模板友Function/Class前向声明

Question

Why do we need template forward declaration for BlobPtr and operator overloading for template.

template <typename> class BlobPtr;
template <typename> class Blob; 

template <typename T>
    bool operator==(const Blob<T>&, const Blob<T>&); 

template <typename T> 
class Blob {
    friend class BlobPtr<T>; 
    friend bool operator==<T>
        (const Blob<T>&, const Blob<T>&); 

};
class BlobPtr<T>{};

Blob<char> ca; 

Blob<int> ia; 

For for nontemplate class, forward declaration is not needed.

class Blob {
type
    
   class BlobPtr; 
   friend bool operator==
        (const Blob&, const Blob&); 

};
class BlobPtr{};

Answer 1

If you want to declare the template in a friend declaration, you can just do that, you don't need forward declarations:

template <typename T> 
class Blob {
    template <typename U>
    friend class BlobPtr;     

    template <typename U>
    friend bool operator==(const Blob<U>&, const Blob<U>&); 
};

This declares all instances of the templates as friends, not just the matching one. That is, BlobPtr<int> and BlobPtr<long> (and generally BlobPtr<Anything> ) are all friends of Blob<int> .

If you want to declare a particular specialization of a template as a friend (so that BlobPtr<int> is a friend of Blob<int> but BlobPtr<long> is not), then you first need to tell the compiler that BlobPtr is a template in the first place - that's what you need forward declaration for.