Difference between revisions of "cpp/memory/addressof"
From cppreference.com
| Line 11: | Line 11: | ||
template< class T > | template< class T > | ||
constexpr T* addressof(T& arg) noexcept; | constexpr T* addressof(T& arg) noexcept; | ||
| + | }} | ||
| + | {{dcl rev end}} | ||
| + | {{dcl | since=c++17 | num = 2 | 1= | ||
| + | template <class T> | ||
const T* addressof(const T&&) = delete; | const T* addressof(const T&&) = delete; | ||
}} | }} | ||
| Line 31: | Line 35: | ||
===Return value=== | ===Return value=== | ||
Pointer to {{tt|arg}}. | Pointer to {{tt|arg}}. | ||
| − | |||
| − | |||
| − | |||
===Possible implementation=== | ===Possible implementation=== | ||
| − | {{ | + | {{eq fun |
| − | | | + | | 1= |
template<class T> | template<class T> | ||
| − | + | typename std::enable_if<std::is_object<T>::value, T*>::type addressof(T& arg) noexcept | |
{ | { | ||
| − | return | + | return reinterpret_cast<T*>( |
| + | &const_cast<char&>( | ||
| + | reinterpret_cast<const volatile char&>(arg))); | ||
} | } | ||
| − | |||
| − | |||
| − | Note: the above implementation requires compiler support. | + | template<class T> |
| + | typename std::enable_if<!std::is_object<T>::value, T*>::type addressof(T& arg) noexcept | ||
| + | { | ||
| + | return &arg; | ||
| + | } | ||
| + | }} | ||
| + | Note: the above implementation is not {{tt|constexpr}} (which requires compiler support). | ||
===Example=== | ===Example=== | ||
Revision as of 14:15, 23 July 2021
| Defined in header <memory>
|
||
| (1) | ||
| template< class T > T* addressof(T& arg) noexcept; |
(since C++11) (until C++17) |
|
| template< class T > constexpr T* addressof(T& arg) noexcept; |
(since C++17) | |
| template <class T> const T* addressof(const T&&) = delete; |
(2) | (since C++17) |
1) Obtains the actual address of the object or function
arg, even in presence of overloaded operator&2) Rvalue overload is deleted to prevent taking the address of const rvalues.
|
The expression |
(since C++17) |
Contents |
Parameters
| arg | - | lvalue object or function |
Return value
Pointer to arg.
Possible implementation
template<class T> typename std::enable_if<std::is_object<T>::value, T*>::type addressof(T& arg) noexcept { return reinterpret_cast<T*>( &const_cast<char&>( reinterpret_cast<const volatile char&>(arg))); } template<class T> typename std::enable_if<!std::is_object<T>::value, T*>::type addressof(T& arg) noexcept { return &arg; } |
Note: the above implementation is not constexpr (which requires compiler support).
Example
operator& may be overloaded for a pointer wrapper class to obtain a pointer to pointer:
Run this code
#include <iostream> #include <memory> template<class T> struct Ptr { T* pad; // add pad to show difference between 'this' and 'data' T* data; Ptr(T* arg) : pad(nullptr), data(arg) { std::cout << "Ctor this = " << this << std::endl; } ~Ptr() { delete data; } T** operator&() { return &data; } }; template<class T> void f(Ptr<T>* p) { std::cout << "Ptr overload called with p = " << p << '\n'; } void f(int** p) { std::cout << "int** overload called with p = " << p << '\n'; } int main() { Ptr<int> p(new int(42)); f(&p); // calls int** overload f(std::addressof(p)); // calls Ptr<int>* overload, (= this) }
Possible output:
Ctor this = 0x7fff59ae6e88 int** overload called with p = 0x7fff59ae6e90 Ptr overload called with p = 0x7fff59ae6e88
See also
| the default allocator (class template) | |
| [static] |
obtains a dereferenceable pointer to its argument (public static member function of std::pointer_traits<Ptr>)
|
