std::experimental::source_location
will probably be added to the C++ standard at some point. I'm wondering if it possible to get the location information into the compile-time realm. Essentially, I want a function that returns different types when called from different source locations. Something like this, although it doesn't compile because the location
object isn't constexpr
as it's a function argument:
#include <experimental/source_location>
using namespace std::experimental;
constexpr auto line (const source_location& location = source_location::current())
{
return std::integral_constant<int, location.line()>{};
}
int main()
{
constexpr auto ll = line();
std::cout << ll.value << '\n';
}
This doesn't compile, with a message about
expansion of [...] is not a constant expression
regarding the return std::integral_constant<int, location.line()>{}
line. What good it is to have the methods of source_location
be constexpr
if I can't use them?
As Justin pointed the issue with your code is that function argument are not constexpr but the problem of using source_location in a constexpr function in a more useful way is mentioned in the constexpr! functions proposal which says:
So this is currently an open problem.