实例化模板成员函数

Question

I have template member functions, but I cannot figure out how I can instantiate them when I am using them properly. The structure is as follows:

// foo.h
class Foo {
template<typename T>
T Func(T);
}
#include "foo.tpp"

// foo.tpp
template<typename T>
T Func(T input) {return(input);}

How should I instantiate the template if I want to realize the following code in the main function?

//main.cpp
#include "foo.h"
int main()
{
    Foo obj;
    int int_input, int_output;
    int_output = obj.Func(int_input);
    double double_input, double_output;
    double_output = obj.Func(double_input);
    return 0;
}

I have checked answers for similar questions such as Instantiation of template member function , but I do not want to instantiate all the member functions for all combinations every time I use them. Another option might be defining the class as a template class and instantiate different classes using template class Foo<int>; . However, this forces me to define different classes for different type of member functions, which I do not prefer. Is there other solution? Thank you very much!

Answer 1

template<typename T>
T Func(T input) {return(input);}

This declares a free-standing function template in the global scope that is unrelated to your Foo class. To implement the member function, you need to prefix the function definition's name with Foo:: , as you would for any other member function implementation.

template<typename T>
T Foo::Func(T input){ return input; }

Also note that return(value); as opposed to return value; is a slight anti-pattern. While this doesn't usually matter, it can make a difference when using things like the decltype(auto) inferred return type and cause an accidental dangling reference.