How to declare constexpr extern?

Question

Is it possible to declare a variable extern constexpr and define it in another file?

I tried it but the compiler gives error:

De

Answer 1

I agree with 'swang' above, but there is a consequence. Consider:

ExternHeader.hpp

extern int e; // Must be extern and defined in .cpp otherwise it is a duplicate symbol.

ExternHeader.cpp

#include "ExternHeader.hpp"
int e = 0;

ConstexprHeader.hpp

int constexpr c = 0; // Must be defined in header since constexpr must be initialized.

Include1.hpp

void print1();

Include1.cpp

#include "Include1.hpp"
#include "ExternHeader.hpp"
#include "ConstexprHeader.hpp"
#include 

void print1() {
    std::cout << "1: extern = " << &e << ", constexpr = " << &c << "\n";
}

Include2.hpp

void print2();

Include2.cpp

#include "Include2.hpp"
#include "ExternHeader.hpp"
#include "ConstexprHeader.hpp"
#include 

void print2() {
    std::cout << "2: extern = " << &e << ", constexpr = " << &c << "\n";
}

main.cpp

#include 
#include "Include1.hpp"
#include "Include2.hpp"

int main(int argc, const char * argv[]) {
    print1();
    print2();
    return 0;
}

Which prints:

1: extern = 0x1000020a8, constexpr = 0x100001ed0
2: extern = 0x1000020a8, constexpr = 0x100001ed4

IE the constexpr is allocated twice whereas the extern is allocated once. This is counterintuitive to me, since I 'expect' constexpr to be more optimized than extern.

Edit: const and constexpr have the same behaviour, with regard to allocation, therefore from that point of view the behaviour is as expected. Though, as I said, I was surprised when I came across the behaviour of constexpr.