std::addressof
| Defined in header <memory>
|
||
| template< class T > T* addressof( T& arg ) noexcept; |
(1) | (since C++11) (constexpr since C++17) |
| template< class T > const T* addressof( const T&& ) = delete; |
(2) | (since C++11) |
|
The expression |
(since C++17) |
Contents |
[edit] Parameters
| arg | - | lvalue object or function |
[edit] Return value
Pointer to arg.
[edit] Possible implementation
The implementation below is not constexpr, because reinterpret_cast is not usable in a constant expression. Compiler support is needed (see below).
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; } |
Correct implementation of this function requires compiler support: GNU libstdc++, LLVM libc++, Microsoft STL.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_addressof_constexpr |
201603L | (C++17) | constexpr std::addressof
|
constexpr for addressof is added by LWG2296, and MSVC STL applies the change to C++14 mode as a defect report.
There are some weird cases where use of built-in operator& is ill-formed due to argument-dependent lookup even if it is not overloaded, and std::addressof can be used instead.
template<class T> struct holder { T t; }; struct incomp; int main() { holder<holder<incomp>*> x{}; // &x; // error: argument-dependent lookup attempts to instantiate holder<incomp> std::addressof(x); // OK }
[edit] Example
operator& may be overloaded for a pointer wrapper class to obtain a pointer to pointer:
#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 << '\n'; } ~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
[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 2598 | C++11 | std::addressof<const T> could take address of rvalues | disallowed by a deleted overload |
[edit] See also
| the default allocator (class template) | |
| [static] |
obtains a dereferenceable pointer to its argument (public static member function of std::pointer_traits<Ptr>)
|
