std::add_lvalue_reference, std::add_rvalue_reference
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <type_traits> | ||
| template< class T > struct add_lvalue_reference; | (1) | (since C++11) | 
| template< class T > struct add_rvalue_reference; | (2) | (since C++11) | 
Creates a lvalue or rvalue reference type of T.
1) If 
T is an object or function that isn't cv- or ref- qualified (since C++17), provides a member typedef type which is T&. If T is an rvalue reference to some type U, then type is U&. Otherwise, type is T.2) If 
T is an object or function that isn't cv- or ref- qualified (since C++17), provides a member typedef type which is T&&, otherwise type is T.| Contents | 
[edit] Member types
| Name | Definition | 
| type | reference to T, orTif not allowed | 
[edit] Helper types
| template< class T > using add_lvalue_reference_t = typename add_lvalue_reference<T>::type; | (since C++14) | |
| template< class T > using add_rvalue_reference_t = typename add_rvalue_reference<T>::type; | (since C++14) | |
[edit] Notes
These type transformations honor reference collapse rules:
std::add_lvalue_reference<T&>::type is T&
std::add_lvalue_reference<T&&>::type is T&
std::add_rvalue_reference<T&>::type is T&
std::add_rvalue_reference<T&&>::type is T&&
[edit] Example
Run this code
#include <iostream> #include <type_traits> int main() { using nonref = int; using lref = typename std::add_lvalue_reference<nonref>::type; using rref = typename std::add_rvalue_reference<nonref>::type; std::cout << std::boolalpha; std::cout << std::is_lvalue_reference<nonref>::value << '\n'; std::cout << std::is_lvalue_reference<lref>::value << '\n'; std::cout << std::is_rvalue_reference<rref>::value << '\n'; }
Output:
false true true
[edit] See also
| (C++11) | checks if a type is either lvalue reference or rvalue reference (class template) | 
| (C++11) | removes reference from the given type (class template) | 


