union issues.

[resumurof]

#include <iostream.h>

class vehicle
{
public : vehicle()
{
}
int i;
float e;
};

main()
{
union a
{
char k;
int j;
vehicle obj;
};
cout<<sizeof(a)<<endl;
return 0;
}
this code gives compilation error as shown below error C2620: union 'a' : member 'obj' has user-defined constructor or non-trivial default constructor.

But without constructor same code prints 8 bytes...Need to know why defining constructor gives compilation problem ..pls clarify..

[VictorN]

Are you banned by MSDN?
Compiler Error C2620

[resumurof]

I know this is a restriction....but i want to know why it has been restricted...the reason behind it...

[VictorN]

http://c.ittoolbox.com/groups/techni...classes-197889
http://www.gamedev.net/topic/493162-...n-declaration/
http://stackoverflow.com/questions/3...s-into-a-union

[Igor Vartanov]

I know this is a restriction....but i want to know why it has been restricted...the reason behind it...

Well, the answer is pretty apparent. Union is a way for interpreting the same memory region, therefore there should be the only and default way for constructing it.

[Chris_F]

In C++0x, the union example you posted is not restricted, since vehicle is a POD by C++0x standards.