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std::as_const - cppreference.com

From cppreference.com

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;
}

Notes

Feature-test macro Value Std Feature
__cpp_lib_as_const 201510L (C++17) std::as_const

Example

#include <cassert>
#include <string>
#include <type_traits>
#include <utility>

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