When and Why Do You Need the 'typename' Keyword in C Templates?

Syntax for "typename" Keyword

The "typename" keyword is used in C when referring to a nested name that is a dependent name, meaning it's nested inside a template instance with an unknown parameter. This keyword explicitly specifies that the name represents a type, particularly when the inferred entity (value, type, or template) is ambiguous.

Usage of "typename" in Nested Names

Consider the following code snippet:

template<class K>
class C {
    struct P {};
    vector<P> vec;
    void f();
};

template<class K> 
void C<K>::f() {
    typename vector<P>::iterator p = vec.begin();
}

Here, the "typename" keyword is necessary to declare p as a type, namely an iterator for a vector of P structs. Without "typename," the compiler would interpret the sequence vector

::iterator as an expression that represents a value or a function, which would be incorrect.

Additional Cases Requiring "typename"

Apart from nested names, the "typename" keyword is also required in the following scenarios:

• When referring to the type of a template template argument
• When declaring a template class's member function as a friend of another template class
• When specifying the type of a template parameter pack
• When casting a dependent name to a specific type

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