Can I put `extern template` into a header file?

Is putting an extern template in a header file and then do the explicit template instantiation in a unit compilation file valid ?

For example in the compiling example for g++, is this working to avoid the instancation of nothing<int> twice ? Why doesn't anybody write it like this and prefer to copy pase the extern template line in each .cpp file ?

A.hpp:

#ifndef HEADERC_A
#define HEADERC_A

template< typename T > struct nothing {};
extern template struct nothing<int>;

#endif

A.cpp:

#include "A.hpp"

template struct nothing<int>;

main.cpp:

#include "A.hpp"
#include <iostream>

int main()
{
    nothing<int> n;
    return 0;
}