Use of Union with reference

Question

At work I\'ve been using linux and the GCC compiler for C++11 and C++14. In some of the code at work, I\'ve used a union to store both a reference and a pointer, as so: (Sim

Answer 1

It is illegal for a union to contain a reference member. This is presumably because references are not objects and it's unspecified whether or not they occupy storage---so it makes little sense for a reference to share its storage with other variables.

A union can have member functions (including constructors and destructors), but not virtual (10.3) functions. A union shall not have base classes. A union shall not be used as a base class. If a union contains a non- static data member of reference type the program is ill-formed.

([class.union]/2)

Answer 2

In addition to @Brian: You can make it compile by using e.g. std::reference_wrapper instead of a plain reference:

#include <functional>

struct MyStruct
{
    //Stuff
    union { std::reference_wrapper<double> x; double* x_ptr; };
    MyStruct(double& value) : x(value) {}
    //More stuff
};

int main()
{
    double value = 123;
    MyStruct myStruct(value);
}

live example