std:: is_const

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

Helper variable template

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

#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