When is the `typename` Keyword Necessary in C Templates?

When is the "typename" keyword necessary?

In C , the "typename" keyword is used to disambiguate nested names that are dependent on template parameters.

Why is "typename" necessary in the example code?

The example code defines a class C with a nested struct P. Inside the member function f(), the line:

typename vector<P>::iterator p = vec.begin();

requires the use of "typename" because:

• Dependent name: vector

  is a nested name that depends on the template parameter K of class C.

• Ambiguity: Without "typename", the compiler cannot determine whether vector

  ::iterator is a type or a template.

Other cases where "typename" is necessary:

"typename" is also required in the following situations:

• Referencing a dependent member function:

template<typename T>
class A {
public:
    void f(typename T::P& p); // Requires "typename"
};

• Referencing a dependent nested template:

template<typename T>
class A {
public:
    template<typename U>
    void g(typename T::template F<U>& f); // Requires "typename"
};

• Referencing a dependent template specialization:

template<typename T>
class A {
public:
    template<>
    void g<int>(typename T::F<int>& f); // Requires "typename"
};

In general, any time you need to refer to a dependent nested name, the "typename" keyword is necessary to disambiguate the type from the template.

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