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.