std::is_scalar

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Defined in header <type_traits>
template< class T >
struct is_scalar;
 (since C++11)

std::is_scalar is a UnaryTypeTrait.

If T is a scalar type, provides the member constant value equal true. For any other type, value is false.

If the program adds specializations for std::is_scalar or std::is_scalar_v, the behavior is undefined.

Template parameters

T - a type to check

Helper variable template

template< class T >
constexpr bool is_scalar_v = is_scalar<T>::value;
 (since C++17)

Inherited from std::integral_constant

Member constants

value
[static]
 true if T is a scalar 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>

Notes

Each individual memory location in the C++ memory model, including the hidden memory locations used by language features (e.g. virtual table pointer), has scalar type (or is a sequence of adjacent bit-fields of non-zero length). Sequencing of side-effects in expression evaluation, inter-thread synchronization, and dependency ordering are all defined in terms of individual scalar objects.

Possible implementation

template<class T>
struct is_scalar : std::integral_constant<bool, std::is_arithmetic<T>::value
                                             || std::is_enum<T>::value
                                             || std::is_pointer<T>::value
                                             || std::is_member_pointer<T>::value
                                             || std::is_null_pointer<T>::value>
{};

Example

Run this code
#include <iostream>
#include <type_traits>
#include <typeinfo>
#include <utility>
 
template<typename Head, typename... Tail>
void are_scalars(Head&& head, Tail&&... tail)
{
    using T = std::decay_t<decltype(head)>;
 
    std::cout << typeid(T).name() << " is "
              << (std::is_scalar_v<T> ? "" : "not ")
              << "a scalar\n";
 
    if constexpr (sizeof... (Tail))
    {
        are_scalars(std::forward<decltype(tail)>(tail)...);
    }
}
 
int main()
{
    struct S { int m; } s;
    int S::* mp = &S::m;
    enum class E { e };
 
    are_scalars(42, 3.14, E::e, "str", mp, nullptr, s);
}

Possible output: 

int is a scalar
double is a scalar
main::E is a scalar
char const* is a scalar
int main::S::* is a scalar
nullptr is a scalar
main::S is not a scalar

See also

(C++11)
 checks if a type is an arithmetic type
(class template)
(C++11)
 checks if a type is an enumeration type
(class template)
(C++11)
 checks if a type is a pointer type
(class template)
(C++11)
 checks if a type is a pointer to a non-static member function or object
(class template)
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