std::as_const (3) - Linux Manuals

std::as_const: std::as_const

NAME

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." );
  }