How to give a friend function a default argument while separating declaration and definition

Question

I have a function template that I'm declaring as a friend of my class. The thing is, I want to keep the declaration and definition separate because I don't want to clutter the body of X, but I'm running into problems when using the default argument of T():

struct X
{
    template <class T>
    friend void f(T t);
};

template <class T>
void f(T t = T()) {}

// error: default arguments cannot be added to a function template that has
// already been declared
// void f(T t = T()) {}
//          ^   ~~~
// main.cpp:9:17: note: previous template declaration is here
//     friend void f(T t);
//                 ^

If I switch it around to use default arguments in the declaration instead, then the compiler outputs a message saying I have to define the function there, but I don't want to do that.

How do I give a default argument to a friend function template while keeping the definition and declaration separate?

Answer 1

For function templates you may only write the default arguments in the first declaration in any given scope; and, indeed, there is another rule that prohibits writing the default arguments in the friend declaration:

[C++11: 8.3.6/4]: For non-template functions, default arguments can be added in later declarations of a function in the same scope. [..] If a friend declaration specifies a default argument expression, that declaration shall be a definition and shall be the only declaration of the function or function template in the translation unit.

However, there's nothing stopping you from adding yet another declaration, this time not a friend declaration, before everything else, and put your default arguments on that.

So, in order:

1. Declaration of function template, with default arguments
2. Class definition with friend declaration
3. Definition of function template

Answer 2

I think if you declare the template function before friending it that would work, but I can't figure out what your main.cpp has to test. Did you try this (EDIT: this seems to work on coliru but I can't test your real-world scenario).

template <class T>
void f(T t = T());

struct X
{
    template <class T>
    friend void f(T t);
};

template <class T>
void f(T t) {}