Defined in header <utility>
template< class T >
constexpr std::add_const_t<T>& as_const( T& t ) noexcept;
(1) (since C++17)
template< class T >
void as_const( const T&& ) = delete;
(2) (since C++17)
1) Forms lvalue reference to const type of t.
2) const rvalue reference overload is deleted to disallow rvalue arguments.

Possible implementation

template <class T>
constexpr std::add_const_t<T>& as_const(T& t) noexcept
    return t;


Feature-test macro


#include <string>
#include <cassert>
#include <utility>
#include <type_traits>
int main()
    std::string mutableString = "Hello World!";
    auto&& constRef = std::as_const(mutableString);
//  mutableString.clear(); // OK
//  constRef.clear(); // error: 'constRef' is 'const' qualified,
                      //        but 'clear' is not marked const
    assert( &constRef == &mutableString );
    assert( &std::as_const( mutableString ) == &mutableString );
    using ExprType = std::remove_reference_t<decltype(std::as_const(mutableString))>;
    static_assert(std::is_same_v<std::remove_const_t<ExprType>, std::string>,
            "ExprType should be some kind of string." );
    static_assert(!std::is_same_v<ExprType, std::string>,
            "ExprType shouldn't be a mutable string." );

See also

checks if a type is const-qualified
(class template)
adds const or/and volatile specifiers to the given type
(class template)
removes const or/and volatile specifiers from the given type
(class template)

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