Why does this compile?

[EwCjE3v7]

Why does this compile? Value isn`t known till run time? What if this was to return a variable that was not a constexpr?

#include <iostream>
using std::cout; using std::endl; using std::cin;

int txt_size()
{
    return 0;
}
int main()
{
    int ia[txt_size()];
    return 0;
}

[Yanson]

it doesn't compile at least not on my compiler. Must be a quirk of the compiler you are using.

[keskiverto]

That value is known at compile-time, if the compiler is aggressive enough in optimization. Depends on compiler.

Similarly, template metaprogramming could do rather complex calculations.

A support for VLA (variable lenght arrays) would allow runtime size determination. C has that. Next C++ standard might get it too (whether it is sane or not).