Order of evaluation in C++ function parameters

问题:

If we have three functions (foo, bar, and baz) that are composed like so...

foo(bar(), baz())

Is there any guarantee by the C++ standard that bar will be evaluated before baz?

回答1:

No, there's no such guarantee. It's unspecified according to the C++ standard.

Bjarne Stroustrup also says it explicitly in "The C++ Programming Language" 3rd edition section 6.2.2, with some reasoning:

Better code can be generated in the absence of restrictions on expression evaluation order

Although technically this refers to an earlier part of the same section which says that the order of evaluation of parts of an expression is also unspecified, i.e.

int x = f(2) + g(3);   // unspecified whether f() or g() is called first

回答2:

There's no specified order for bar() and baz() - the only thing the Standard says is that they will both be evaluated before foo() is called. From the C++ Standard, section 5.2.2/8:

The order of evaluation of arguments is unspecified.

回答3:

From [5.2.2] Function call,

The order of evaluation of arguments is unspecified. All side effects of argument expression evaluations take effect before the function is entered.

Therefore, there is no guarantee that bar() will run before baz(), only that bar() and baz() will be called before foo.

Also note from [5] Expressions that:

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.

so even if you were asking whether bar() will run before baz() in foo(bar() + baz()), the order is still unspecified.

回答4:

C++17 specifies evaluation order for operators that was unspecified until C++17. See the question What are the evaluation order guarantees introduced by C++17? But note your expression

foo(bar(), baz())

has still unspecified evaluation order.

回答5:

In C++11, the relevant text can be found in 8.3.6 Default arguments/9 (Emphasis mine)

Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspecified. Consequently, parameters of a function shall not be used in a default argument, even if they are not evaluated.

The same verbiage is used by C++14 standard as well, and is found under the same section.

回答6:

As others have already pointed out, the standard does not give any guidance on order of evaluation for this particular scenario. This order of evaluation is then left to the compiler, and the compiler might have a guarantee.

It's important to remember that the C++ standard is really a language to instruct a compiler on constructing assembly/machine code. The standard is only one part of the equation. Where the standard is ambiguous or is specifically implementation defined you should turn to the compiler and understand how it translates C++ instructions into true machine language.

So, if order of evaluation is a requirement, or at least important, and being cross-compiler compatible is not a requirement, investigate how your compiler will ultimately piece this together, your answer could ultimate lie there. Note that the compiler could change it's methodology in the future