std::as_const(3)

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;
   }

Notes
   Feature-test macro: __cpp_lib_as_const

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