Initializing aggregate unions

Question

I've got a union :

union my_union 
{ short int Int16; float Float; };

I'd like to create :

const my_union u1 = ???;
const my_union u2 = ???;

and initialize their values to be of different types respectively : u1 -> int16 u2 -> float

How do I do that ? If the above is not possible, are there any workarounds?

Answer 1

Unions cannot contain non-POD data types such as string, so your question is meaningless.

Answer 2

Notwithstanding the ban on non-POD member data (as elaborated above) the Standard says:

at 8.5.1.15: When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer for the first member of the union.

so

const my_union u1 = {1};

should work but this form cannot be used for the second (and subsequent) members.

Answer 3

union can have any number of constructors - this will work for any datatypes without constructor, so your example is well if exclude string (or make pointer to string)

#include <string>
using namespace std;
union my_union 
{ 
    my_union(short i16):
        Int16(i16){}
    my_union(float f):
        Float(f){}
    my_union(const string *s):
        str(s){}

    short int Int16; float Float; const string *str;
};

int main()
{
    const my_union u1 = (short)5;
    const my_union u2 = (float)7.;
    static const string refstr= "asdf";
    const my_union u3 = &refstr;
}

There is more complicated way to create class, that owns by union, class must have a selector (scalar or vector datatype used) - to correctly destroy string.

Answer 4

You can do it, as described in the flagged answer. Unions however are there to alias memory - why would you want to use it in this way?