typename keyword

I used this keyword to specify that we are using a type, as opposed to an actual value.

For example, consider (NVM bad example)

• though it showcases how we ca have a variable be a “type” or a “value”

#include <iostream>
 
template <typename T, int Value>
struct A {
    using t1 = T;
    static constexpr int t2 = Value;
};
 
template <typename T>
struct A<T, 5> {
    using t2 = T;
    static constexpr int t1 = 6;
};
 
int main() {
    A<int, 5> a;
    typename A<int, 5>::t2 v = A<int, 5>::t1; // typename is actually not needed here, because we have a full specialization
    std::cout << v << std::endl;
}

At the root of it

Structure templates can have different data members for each specialization.

The compiler needs to know in advance if a symbol within a structure is a type or a static member when the structure template depends on another template parameter.

The keyword typename placed before a structure template solves this ambiguity.

Below is a a better example:

template<typename T>
struct A {
using type = int;
};
 
template<typename R>
void g() {
using X = typename A<R>::type; // "type" is a typename or a data member depending on R
}