std::is_same - cppreference.com (original) (raw)
| | | | | ---------------------------------------------- | | ------------- | | 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
| | true if T and U are the same type, false otherwise (public static member constant) | | ------------------------------------------------------------------------------------- |
Member functions
| | converts the object to bool, returns value (public member function) | | ---------------------------------------------------------------------- | | | returns value (public member function) |
Member types
[edit] Possible implementation
[edit] Example
#include #include #include #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
Possible output:
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