How to Declare a Templated Struct or Class as a Friend in C ?

Declaring a Templated Struct or Class as a Friend

In C programming, it is possible to declare a templated struct or class as a friend to another class, effectively allowing access to private members. However, some compilers, such as VC8, may encounter errors when implementing this feature.

Incorrect Syntax:

The following code attempts to declare a templated struct foo as a friend to all other instantiations of foo:

template <typename T>
struct foo
{
    template <typename S>
    friend struct foo<S>;

   private:
    // ...
};

However, this will result in the error:

error C3857: 'foo<T>': multiple template parameter lists are not allowed

Correct Syntax:

To correctly declare a templated struct or class as a friend, use the following syntax:

template <typename> friend class foo;

This will allow all template instantiations of foo to be friends of each other, as desired. Therefore, the correct code should be:

template <typename T>
struct foo
{
   template <typename>
   friend class foo;

   private:
    // ...
};

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