decay and remove_reference
- C++ course
- Advanced
- Lesson 17/83
- 5 min
Learn type transformation with decay and remove_reference.
decay and remove_reference
std::decay_t<T> strips references, const and arrays from a type and turns functions into pointers. std::decay_t<int&> becomes int, std::decay_t<const int&> becomes int, std::decay_t<int[5]> becomes int*.
std::remove_reference_t<T> strips only references; it keeps const and arrays. std::remove_reference_t<int&> becomes int but std::remove_reference_t<const int&> stays const int. These traits normalize template parameters.
Use in template parameters
When a template takes T&& as a universal reference, T may itself be a reference. To get the bare type use std::decay_t<T>. Example: template <typename T> struct Store { using value_type = std::decay_t<T>; }; makes Store<int&>::value_type equal to int.
The standard library applies decay in std::tuple, std::function and std::make_pair. This keeps the stored type reference-free even when the user calls with a reference. It is a standard pattern in template design.
Practice Write the CleanType template
Write template <typename T> struct CleanType with using type = std::decay_t<T>; inside. In main write std::cout << std::is_same_v<CleanType<int&>::type, int>;.
Starter code
#include <iostream>
#include <type_traits>
// Write the CleanType template
int main() {
// Print the result
return 0;
}Sample solution
#include <iostream>
#include <type_traits>
template <typename T>
struct CleanType {
using type = std::decay_t<T>;
};
int main() {
std::cout << std::is_same_v<CleanType<int&>::type, int>;
return 0;
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the remove_reference line
template <typename T>
using Clean = std::____<T>;template <typename T>
using Clean = std::remove_reference_t<T>;Explanation: remove_reference_t<T> strips only references.
2 What does decay_t<int&> produce?
- int Correct answer
- int&
- const int
- int*
Explanation: decay_t strips references and const; the result is int.
3 Match each trait to its result
| Term | Meaning |
|---|---|
decay_t<int&> |
int (decay) |
decay_t<const int&> |
int (const dropped) |
remove_reference_t<int&> |
int (ref only) |
remove_reference_t<const int&> |
const int (ref dropped) |