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

Example¶

// Run this code

#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¶

is_const checks if a type is const-qualified
(C++11) (class template)
add_cv
add_const
add_volatile adds const and/or volatile specifiers to the given type
(C++11) (class template)
(C++11)
(C++11)
remove_cv
remove_const
remove_volatile removes const and/or volatile specifiers from the given type
(C++11) (class template)
(C++11)
(C++11)
ranges::as_const_view converts a view into a constant_range
views::as_const (class template) (range adaptor object)
(C++23)