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std::as_const

From cppreference.com
< cpp‎ | utility
 
 
Utilities library
Type support (basic types, RTTI)
(C++20)
(C++20)
(C++20)(C++20)(C++20)  
(C++20)(C++20)(C++20)

General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)  
(C++20)
Swap and type operations
(C++20)
(C++14)
(C++11)
(C++11)
(C++23)
(C++11)
as_const
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)


 
Defined in header <utility>
template< class T >
constexpr std::add_const_t <T>& as_const( T& t ) noexcept;
(1) (since C++17)
template< class T >
void as_const( const T&& ) = delete;
(2) (since C++17)
1) Forms lvalue reference to const type of t.
2) const rvalue reference overload is deleted to disallow rvalue arguments.

[edit] Possible implementation

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

[edit] Notes

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

[edit] 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.");
}

[edit] See also

(C++11)
checks if a type is const-qualified
(class template) [edit]
(C++11)(C++11)(C++11)
adds const and/or volatile specifiers to the given type
(class template) [edit]
removes const and/or volatile specifiers from the given type
(class template) [edit]
converts a view into a constant_range
(class template) (range adaptor object)[edit]

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