CODEWITHSUNDEEP
c++cc++11expressionoperator-precedence
fredoverflow
fredoverflow

Reputation: 263098

order of evaluation of operands

In the expression a + b, is a guaranteed to be evaluated before b, or is the order of evaluation unspecified? I think it is the latter, but I struggle to find a definite answer in the standard.

Since I don't know whether C handles this different from C++, or if evaluation order rules were simplified in C++11, I'm gonna tag the question as all three.

Upvotes: 16

Views: 5113

Answers (6)

dbush
dbush

Reputation: 223699

According to the current C standard, C11, it also specifies that the order of evaluation of subexpressions (a and b in this case) is indeterminate. In fact, this order doesn't even have to be the same if the same expression is evaluated multiple times.

From section 6.5:

The grouping of operators and operands is indicated by the syntax.85) Except as specified later, side effects and value computations of subexpressions are unsequenced.86)

86) In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed consistently in different evaluations.

Upvotes: 0

Yann Ramin
Yann Ramin

Reputation: 33167

It is unspecified. C and C++ follow the same logic in selecting sequence points:

https://en.wikipedia.org/wiki/Sequence_point

Upvotes: 1

Peter Alexander
Peter Alexander

Reputation: 54270

In C++, for user-defined types a + b is a function call, and the standard says:

§5.2.2.8 - [...] The order of evaluation of function arguments is unspecified. [...]

For normal operators, the standard says:

§5.4 - Except where noted, the order of evaluation of operands of individual operators and subexpressions of individual expressions, and the order in which side effects take place, is unspecified. [...]

These haven't been changed for C++11. However, the wording changes in the second one to say that the order is "unsequenced" rather than unspecified, but it is essentially the same.

I don't have a copy of the C standard, but I imagine that it is the same there as well.

Upvotes: 13

Alok Save
Alok Save

Reputation: 206508

It is Unspecified.

Reference - C++03 Standard:

Section 5: Expressions, Para 4:

except where noted [e.g. special rules for && and ||], the order of evaluation of operands of individual operators and subexpressions of individual expressions, and the order in which side effects take place, is Unspecified.

Upvotes: 11

fredoverflow
fredoverflow

Reputation: 263098

C++0x FDIS section 1.9 "Program Execution" §15 is similar to the corresponding paragraph in C++03, just reworded to accommodate the conceptual change from "sequence points" to "being sequenced":

Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced.

Upvotes: 1

Janick Bernet
Janick Bernet

Reputation: 21184

For C: "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."

Source: http://msdn.microsoft.com/en-us/library/2bxt6kc4.aspx

The way this is organized in that article, it seems to indicate this applies to C++ too, which is confirmed by the answer to this question: Operator Precedence vs Order of Evaluation.

Upvotes: 0

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