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Question #192 Difficulty: 2

According to the C++23 standard, what is the output of this program? 

    #include <vector>
#include <iostream>

std::vector<int> v;

int f1() {
  v.push_back(1);
  return 0;
}

int f2() {
  v.push_back(2);
  return 0;
}

void g(int, int) {}

void h() {
  g(f1(), f2());
}

int main() {
  h();
  h();
  std::cout << (v[0] == v[2]);
}

Correct!

The evaluation order of function argument expressions is unspecified, all we know is that they will all happen before ("be sequenced before") the contents of the called function.

In particular, in the expression g(f1(), f2()), we don't know whether f1 or f2 will be sequenced first, we only know that they will both be sequenced before the body of g.

Also: There isn't even a requirement on the implementation that f1 and f2 gets evaluated in the same order each time. So after calling h twice, v can contain 1212, 2121, 1221 or 2112.

§[expr.call]¶7:

The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.

And the helpful note in §[intro.execution]¶note-5:

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.

You can explore this question further on C++ Insights or Compiler Explorer!

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