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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 |
| arg | - | lvalue object or function |
Pointer toarg.
The implementation below is notconstexpr, becausereinterpret_cast is not usable in a constant expression. Compiler support is needed (see below).
template<class T>typenamestd::enable_if<std::is_object<T>::value, T*>::type addressof(T& arg)noexcept{returnreinterpret_cast<T*>(&const_cast<char&>(reinterpret_cast<constvolatilechar&>(arg)));} template<class T>typenamestd::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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_addressof_constexpr | 201603L | (C++17) | constexprstd::addressof |
constexpr foraddressof is added byLWG2296, and MSVC STL applies the change to C++14 mode as a defect report.
There are some weird cases where use of built-inoperator& is ill-formed due toargument-dependent lookup even if it is not overloaded, andstd::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}
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(newint(42)); f(&p);// calls int** overload f(std::addressof(p));// calls Ptr<int>* overload, (= this)}
Possible output:
Ctor this = 0x7fff59ae6e88int** overload called with p = 0x7fff59ae6e90Ptr overload called with p = 0x7fff59ae6e88
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 |
| the default allocator (class template)[edit] | |
[static] | obtains a dereferenceable pointer to its argument (public static member function of std::pointer_traits<Ptr>)[edit] |
