NAME¶

std::is_same - std::is_same

Synopsis¶

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

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 > (since C++17)
inline constexpr bool is_same_v = is_same<T, U>::value;

Inherited from std::integral_constant

Member constants¶

value true if T and U are the same type , false otherwise
[static] (public static member constant)

Member functions¶

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

// Run this code

#include <iostream>
#include <type_traits>
#include <cstdint>

void print_separator()
{
std::cout << "-----\n";
}

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 << ' '; // ~ true
// possibly true if ILP64 data model is used
std::cout << std::is_same<int, std::int64_t>::value << ' '; // ~ 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> << ' '; // ~ true
std::cout << std::is_same_v<int, std::int64_t> << '\n'; // ~ false

print_separator();

// compare the types of a couple variables
long double num1 = 1.0;
long double num2 = 2.0;
std::cout << std::is_same_v<decltype(num1), decltype(num2)> << '\n'; // true

print_separator();

// 'float' is never an integral type
std::cout << std::is_same<float, std::int32_t>::value << '\n'; // false

print_separator();

// 'int' is implicitly 'signed'
std::cout << std::is_same<int, int>::value << ' '; // true
std::cout << std::is_same<int, unsigned int>::value << ' '; // false
std::cout << std::is_same<int, signed int>::value << '\n'; // true

print_separator();

// unlike other types, 'char' is neither 'unsigned' nor 'signed'
std::cout << std::is_same<char, char>::value << ' '; // true
std::cout << std::is_same<char, unsigned char>::value << ' '; // false
std::cout << std::is_same<char, signed char>::value << '\n'; // false

// const-qualified type T is not same as non-const T
static_assert( not std::is_same<const int, int>() );
}

Possible output:¶

true false true false
-----
true
-----
false
-----
true false true
-----
true false false

See also¶

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