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    std::is_reference

    From cppreference.com
     
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    Defined in header <type_traits>
    template< class T > struct is_reference;
    		 (since C++11)

    std::is_reference is a UnaryTypeTrait.

    If T is a reference type (lvalue reference or rvalue reference), provides the member constant value equal true. For any other type, value is false.

    If the program adds specializations for std::is_reference or std::is_reference_v, the behavior is undefined.

    Template parameters

    T - a type to check

    Helper variable template

    template< class T > constexpr bool is_reference_v = is_reference<T>::value;
    		 (since C++17)
    [edit]

    Inherited from std::integral_constant

    Member constants

    value
    [static]
    		 true if T is a reference type, false otherwise
    (public static member constant)

    Member functions

    operator bool
    		 converts the object to bool, returns value
    (public member function)
    operator()
    (C++14)
    		 returns value
    (public member function)

    Member types

    Type Definition
    value_type bool
    type std::integral_constant<bool, value>

    Possible implementation

    template<class T> struct is_reference : std::false_type {};
template<class T> struct is_reference<T&> : std::true_type {};
template<class T> struct is_reference<T&&> : std::true_type {};

    Example

    Run this code
    #include <iostream>
#include <type_traits>

class A {};

int main()
{
#   define REF(x) << #x " ?: " << x << '\n'
    std::cout << std::boolalpha
    REF(std::is_reference_v<A>)
    REF(std::is_reference_v<A&>)
    REF(std::is_reference_v<A&&>)
    REF(std::is_reference_v<long>)
    REF(std::is_reference_v<long&>)
    REF(std::is_reference_v<long&&>)
    REF(std::is_reference_v<double*>)
    REF(std::is_reference_v<double*&>)
    REF(std::is_reference_v<double*&&>);
#   undef REF
}

    Output: 

    std::is_reference_v<A> ?: false
std::is_reference_v<A&> ?: true
std::is_reference_v<A&&> ?: true
std::is_reference_v<long> ?: false
std::is_reference_v<long&> ?: true
std::is_reference_v<long&&> ?: true
std::is_reference_v<double*> ?: false
std::is_reference_v<double*&> ?: true
std::is_reference_v<double*&&> ?: true

    See also

    (C++11)
    		 checks if a type is an lvalue reference
    (class template) [edit]
    (C++11)
    		 checks if a type is an rvalue reference
    (class template) [edit]