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Address Of

cppreference C++ named requirements: CopyConstructible # Notes

Until C++11, classes that overloaded operator& were not CopyConstructible and thus weren't usable in the standard library containers. As of C++11, the standard library uses std::addressof whenever the address of an object is needed.

More C++ Idioms/Address Of

NOTE: addressof可以看做是一个专门的用于获取object address的operator。获取object address不再使用&，而是使用addressof。

Intent

Find address of an object of a class that has an overloaded unary ampersand (&) operator.

Motivation

C++ allows overloading of unary ampersand (&) operator for class types. The return type of such an operator need not be the actual address of the object. Intentions(意图) of such a class are highly debatable but the language allows it nevertheless. Address-of idiom is a way to find the real address of an object irrespective(不考虑) of the overloaded unary ampersand（符号） operator and its access protection.

NOTE: 如果class已经overload了&运算符，并且这个overload &不是获取地址的，那么此时就需要使用addressof idiom。

In the example below, the main function fails to compile because operator & of nonaddressable class is private. Even if it were accessible, a conversion from its return type double to a pointer would not have been possible or meaningful.

class nonaddressable 
{
public:
    typedef double useless_type;
private:
    useless_type operator&() const;
};

int main()
{
  nonaddressable na;
  nonaddressable * naptr = &na; // Compiler error here.
}

Solution and Sample Code

The Address-of idiom retrieves the address of an object using a series of casts.

#include<iostream>
class nonaddressable 
{
public:
    typedef double useless_type;
private:
    useless_type operator&() const;
};

template <class T>
T * addressof(T & v)
{
  return reinterpret_cast<T *>(& const_cast<char&>(reinterpret_cast<const volatile char &>(v)));
}
int main()
{
  nonaddressable na;
  nonaddressable * naptr = addressof(na); // No more compiler error.
}

NOTE: 上述实现，采用的是interpretation model、aliasing、serialization的思路，它是实现是考虑的了较多问题:

1、T可能是CV的，它的最内层reinterpret_cast<const volatile char &>(v)

2、第二层使用const_cast<char&>去除CV，这就得到了binary representation

3、& const_cast<char&> 用于取binary representation的地址

4、reinterpret_cast<T *>实现aliasing

C++11

In C++11 the template std::addressof, in the <memory> header, was added to solve this problem. In C++17, the template is also constexpr.