Why is std::string incompatible with C unions?

Question

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Why compiler doesn't allow std::string inside union ?

I knew that I had this problem when I started with C++: The compiler wouldn't allow me to put a variable of the type std::string into unions.

That was years ago, but actually I still don't know the exact answer. I read something related to a copy function with the string that the union didn't like, but that's pretty much all.

• Why are C++ STL strings incompatible with unions?

Answer 1

From Wikipedia:

C++ does not allow for a data member to be any type that has a full fledged constructor/destructor and/or copy constructor, or a non-trivial copy assignment operator. In particular, it is impossible to have the standard C++ string as a member of a union.

Think about it this way: If you have a union of a class type like std::string and a primitive type (let's say a long), how would the compiler know when you are using the class type (in which case the constructor/destructor will need to be called) and when you are using the simple type? That's why full-fledged class types are not allowed as members of a union.

Answer 2

Class which have user-defined constructor or user-defined destructor is not allowed in union.

You can have pointer of such class as member of union, though.

struct X
{
   X() {}
   ~X() {}
};

union A
{
   X x; // not allowed - X has constructor (and destructor too)
   X *px; //allowed!
};

Or you can use boost::variant which is a safe, generic, stack-based discriminated union container.

§9.5/1 says (formatting and emphasize is mine)

• 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.
• An object of a class with a non-trivial constructor (12.1), a non-trivial copy constructor (12.8), a non-trivial destructor (12.4), or a non-trivial copy assignment operator (13.5.3, 12.8) cannot be a member of a union, nor can an array of such objects.
• If a union contains a static data member, or a member of reference type, the program is ill-formed.

Interesting!