Correct syntax for friend template function

Question

In The C++ Programming Language, Fourth Edition - chapter 23.4.7 Friends, I found following example (I have slightly modified it to show only relevant part):

template<typename T>
class Vector {
public:
    friend Vector operator*<>(const Vector& v, int f); 
                           ^^ ~~~~ ?
};

template<typename T>
Vector<T> operator*(const Vector<T>& v, int f) {
    return v;
}

I tried to compile it, but I get following error (clang):

main.cpp:8:20: error: friends can only be classes or functions
        friend Vector operator*<>(const Vector& v, int f); 
                      ^
main.cpp:8:29: error: expected ';' at end of declaration list
        friend Vector operator*<>(const Vector& v, int f); 
                               ^
                               ;
2 errors generated.

Book explains that :

The <> after the name of the friend function is needed to make clear that the friend is a template function. Without the <>, a non template function would be assumed.

And that is all on this.

Without <> this code compiles, but when operator* is used (ex.: Vector<int> v; v*12;) then linker error appears:

main.cpp:(.text+0xb): undefined reference to `operator*(Vector<int> const&, int)'

So I assume that <> is needed to tell compiler that function template for operator* should be generated each time Vector template is instantiated for given type.

But what am I doing wrong in the example from the book, and why?

Answer 1

As the book said,

the <> after the name of the friend function is needed to make clear that the friend is a template function.

That means, the name should refer to a function template, which should be declared (as template) in advance. e.g.

// forward declaration of the class template
template<typename T>
class Vector;

// declaration of the function template
template<typename T>
Vector<T> operator*(const Vector<T>& v, int f);

template<typename T>
class Vector {
public:
    // friend declaration
    friend Vector operator*<>(const Vector& v, int f); 
};

// definition of the function template
template<typename T>
Vector<T> operator*(const Vector<T>& v, int f) {
    return v;
}

Answer 2

In your case, you're declaring operator* as a friend directly inside Vector, without any previous declaration. Therefore the correct syntax is:

template<typename T>
class Vector {
public:
    template<typename>
    friend Vector operator*(const Vector& v, int f);               
};

template<typename T>
Vector<T> operator*(const Vector<T>& v, int f) {
    return v;
}

live example on wandbox