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std::is_same

Defined in header `<type_traits>`
template< class T, class U > struct is_same;		(since C++11)

If T and U name the same type (taking into account const/volatile qualifications), provides the member constant value equal to true. Otherwise value is false.

Commutativity is satisfied, i.e. for any two types T and U, is_same<T, U>::value == true if and only if is_same<U, T>::value == true.

The behavior of a program that adds specializations for is_same or is_same_v (since C++17) is undefined.

Helper variable template

template< class T, class U >
inline constexpr bool is_same_v = is_same<T, U>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T and U are the same 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, class U>
struct is_same : std::false_type {};
 
template<class T>
struct is_same<T, T> : std::true_type {};

Example

#include <cstdint>
#include <iostream>
#include <type_traits>
 
int main()
{
    std::cout << std::boolalpha;
 
    // some implementation-defined facts
 
    // usually true if 'int' is 32 bit
    std::cout << std::is_same<int, std::int32_t>::value << ' '; // maybe true
    // possibly true if ILP64 data model is used
    std::cout << std::is_same<int, std::int64_t>::value << ' '; // maybe false
 
    // same tests as above, except using C++17's std::is_same_v<T, U> format
    std::cout << std::is_same_v<int, std::int32_t> << ' ';  // maybe true
    std::cout << std::is_same_v<int, std::int64_t> << '\n'; // maybe false
 
    // compare the types of a couple variables
    long double num1 = 1.0;
    long double num2 = 2.0;
    static_assert( std::is_same_v<decltype(num1), decltype(num2)> == true );
 
    // 'float' is never an integral type
    static_assert( std::is_same<float, std::int32_t>::value == false );
 
    // 'int' is implicitly 'signed'
    static_assert( std::is_same_v<int, int> == true );
    static_assert( std::is_same_v<int, unsigned int> == false );
    static_assert( std::is_same_v<int, signed int> == true );
 
    // unlike other types, 'char' is neither 'unsigned' nor 'signed'
    static_assert( std::is_same_v<char, char> == true );
    static_assert( std::is_same_v<char, unsigned char> == false );
    static_assert( std::is_same_v<char, signed char> == false );
 
    // const-qualified type T is not same as non-const T
    static_assert( !std::is_same<const int, int>() );
}

Possible output:

true false true false

same_as (C++20)	specifies that a type is the same as another type (concept)
`decltype` specifier(C++11)	obtains the type of an expression or an entity