I'm trying to figure out why my code compiles, when it shouldn't:
#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
constexpr int ret_one()
{
return 1;
}
constexpr int f(int p)
{
return ret_one() * p;
}
int main() {
int i = 2;
srand(time(0));
int j = rand();
int first_array[f(10)]; // OK - 10 is a constant expression
int second_array[f(j)]; // Error - the parameter is not a constant expression
j = f(i); // OK - doesn't need to be constexpr
std::cout << sizeof(second_array);
return 0;
}
So the first_array
definition is OK. But because j
is not a constant expression, the second_array
definition should be wrong. On each program run I'm getting different array sizes. Is that how it's supposed to work? In my book the author clearly states that a constepxr is an expression whose value can be evaluated at compile time. Can rand()
be evaluated at compile time? I think it can't be.
Some compilers, such as GCC, allow C-style variable-length arrays as an extension to C++. If your compiler does that, then your code will compile.
If you're using GCC, then you can enable a warning with -Wvla
or -pedantic
.
in fact,
int second_array[f(j)];
will use non standard VLA (Varaible length array) extension.
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