How to use explicit template instantiation to reduce compilation time?

Question

It was suggested to use explicit template instantiation to reduce compilation time. I am wondering how to do it. For example

// a.h
template         


        

Answer 1

Declare the instantiation in the header:

extern template class A<int>;

and define it in one source file:

template class A<int>;

Now it will only be instantiated once, not in every translation unit, which might speed things up.

Answer 2

If you know that your template will be used only for certain types, lets call them T1,T2, you can move implementation to source file, like normal classes.

//foo.hpp
template<typename T>
struct Foo {
    void f();
};

//foo.cpp
template<typename T>
void Foo<T>::f() {}

template class Foo<T1>;
template class Foo<T2>;