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 typename std::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.
Nested types
| Name | Definition |
type
|
determined as above |
Helper types
template< class T > using add_pointer_t = typename add_pointer<T>::type; |
(since C++14) | |
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<typename std::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)) {};
|
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");
typedef std::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
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
|
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) |