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 > constexprbool 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> #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 variableslongdouble num1 =1.0;longdouble 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, unsignedint>==false); static_assert( std::is_same_v<int, signedint>==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, unsignedchar>==false); static_assert( std::is_same_v<char, signedchar>==false); // const-qualified type T is not same as non-const T static_assert(!std::is_same<constint, 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
[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 |
