std::as_const

Since C++17

Header: <utility>

  1. Forms lvalue reference to const type of t.

# Declarations

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

(since C++17)

template< class T >
void as_const( const T&& ) = delete;

(since C++17)

# 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