Question

I have a question for Specific Template Friendship in C++.
in the book C++ Primer, the specific template friendship is written like this:

 template <class T> class Foo3;
 template <class T> void templ_fcn3(const T&);
 template <class Type> class Bar {
     // each instantiation of Bar grants access to the
     // version of Foo3 or templ_fcn3 instantiated with the same type
     friend class Foo3<Type>;
     friend void templ_fcn3<Type>(const Type&);
     // ...
 };

The special point is that there is

<Type>

after the class or function name in the friend statement.

However,in practice, if I write this:

template <class Type> class T_CheckPointer;
template <class T> T_CheckPointer<T> operator+(const T_CheckPointer<T> &, const size_t n);

template <typename Type>
class T_CheckPointer {

    // Specific Template Friendship
    friend T_CheckPointer<Type>
    operator+ <Type> (const T_CheckPointer<Type> &, const size_t n);

// other code...

}

There will be a error during instantiations for the template function.

And if I change

// Specific Template Friendship
friend T_CheckPointer<Type>
    operator+ <Type> (const T_CheckPointer<Type> &, const size_t n);

to

// Specific Template Friendship
friend T_CheckPointer<Type>
    operator+ <> (const T_CheckPointer<Type> &, const size_t n);

by deleting the word type after the function name, then all will be okay.

Anyone can tell me the reason?

---

For information, there is the error message when I call

int iarr[] = {1, 2, 3, 4};
T_CheckPointer<int> itcp(iarr, iarr+4);

error message:

/usr/include/c++/4.4/bits/stl_iterator_base_types.h: In instantiation of ‘std::iterator_traits<int>’:
/usr/include/c++/4.4/bits/stl_iterator.h:96:   instantiated from ‘std::reverse_iterator<int>’
../Classes/T_CheckPointer.hpp:31:   instantiated from ‘T_CheckPointer<int>’
../PE16.cpp:520:   instantiated from here
/usr/include/c++/4.4/bits/stl_iterator_base_types.h:127: error: ‘int’ is not a class, struct, or union type
/usr/include/c++/4.4/bits/stl_iterator_base_types.h:128: error: ‘int’ is not a class, struct, or union type
/usr/include/c++/4.4/bits/stl_iterator_base_types.h:129: error: ‘int’ is not a class, struct, or union type
/usr/include/c++/4.4/bits/stl_iterator_base_types.h:130: error: ‘int’ is not a class, struct, or union type
/usr/include/c++/4.4/bits/stl_iterator_base_types.h:131: error: ‘int’ is not a class, struct, or union type

Answer 1

Here’s a minimal example:

template<typename T> struct U { typedef typename T::X X; };
template<typename T> void foo(typename U<T>::X);

template<typename T> struct S;
template<typename T> void foo(S<T>);
template<typename T> struct S { friend void foo<T>(S<T>); };

template struct S<int>;

The reason the friend declaration fails is that by providing a full list of template arguments you are requesting the compiler specialize all available function templates and pick the one that best matches the signature. Specialization of the first definition of foo results in specializing U with an argument that results in an ill-formed program.

If instead you omit the template argument, it will be deduced from the arguments. As such template argument deduction is performed according to 14.8.2 [temp.deduct], and in particular 14.8.2p8 applies, which means that the substitution failure in the specialization of U is not an error (SFINAE).

This is a good reason to omit template arguments anywhere they can be deduced from context (e.g. here the function or operator parameter types). Note that you still need to provide the <> brackets to ensure that the operator + is read as a template-id (14.5.4 [temp.friend]).