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Comment by fooker | Hacker News Reader

That’s not a valid evaluation order, afaik. You can not start evaluating one argument, stop and switch to the other, then come back to the earlier one.

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Do take a look at dvt's comment. Prior to C++17, it was a valid evaluation order.

Prior to C++17 such an evaluation order would have violated the standard. ISO/IEC 14882:2011 1.9/13 [Note: Indeterminately sequenced evaluations cannot overlap, but either could be executed first. — end note]. The arguments in a faction call are a comma-separated indeterminate sequence (5.2.2/4 [Note: Such initializations are indeterminately sequenced with respect to each other (1.9) — end note]).

Is there any version of C++ in which this interleaving is permitted?

The full context to that second quote is:

> When a function is called, each parameter ([dcl.fct]) shall be initialized ([dcl.init], [class.copy], [class.ctor]) with its corresponding argument. [ Note: Such initializations are indeterminately sequenced with respect to each other ([intro.execution]) — end note ]

So it's only about initialisation of parameters from arguments, not to the evaluation of arguments. (For example if foo() takes a std::string parameter, and bar() returns a const char*, then in the expression foo(bar()) the above quote refers to the call to the string constructor, not to the call to bar().)

The more relevant part is 5.2.2/8:

> [ Note: The evaluations of the postfix expression and of the argument expressions are all unsequenced relative to one another. All side effects of argument expression evaluations are sequenced before the function is entered (see [intro.execution]). — end note ]

> Prior to C++17 such an evaluation order would have violated the standard

Herb Sutter disagrees. [0] The C++ legalese you've quoted seems decisive, unfortunately Sutter's post doesn't mention it.