了解c ++中的extern模板

Question

I am trying to understand extern templates, but i can't get it to work. My goal is to compile some instanciations if Foo<> in a seperate compilation unit in order to save compilation time. In my code base, the template argument is a enum class , so in theory I am able to compile all instanciations in a compilation unit and link them with the rest of the project.

Here is a little example I come up with:

//Foo.hpp
#pragma once

template <class T>
struct Foo {
    Foo();
    ~Foo();
};

extern template struct Foo<int>;

---

//Foo.cpp
#include "Foo.hpp"

template struct Foo<int>;

---

//main.cpp
#include <iostream>

#include "Foo.hpp"

int main(int argc, char **argv) {
   Foo<int> foo;

    return 0;
}

To for compilation i used a makefile, that boils down to this:

g++ -c ../Foo.cpp ../main.cpp
g++ Foo.o main.o 

The output I get with gcc 7.1.1 and basically the same with clang 4.0.1 is:

main.o: In function `main':
main.cpp:(.text+0x27): undefined reference to `Foo<int>::Foo()'
main.cpp:(.text+0x38): undefined reference to `Foo<int>::~Foo()'

My main question is, why is Foo<int>::Foo() and Foo<int>::~Foo() not compiled into Foo.o ?

Answer 1

Because the constructor and destructor are not defined anywhere, you only declare them. You correctly explicitly instantiate the template in the Foo.cpp file, but the functions are still not defined.

If you are only going to use Foo<int> , then you can define the constructor and destructor in Foo.cpp ,

template<typename T>
Foo<T>::Foo(){...} // or = default

and

template<typename T>
Foo<T>::~Foo(){...}

and due to the fact that you explicitly instantiate the template in the Foo.cpp the linker will find the definition. Otherwise, you need to provide the definition in the header file.