Why Does Partial Specialization of a Template Function Member Cause 'Invalid Use of Incomplete Type'?

"Invalid Use of Incomplete Type" Error in Partial Template Specialization

When attempting to create a partial specialization of a template function with the syntax:

template <typename T>
void foo<int, T>::bar() {
}

you may receive the error "invalid use of incomplete type 'struct foo'." This occurs because C requires you to partially specialize the entire template when specializing a member function.

To resolve this, specify the complete template signature when partially specializing the class:

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

template <>
struct foo<int, T> {
   void bar() {
   }
};

Alternatively, you can use a workaround such as creating a nested template struct or deriving from another template that partially specializes.

Note: This applies only to partial specialization of member functions. Fully specializing a template function does not require specializing the entire template.

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