ++ ; // undefined behavior

[ edit ] Sequence point rules (until C++11)

Evaluation of an expression might produce side effects, which are: accessing an object designated by a volatile lvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations.

A sequence point is a point in the execution sequence where all side effects from the previous evaluations in the sequence are complete, and no side effects of the subsequent evaluations started.

1) There is a sequence point at the end of each full expression (typically, at the semicolon)

2) When calling a function (whether or not the function is inline and whether or not function call syntax was used), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body

3) There is a sequence point after the copying of a returned value of a function and before the execution of any expressions outside the function.

4) Once the execution of a function begins, no expressions from the calling function are evaluated until execution of the called function has completed (functions cannot be interleaved)

5) In the evaluation of each of the following four expressions, using the built-in (non-overloaded) operators, there is a sequence point after the evaluation of the expression a .

a && b a || b a ? b : c a , b

1) Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior i = ++ i + 1 ; // undefined behavior (well-defined in C++11) ++ ++ i ; // undefined behavior (well-defined in C++11) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) Between the previous and next sequence point , the prior value of a scalar object that is modified by the evaluation of the expression, shall be accessed only to determine the value to be stored.

[ edit ] See also

  • Operator precedence which defines how expressions are built from their source code representation.

[ edit ] References

  • C++11 standard (ISO/IEC 14882:2011):
  • 1.9 Program execution [intro.execution]
  • 5.2.6 Increment and decrement [expr.post.incr]
  • 5.3.4 New [expr.new]
  • 5.14 Logical AND operator [expr.log.and]
  • 5.15 Logical OR operator [expr.log.or]
  • 5.16 Conditional operator [expr.cond]
  • 5.17 Assignment and compound assignment operators [expr.ass]
  • 5.18 Comma operator [expr.comma]
  • 8.5.4 List-initialization [dcl.init.list]

Order of evaluation

Order of evaluation of the operands of any C operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.

There is no concept of left-to-right or right-to-left evaluation in C, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression f1() + f2() + f3() is parsed as (f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to f3 may be evaluated first, last, or between f1() or f2() at run time.

[ edit ] Definitions

[ edit ] evaluations.

There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):

  • value computation : calculation of the value that is returned by the expression. This may involve determination of the identity of the object ( lvalue evaluation ) or reading the value previously assigned to an object (rvalue evaluation)
  • side effect : access (read or write) to an object designated by a volatile lvalue, modification (writing) to an object , atomic synchronization (since C11) , modifying a file, modifying the floating-point environment (if supported), or calling a function that does any of those operations.

If no side effects are produced by an expression and the compiler can determine that the value is not used, the expression may not be evaluated .

[ edit ] Ordering

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types and memory barriers are involved).

  • If a sequence point is present between the subexpressions E1 and E2, then both value computation and side effects of E1 are sequenced-before every value computation and side effect of E2

[ edit ] Rules

[ edit ] undefined behavior, [ edit ] see also.

Operator precedence which defines how expressions are built from their source code representation.

MC++ BLOG

C++ Core Guidelines: Rules for Expressions

Today’s post is about expressions. You should avoid complicated expressions, know the precedence rules for arithmetic or logical expressions, and know the order of evaluation of expressions. The main reasons for undefined behavior are having the wrong precedence rules for expressions in mind or assuming an evaluation order for expressions that is just wrong or not guaranteed. I know that’s a lot to digest. Let’s start.

Polynomial expansion

Here are the four rules for today.

ES.40: Avoid complicated expressions

Es.41: if in doubt about operator precedence, parenthesize, es.43: avoid expressions with undefined order of evaluation, es.44: don’t depend on order of evaluation of function arguments.

The rules for the precedence and the evaluation are not as easy as it sounds. They even change with C++17; therefore, we should start simple.

What does complicated mean? Here is the original example of the guidelines:

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I added a few (numbers) to it. First, all expressions having a (1) are bad style and should not pass a code review. For example, do you know what is happening here:  x = a & b + c * d && e ^ f == 7;. Of course, you have to look up the precedences of the operators. I will come to it in the following rule. The expression (2) may be acceptable if the conditions hold. i and j must be disjunct, and the indices i,j, and i,k must be pairwise disjunct.

(3) is undefined behavior, because it is not defined which x will be evaluated first. Why? The argument is that the final semicolon “;” is a sequence point, and now we have the guarantee that all side effects from the previous evaluations in the sequence are complete.

With C++17, the rules for operator precedence changed: left-to-right for expressions except for right-to-left in assignments. I will write about it in ES.43.

On the one hand, the guidelines say: If you doubt operator precedence, use parenthesis (1). On the other hand, they state: You should know enough not to need parentheses here (2):

Okay. For an expert, expression (1) may be obvious, but for a beginner, expression (2) may be a challenge. 

I have only two tips in mind according to the guidelines:

  • If in doubt about precedence, use parentheses. Do not forget the beginners!
  • Keep this precedence table from cppreference.com under your pillow.

OperatorPrecedence

I will jump right to the rules ES.43 and ES.44 and write about the rule ES.42 in my next post. With C++17, the order of evaluation of expressions changed.

In C++14, the following expression has undefined behavior. 

This will not hold for C++17. With C++17, the order of evaluation of the last code snippet is right to left; therefore, the expression has well-defined behavior.

Here are the additional guarantees we have with C++17:

  • Postfix expressions are evaluated from left to right. This includes function calls and member selection expressions.
  • Assignment expressions are evaluated from right to left. This includes compound assignments.
  • Operands to shift operators are evaluated from left to right. 

This was the wording of the original proposal . They also provided a few examples. Here are they:

How should you read these examples? Quite simple. First, a will be evaluated, then b , c, and d .

Expression (1) is a bit tricky. With C++17, we can only guarantee that the function is evaluated before its arguments, but the order of the evaluation of the arguments is still unspecified.

I know the last sentence was not easy. Let’s elaborate a little bit more.

In the last years,  I saw many errors because developers assumed that the order of the evaluation of function arguments is left to right. Wrong! You have no guarantees!

Here is my proof. The output from Gcc and clang differs:

gcc

With C++17, this behavior didn’t change. The order of evaluation is unspecified. But at least the order of evaluating the following expressions is specified with C++17.

Here is the reason why:

(1): Postfix expressions are evaluated from left to right. This includes function calls and member selection expressions.

(2): Operands to shift operators are evaluated from left to right.

(3): Assignment expressions are evaluated from right to left.

Only to remind you. With C++14, the last three expressions have undefined behavior.

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declarationSmall

Order of evaluation

Order of evaluation of the operands of any C++ operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the same expression is evaluated again.

There is no concept of left-to-right or right-to-left evaluation in C++, which is not to be confused with left-to-right and right-to-left associativity of operators: the expression f1() + f2() + f3() is parsed as (f1() + f2()) + f3() due to left-to-right associativity of operator+, but the function call to f3 may be evaluated first, last, or between f1() or f2() at run time.

[ edit ] Sequenced-before rules (since C++11)

[ edit ] definitions, [ edit ] evaluations.

There are two kinds of evaluations performed by the compiler for each expression or subexpression (both of which are optional):

  • value computation : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, e.g. if the expression returns a number, or some other value)
  • side effect : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

[ edit ] Ordering

"sequenced-before" is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread (it may extend across threads if atomic types are involved with suitable std:: memory_order ).

  • If A is sequenced before B, then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • evaluations of A and B are unsequenced: they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B)
  • evaluations of A and B are indeterminably-sequenced: they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

[ edit ] Rules

1) Each value computation and side effect of a full expression (an expression that is not part of another expression, typically one that ends with a semicolon) is sequenced before each value computation and side effect of the next full expression.

2) The value computations (but not the side-effects) of the operands to any operator are sequenced before the value computation of the result of the operator (but not its side-effects).

3) When calling a function (whether or not the function is inline, and whether or not explicit function call syntax is used), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function.

4) The value computation of the built-in postincrement and postdecrement operators is sequenced before its side-effect.

5) The side effect of the built-in preincrement and predecrement operators is sequenced before its value computation (implicit rule due to definition as compound assignment)

6) Every value computation and side effect of the first (left) argument of the built-in logical AND operator && and the built-in logical OR operator || is sequenced before every value computation and side effect of the second (right) argument.

7) Every value computation and side effect associated with the first expression in the logical :? operator is sequenced before every value computation and side effect associated with the second or third expression.

8) The side effect (modification of the left argument) of the built-in assignment operator and of all built-in compound assignment operators is sequenced after the value computation (but not the side effects) of both left and right arguments, and is sequenced before the value computation of the assignment expression (that is, before returning the reference to the modified object)

9) Every value computation and side effect of the first (left) argument of the built-in comma operator is sequenced before every value computation and side effect of the second (right) argument.

10) In list-initialization , every value computation and side effect of a given initializer clause is sequenced before every value computation and side effect associated with any initializer clause that follows it in the comma-separated list of the initializer list.

11) A function call that is not sequenced before or sequenced after another function call is indeterminately sequenced (the program must behave as if the CPU instructions that constitute different function calls were not interleaved, even if the functions were inlined)

12) The call to the allocation function (operator new) is indeterminately sequenced with respect to the evaluation of the constructor arguments in a new-expression

[ edit ] Undefined behavior

1) If a side effect on a scalar object is unsequenced relative to another side effect on the same scalar object, the behavior is undefined.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior (but i = ++i + 1; is well-defined) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) If a side effect on a scalar object is unsequenced relative to a value computation using the value of the same scalar object, the behavior is undefined.

cout << i << i ++ ; // undefined behavior a [ i ] = i ++ ; // undefined bevahior

[ edit ] Sequence point rules (until C++11)

Evaluation of an expression might produce side effects, which are: accessing an object designated by a volatile lvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations.

A sequence point is a point in the execution sequence where all side effects from the previous evaluations in the sequence are complete, and no side effects of the subsequent evaluations started.

1) There is a sequence point at the end of each full expression (typically, at the semicolon)

2) When calling a function (whether or not the function is inline and whether or not function call syntax was used), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body

3) There is a sequence point after the copying of a returned value of a function and before the execution of any expressions outside the function.

4) Once the execution of a function begins, no expressions from the calling function are evaluated until execution of the called function has completed (functions cannot be interleaved)

5) In the evaluation of each of the following four expressions, using the built-in (non-overloaded) operators, there is a sequence point after the evaluation of the expression a .

a && b a || b a ? b : c a , b

1) Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression.

i = ++ i + i ++ ; // undefined behavior i = i ++ + 1 ; // undefined behavior i = ++ i + 1 ; // undefined behavior (well-defined in C++11) ++ ++ i ; // undefined behavior (well-defined in C++11) f ( ++ i, ++ i ) ; // undefined behavior f ( i = - 1 , i = - 1 ) ; // undefined behavior

2) Between the previous and next sequence point , the prior value of a scalar object that is modified by the evaluation of the expression, shall be accessed only to determine the value to be stored.

[ edit ] See also

  • Operator precedence which defines how expressions are built from their source code representation.
  • Order of evaluation in the C programming language.

Order of evaluation

Order of evaluation of any part of any expression, including order of evaluation of function arguments is unspecified (with some exceptions listed below). The compiler can evaluate operands and other subexpressions in any order, and may choose another order when the same expression is evaluated again.

There is no concept of left-to-right or right-to-left evaluation in C++. This is not to be confused with left-to-right and right-to-left associativity of operators: the expression a() + b() + c() is parsed as (a() + b()) + c() due to left-to-right associativity of operator+, but c() may be evaluated first, last, or between a() or b() at run time:

Possible output:

"Sequenced before" rules (since C++11)

Evaluation of expressions.

Evaluation of each expression includes:

  • Value computations : calculation of the value that is returned by the expression. This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue evaluation, e.g. if the expression returns a number, or some other value).
  • Initiation of side effects : access (read or write) to an object designated by a volatile glvalue, modification (writing) to an object, calling a library I/O function, or calling a function that does any of those operations.

Sequenced before is an asymmetric, transitive, pair-wise relationship between evaluations within the same thread.

  • If A is sequenced before B (or, equivalently, B is sequenced after A), then evaluation of A will be complete before evaluation of B begins.
  • If A is not sequenced before B and B is sequenced before A, then evaluation of B will be complete before evaluation of A begins.
  • Evaluations of A and B are unsequenced : they may be performed in any order and may overlap (within a single thread of execution, the compiler may interleave the CPU instructions that comprise A and B).
  • Evaluations of A and B are indeterminately sequenced : they may be performed in any order but may not overlap: either A will be complete before B, or B will be complete before A. The order may be the opposite the next time the same expression is evaluated.

Undefined behavior

1) If a side effect on a memory location is unsequenced relative to another side effect on the same memory location, the behavior is undefined .

2) If a side effect on a memory location is unsequenced relative to a value computation using the value of any object in the same memory location, the behavior is undefined .

Sequence point rules (until C++11)

Pre-c++11 definitions.

Evaluation of an expression might produce side effects, which are: accessing an object designated by a volatile lvalue, modifying an object, calling a library I/O function, or calling a function that does any of those operations.

A sequence point is a point in the execution sequence where all side effects from the previous evaluations in the sequence are complete, and no side effects of the subsequent evaluations started.

Pre-C++11 Rules

1) There is a sequence point at the end of each full-expression (typically, at the semicolon).

2) When calling a function (whether or not the function is inline and whether or not function call syntax was used), there is a sequence point after the evaluation of all function arguments (if any) which takes place before execution of any expressions or statements in the function body.

3) When returning from a function, there is a sequence point after the copy-initialization of the result of the function call, and before the destruction of all temporary objects at the end of expression in the return statement (if any).

4) There is a sequence point after the copying of a returned value of a function and before the execution of any expressions outside the function.

5) Once the execution of a function begins, no expressions from the calling function are evaluated until execution of the called function has completed (functions cannot be interleaved).

6) In the evaluation of each of the following four expressions, using the built-in (non-overloaded) operators, there is a sequence point after the evaluation of the expression a .

Pre-C++11 Undefined behavior

1) Between the previous and next sequence point, the value of any object in a memory location must be modified at most once by the evaluation of an expression, otherwise the behavior is undefined .

2) Between the previous and next sequence point, for any object in a memory location, its prior value that is modified by the evaluation of the expression must be accessed only to determine the value to be stored. If it is accessed in any other way, the behavior is undefined .

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

  • C++23 standard (ISO/IEC 14882:2023):
  • 6.9.1 Program execution [intro.execution]
  • 7.6.1.6 Increment and decrement [expr.post.incr]
  • 7.6.2.8 New [expr.new]
  • 7.6.14 Logical AND operator [expr.log.and]
  • 7.6.15 Logical OR operator [expr.log.or]
  • 7.6.16 Conditional operator [expr.cond]
  • 7.6.19 Assignment and compound assignment operators [expr.ass]
  • 7.6.20 Comma operator [expr.comma]
  • 9.4.5 List-initialization [dcl.init.list]
  • C++20 standard (ISO/IEC 14882:2020):
  • 7.6.1.5 Increment and decrement [expr.post.incr]
  • 7.6.2.7 New [expr.new]
  • 9.4.4 List-initialization [dcl.init.list]
  • C++17 standard (ISO/IEC 14882:2017):
  • 4.6 Program execution [intro.execution]
  • 8.2.6 Increment and decrement [expr.post.incr]
  • 8.3.4 New [expr.new]
  • 8.14 Logical AND operator [expr.log.and]
  • 8.15 Logical OR operator [expr.log.or]
  • 8.16 Conditional operator [expr.cond]
  • 8.18 Assignment and compound assignment operators [expr.ass]
  • 8.19 Comma operator [expr.comma]
  • 11.6.4 List-initialization [dcl.init.list]
  • C++14 standard (ISO/IEC 14882:2014):
  • 1.9 Program execution [intro.execution]
  • 5.2.6 Increment and decrement [expr.post.incr]
  • 5.3.4 New [expr.new]
  • 5.14 Logical AND operator [expr.log.and]
  • 5.15 Logical OR operator [expr.log.or]
  • 5.16 Conditional operator [expr.cond]
  • 5.17 Assignment and compound assignment operators [expr.ass]
  • 5.18 Comma operator [expr.comma]
  • 8.5.4 List-initialization [dcl.init.list]
  • C++11 standard (ISO/IEC 14882:2011):
  • Operator precedence which defines how expressions are built from their source code representation.

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COMMENTS

  1. Order of evaluation

    Order of evaluation. Order of evaluation of the operands of any C operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another ...

  2. Order of evaluation

    Sequenced before" rules (since C++11) [] Evaluation of ExpressionEvaluation of each expression includes: Value computations: calculation of the value that is returned by the expression.This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue ...

  3. C order of evaluation of assignment statement

    When using the simple assignment operator: =, the order of evaluation of operands is unspecified. There is also no sequence point in between the evaluations. It is not specified in which order they are called at any time, and yet this behavior is completely defined. A function call will introduce a sequence point.

  4. C Operator Precedence

    Precedence and associativity are independent from order of evaluation. The standard itself doesn't specify precedence levels. They are derived from the grammar. In C++, the conditional operator has the same precedence as assignment operators, and prefix ++ and --and assignment operators don't have the restrictions about their operands.

  5. Precedence and order of evaluation

    Sequential evaluation. Left to right. 1 Operators are listed in descending order of precedence. If several operators appear on the same line or in a group, they have equal precedence. 2 All simple and compound-assignment operators have equal precedence. An expression can contain several operators with equal precedence.

  6. Order of evaluation

    order of evaluation: operators: operator precedence: alternative representations: Utilities Types typedef declaration: type alias declaration (C++11) attributes (C++11) Casts implicit conversions: ... copy assignment: move assignment (C++11) destructor: Templates class template : function template :

  7. Stricter Expression Evaluation Order in C++17

    In an expression: f(a, b, c); The order of evaluation of a, b, c is still unspecified in C++17, but any parameter is fully evaluated before the next one is started. It's especially crucial for complex expressions like this: f(a(x), b, c(y)); if the compiler chooses to evaluate x first, then it must evaluate a (x) before processing b, c (y) or y.

  8. Order of evaluation

    The compiler can evaluate operands in any order, and may choose another order when the same expression is evaluated again. There are exceptions to this rule which are noted below. Except where noted below, there is no concept of left-to-right or right-to-left evaluation in C++.

  9. Order of Evaluation in C++17

    In C++ programming, the order of evaluation of expressions can have a significant impact on the behavior and correctness of the code. C++17 introduced changes to the order of evaluation rules, providing clearer guidelines and improving consistency across different compilers. ... Assignment Operator Sequencing: For = and +=, the right value is ...

  10. c ++

    The order of evaluation of an expression (and its sub-expressions) is unspecified.Meaning, it's up to the compiler. If the compiler itself doesn't provide any guarantee on the OoE (and most of the time it doesn't) but your code assumes one, then expressions with side-effects on other operands may will cause undefined behavior, which in turn may cause runtime issues.

  11. Assignment operators

    Assignment performs implicit conversion from the value of rhs to the type of lhs and then replaces the value in the object designated by lhs with the converted value of rhs . Assignment also returns the same value as what was stored in lhs (so that expressions such as a = b = c are possible). The value category of the assignment operator is non ...

  12. Order of evaluation

    Order of evaluation of the operands of any C++ operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the ...

  13. PDF Order of evaluation

    evaluation of function arguments in a function-call expression and the order of evaluation of the subexpressions within any expression) is unspecified. The compiler can evaluate operands in any order, and may choose another order when the same expression is evaluated again. There are exceptions to this rule which are noted below. Except where ...

  14. Order of evaluation

    assignment operators. increment and decrement. function call, comma, conditional operator. sizeof. alignof (C11) cast operators Order of evaluation of the operands of any C operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is ...

  15. C++ Core Guidelines: Rules for Expressions

    This will not hold for C++17. With C++17, the order of evaluation of the last code snippet is right to left; therefore, the expression has well-defined behavior. ... Assignment expressions are evaluated from right to left. This includes compound assignments. Operands to shift operators are evaluated from left to right.

  16. What are the evaluation order guarantees introduced by C++17?

    Related to Order of evaluation of assignment statement in C++ and Does this code from "The C++ Programming Language" 4th edition section 36.3.6 have well-defined behavior? which are both covered by the paper. The first one might make a nice additional examples in your answer below. -

  17. Order of evaluation

    Order of evaluation of the operands of any C++ operator, including the order of evaluation of function arguments in a function-call expression, and the order of evaluation of the subexpressions within any expression is unspecified (except where noted below). The compiler will evaluate them in any order, and may choose another order when the ...

  18. Order of Evaluation

    "Sequenced before" rules (since C++11) Evaluation of Expressions. Evaluation of each expression includes: value computations: calculation of the value that is returned by the expression.This may involve determination of the identity of the object (glvalue evaluation, e.g. if the expression returns a reference to some object) or reading the value previously assigned to an object (prvalue ...

  19. Evaluation order of side effects for assignment operator in C++11

    First of all, note that C++17 introduced quite some changes to expression evaluation order.. Let's first see what the current standard draft has to say. I guess relevant here should be [intro.execution]/7. Evaluation of an expression (or a subexpression) in general includes both value computations (including determining the identity of an object for glvalue evaluation and fetching a value ...