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.
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>
|
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 <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
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
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 |