Templated operator<< explicit instantiation and header

Question

Typically for my templated classes, I include declarations in a .hpp file and templated implementation code in a .t.hpp file. I explicitly instantiate the class in a .cpp file:

template class MyClass< AnotherClass >;

whose object code gets put in a library.

The problem is that if I try to print the object with operator<<, which is declared in the .hpp file and defined in the .t.hpp file as:

template<class T>
std::ostream& operator<<( std::ostream& os, const MyClass<T>& c)
{
    os  << "Hello, I am being output.";
    return os;
}

I get a linker error saying that the right symbol is undefined.

I understand that this is because this templated function is not explicitly instantiated when the class is. Is there a way to get around this other than to include the .t.hpp file any time I want to use operator<< on the class, or to move the templated function code into the .hpp file? Can I explicitly instantiate the function code?

Answer 1

You can explicitly instantiate function templates

template std::ostream& operator<<(std::ostream&, const MyClass<int>&);

To instantiate a specialization with T = int. The template argument brackets can be omitted if all template arguments can be deduced (like in this case, from the type MyClass<int>. If it can't, for example because the template parameters do not occur in a function parameter type, you have to explicitly specify it

template<typename T> void f() { }
template void f<int>();

Answer 2

See litb's solution.

For your specific case, the compiler can deduce all template arguments from parameter and return types, but if it could not you could give them explicitly:

template std::ostream& operator<< <T>(std::ostream&, const MyClass<T>&);

This is still allowed if the template arguments can be deduced.

May I ask: Is there a reason why you prefer to explicitly instantiate everything? You are making quite a lot of work for yourself. You will certainly save compilation time by avoiding including the template source code everywhere, but is it worth it in developer time?