C++ Explicit instantiation

explicit_instantiation.cpp

// foo.h

#pragma once

template<typename A>
class Foo
{
public:
   Foo();
   void DoSomething();
   
   template<typename B>
   void DoSomethingElse(B b);
};

// foo.cpp

#include <iostream>
#include "a.h"

template<typename A> Foo<A>::Foo()
{
   std::cout << "constructor" << std::endl;
}

template<typename A> void Foo<A>::DoSomething()
{
   std::cout << "do something" << std::endl;
}

template<typename A>
template<typename B>
void Foo<A>::DoSomethingElse(B b)
{
   std::cout << "size of " << sizeof(b) << std::endl;
}

// explicit instantiation of the class
template class Foo<int>;

// explicit instantiation of the method with A = int and B = double
template void Foo<int>::DoSomethingElse<double>(double);

// main.cpp

#include "foo.h"

int main()
{
   Foo<int> foo;
   foo.DoSomething();
   foo.DoSomethingElse<double>(3.3);
   return 0;
}