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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../error/../types/is_const.html below:

std::is_const - cppreference.com

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.

[edit] Template parameters [edit] 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 true if T is a const-qualified type, false otherwise
(public static member constant) Member functions converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [edit] 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>.

[edit] Possible implementation [edit] Example

#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.");
}

[edit] See also checks if a type is volatile-qualified
(class template) [edit] obtains a reference to const to its argument
(function template) [edit]

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