# std::is_scalar

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< cpp‎ | types

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

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

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