< cpp‎ | utility
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

[edit] Possible implementation

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

[edit] Example

#include <string>
#include <cassert>
#include <utility>
#include <type_traits>
int main()
    std::string mutableString = "Hello World!";
    const std::string& constView = std::as_const(mutableString);
    assert( &constView == &mutableString );
    assert( &std::as_const( mutableString ) == &mutableString );
    using WhatTypeIsIt = std::remove_reference_t<decltype(std::as_const(mutableString))>;
    static_assert(std::is_same<std::remove_const_t<WhatTypeIsIt>, std::string>::value,
            "WhatTypeIsIt should be some kind of string." );
    static_assert(!std::is_same< WhatTypeIsIt, std::string >::value,
            "WhatTypeIsIt shouldn't be a mutable string." );

[edit] 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)