std:: 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 .

If the program adds specializations for std::is_same or std::is_same_v (since C++17) , the behavior is undefined.

Helper variable template

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>
#define SHOW(...) std::cout << #__VA_ARGS__ << " : " << __VA_ARGS__ << '\n'
int main()
{
    std::cout << std::boolalpha;
    // some implementation-defined facts
    // usually true if 'int' is 32 bit
    SHOW( std::is_same<int, std::int32_t>::value ); // maybe true
    // possibly true if ILP64 data model is used
    SHOW( 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
    SHOW( std::is_same_v<int, std::int32_t> ); // maybe true
    SHOW( std::is_same_v<int, std::int64_t> ); // 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>() );
}
#undef SHOW

std::is_same<int, std::int32_t>::value : true
std::is_same<int, std::int64_t>::value : false
std::is_same_v<int, std::int32_t> : true
std::is_same_v<int, std::int64_t> : false

See also