std::as_const (3) Linux Manual Page

std::as_const – std::as_const

Synopsis

Defined in header<utility>

    template <class T>
    (1)(since C++ 17)

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

template <class T>
(2)(since C++ 17)

    void as_const(const T &&) = delete;

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;
  }

Example

// Run this code

#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.");
}

See also

is_const checks if a type is const-qualified
                (class template)
(C++11)
add_cv
add_const
add_volatile adds const or/and volatile specifiers to the given type
                (class template)
(C++11)
(C++11)
(C++11)
remove_cv
remove_const
remove_volatile removes const or/and volatile specifiers from the given type
                (class template)
(C++11)
(C++11)
(C++11)