Copying (using assignment) a structure to a structure inside a union causing seg fault

Question

I wrote the following code:

#include 
#include 
#include 

struct bar
{
  std::string s3;
  std::string s4;
}Ba         


        

Answer 1

std::string has a non-trivial constructor, that initializes its internal members. Therefore, your struct bar is no POD structure.

Unions only support POD (this is relaxed in C++11). The compiler cannot decide which constructor of which of the union's members to call. Imagine following situation:

unition MyUnion {
  std::string s;
  std::vector v;
};

Should it use vector's or string's constructor to initialize the object?

So in your situation, when you assign the string to the union's string, the internal data is not initalized, which causes random errors.