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.
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
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
| (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 |