std::add_pointer
From cppreference.com
Defined in header <type_traits> | ||
template<class T > struct add_pointer; | (since C++11) | |
If T is a referenceable type or (possibly cv-qualified) void, the member typedef type provided is typenamestd::remove_reference<T>::type*.
Otherwise, the member typedef type provided is T.
If the program adds specializations for std::add_pointer, the behavior is undefined.
Contents |
[edit]Nested types
| Name | Definition |
type | determined as above |
[edit]Helper types
template<class T > using add_pointer_t =typename add_pointer<T>::type; | (since C++14) | |
[edit]Possible implementation
namespace detail {template<class T>struct type_identity {using type = T;};// or use std::type_identity (since C++20) template<class T>auto try_add_pointer(int)-> type_identity<typenamestd::remove_reference<T>::type*>;// usual case template<class T>auto try_add_pointer(...)-> type_identity<T>;// unusual case (cannot form std::remove_reference<T>::type*)}// namespace detail template<class T>struct add_pointer : decltype(detail::try_add_pointer<T>(0)){}; |
[edit]Example
Run this code
#include <iostream>#include <type_traits> template<typename F, typename Class>void ptr_to_member_func_cvref_test(F Class::*){// F is an “abominable function type”using FF = std::add_pointer_t<F>; static_assert(std::is_same_v<F, FF>, "FF should be precisely F");} struct S {void f_ref()&{}void f_const()const{}}; int main(){int i =123;int& ri = i;typedef std::add_pointer<decltype(i)>::type IntPtr;typedef std::add_pointer<decltype(ri)>::type IntPtr2; IntPtr pi =&i;std::cout<<"i = "<< i <<'\n';std::cout<<"*pi = "<<*pi <<'\n'; static_assert(std::is_pointer_v<IntPtr>, "IntPtr should be a pointer"); static_assert(std::is_same_v<IntPtr, int*>, "IntPtr should be a pointer to int"); static_assert(std::is_same_v<IntPtr2, IntPtr>, "IntPtr2 should be equal to IntPtr"); typedefstd::remove_pointer<IntPtr>::type IntAgain; IntAgain j = i;std::cout<<"j = "<< j <<'\n'; static_assert(!std::is_pointer_v<IntAgain>, "IntAgain should not be a pointer"); static_assert(std::is_same_v<IntAgain, int>, "IntAgain should be equal to int"); ptr_to_member_func_cvref_test(&S::f_ref); ptr_to_member_func_cvref_test(&S::f_const);}
Output:
i = 123 *pi = 123 j = 123
[edit]Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2101 | C++11 | the program was ill-formed if T is a function type with cv or ref | the type produced is T in this case |
[edit]See also
(C++11) | checks if a type is a pointer type (class template) |
(C++11) | removes a pointer from the given type (class template) |
