std::is_same
From cppreference.com
| 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.
If the program adds specializations for std::is_same or std::is_same_v(since C++17), the behavior is undefined.
Contents |
[edit] Helper variable template
| template< class T, class U > 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> |
[edit] Possible implementation
template<class T, class U> struct is_same : std::false_type {}; template<class T> struct is_same<T, T> : std::true_type {}; |
[edit] Example
Run this code
#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
[edit] See also
| (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 |
