# std::is_const

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< cpp‎ | types

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

`std::is_const` is a UnaryTypeTrait.

If `T` is a const-qualified type (that is, const, or const volatile), provides the member constant value equal to true. For any other type, value is false.

If the program adds specializations for `std::is_const` or `std::is_const_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_const_v = is_const<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a const-qualified 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

If T is a reference type then is_const<T>::value is always false. The proper way to check a potentially-reference type for constness is to remove the reference: is_const<typename remove_reference<T>::type>.

### Possible implementation

    template<class T> struct is_const          : std::false_type {};
    template<class T> struct is_const<const T> : std::true_type {};

### Example

Run this code

    #include <type_traits>

    static_assert(std::is_same_v<const int*, int const*>,
        "Remember, constness binds tightly inside pointers.");
    static_assert(!std::is_const_v<int>);
    static_assert(std::is_const_v<const int>);
    static_assert(!std::is_const_v<int*>);
    static_assert(std::is_const_v<int* const>,
        "Because the pointer itself can't be changed but the int pointed at can.");
    static_assert(!std::is_const_v<const int*>,
        "Because the pointer itself can be changed but not the int pointed at.");
    static_assert(!std::is_const_v<const int&>);
    static_assert(std::is_const_v<std::remove_reference_t<const int&>>);

    struct S
    {
        void foo() const {}
        void bar() const {}
    };

    int main()
    {
        // A const member function is const in a different way:

        static_assert(!std::is_const_v<decltype(&S::foo)>,
            "Because &S::foo is a pointer.");

        using S_mem_fun_ptr = void(S::*)() const;

        S_mem_fun_ptr sfp = &S::foo;
        sfp = &S::bar; // OK, can be re-pointed
        static_assert(!std::is_const_v<decltype(sfp)>,
            "Because sfp is the same pointer type and thus can be re-pointed.");

        const S_mem_fun_ptr csfp = &S::foo;
        // csfp = &S::bar; // Error
        static_assert(std::is_const_v<decltype(csfp)>,
            "Because csfp cannot be re-pointed.");
    }

### See also

is_volatile (C++11) | checks if a type is volatile-qualified (class template)
as_const (C++17) | obtains a reference to const to its argument (function template)

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