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<typename T> class A {...};
template class A<int>; // explicit template instantiation  to reduce compilation time

But in every translation unit where a.h is included, it seems A<int> will be compiled. The compilation time is not reduced. How to use explicit template instantiation to reduce compilation time?

Answer 1

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>;