C++ Handbook

`typename` — C++ Keyword

`typename` — C++ Keyword

The typename keyword in C++: introduces type template parameters and disambiguates dependent type names.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

typename

Has two uses: (1) introduces a type template parameter (equivalent to class in that context), and (2) disambiguates a dependent name that refers to a type inside a template.

Syntax

template <typename T> ...           // type parameter
template <typename T, typename U> ...

// Disambiguate dependent type inside a template body
typename T::value_type x;

Example

#include <print>
#include <vector>

// (1) Type parameter
template <typename T>
T square(T x) { return x * x; }

// (2) Dependent type disambiguation
template <typename Container>
void print_first(const Container& c) {
    typename Container::value_type first = c.front();
    std::println("{}", first);
}

int main() {
    std::println("{}", square(5));      // 25
    std::println("{:.1f}", square(2.5)); // 6.2

    std::vector<int> v = {10, 20, 30};
    print_first(v);   // 10
}

Notes

Example in practice

int main() {
    // Pick one facility from this reference page.
    // Write the smallest program that exercises its main precondition,
    // complexity rule, or lifetime constraint before scaling up.
    return 0;
}