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

Templated Struct/Class Declaration as Friend

In C programming, one may encounter scenarios where declaring a templated struct or class as a friend is desired. However, when attempting the common approach below, some compilers, such as Visual C 8 (VC8), raise errors:

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

The rationale behind this error is the compiler's inability to handle multiple template parameter lists in a single line. To overcome this hindrance, an alternative syntax is introduced:

template <typename> friend class foo;

By specifying an empty template parameter list, the friend declaration now applies to all potential instantiations of the templated struct or class. However, it's worth noting that this approach makes all templates friends with each other. For instance, foo and foo would both be friends, despite your original intent of declaring instances of foo as friends to foo.

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