Placing C++ template definition for a class member function in CPP file with IMPLICIT instantiation instead of .H allowed?

Question

Given code like the following:

foo.cpp

#include"foo.h"

template <int X>
void bar::foo(){
 cout << X << endl;
}

static void tea(){
 bar().foo<1>();
 bar().foo<2>();
 bar().foo<3>();
}

foo.h

class bar {
 public:
  template <int X>
  void foo();
};

main.cpp

...
bar().foo<1>();
bar().foo<2>();
bar().foo<3>();
...

Is such a pattern allowed in any version of C++?

Edit:

Why is this disallowed when explicit instantiation with template void bar::foo<1>() (and etc) would allow this to link correctly?

Answer 1

C++ compilers are not required to actually generate symbols for anything unless you force it with an extern declaration. They aggresively inline functions, which might remove all evidence of your instantiation. I tried out your example, and it linked with -O0 , but not with -O3 .