More C++ Idioms/Address Of

Address Of

Intent

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

Also Known As

This idiom has no known alias.

Motivation

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

In the example below, the main function fails to compile because the operator & of the class nonaddressable 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;  // Error: operator & of type nonadressable is private.
}

Solution and Sample Code

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

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.
}

C++11

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

Known Uses

• Boost's addressof utility.