|  | Why union cannot have member having user defined constructor | | |
| | | ashwin |  |
|  Posted: Fri Aug 22, 2008 6:50 am Post subject: Why union cannot have member having user defined constructor |  |
In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
#include<iostream>
using namespace std;
class A
{
public:
//union 'Union' : member 'iObj' has user-defined constructor or
//non-trivial default constructor ??
A():
i(2)
{
cout<<"A::A()"<<endl;
}
private:
int i;
};
union Union
{
int Val;
A Obj;
};
int main()
{
Union un;
cout<<sizeof(un)<<endl;
return 0;
}
Why this restriction ?
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| |
| | | Ulrich Eckhardt | |
| | Posted: Fri Aug 22, 2008 12:55 pm Post subject: Re: Why union cannot have member having user defined constru | |
| |  | |
ashwin wrote:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
[...]
Why this restriction ?
|
All members of a union live in the same memory. Constructing multiple
objects on that memory doesn't work, since each of them would overwrite the
constructor changes of the other. This leaves the option of only
constructing one of them, but the union doesn't know which one (there are
alternative variant classes which do), just as it doesn't know which one to
destroy.
Try running this code and examine the content of the two objects in a
debugger. You should see that constructing one damages the other and vice
versa. Destroying the last created one will probably work still, but
destroying the other will surely fail.
// reserve memory
size_t max_size = max<size_t>( sizeof (vector<int>),
sizeof (string));
char memory[max_size];
// create objects with placement new
vector<int>* pv = new (memory) vector<int>;
string* ps = new (memory) string;
Uli
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| |
| | | Guest | |
| | Posted: Fri Aug 22, 2008 12:56 pm Post subject: Re: Why union cannot have member having user defined constru | |
| | | |
On Aug 21, 11:50 pm, ashwin <ashwind...@gmail.com> wrote:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
[snip]
Why this restriction ?
|
int main()
{ struct A
{ A() : x(1) {}
int x;
};
struct B
{ B() : x(2) {}
int x;
};
union C
{ A a;
B b;
};
C c;
return c.b.x;
}
What would you like this to return? 1? 2? Unspecified? Undefined?
C++ generally has strict rules governing the lifetime of objects. An
obect exists as soon as the constructor exits until the destructor. It
doesn't mesh well when you start calling multiple constructors on the
same memory location, or by not calling a constructor at all for an
object of a type which has a constructor. It doesn't mix with the C++
basic idioms well to be able to access an object without its
constructor setting up its state.
Now, this isn't to say you can't do this. It's just that C++ tries to
make it somewhat hard to shoot yourself in the foot if at all
possible. This is one of those cases where the language rules are
trying to prevent you from shooting yourself in the foot.
If you REALLY want to do this, you can. You can explicitly allocate a
piece of memory, and reinterpret_cast it back and forth. You can
control exactly when the constructor of a type is called with
placement new, and you can explicitly call a destructor as well. You
probably do not want to be doing these things.
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| |
| | | Nicola Musatti | |
| | Posted: Fri Aug 22, 2008 1:01 pm Post subject: Re: Why union cannot have member having user defined constru | |
On Aug 22, 8:50 am, ashwin <ashwind...@gmail.com> wrote:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
#include<iostream
using namespace std;
class A {
public:
A(): i(2) { cout<<"A::A()"<<endl; }
private:
int i;
};
union Union {
int Val;
A Obj;
};
int main() {
Union un;
cout<<sizeof(un)<<endl;
}
Why this restriction ?
|
suppose you do:
un.Val = 42;
A a = un.Obj;
un.Obj's constructor never ran, so a is assigned the value of an
uninitialized object.
Cheers,
Nicola Musatti
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| |
| | | Pete Becker | |
| | Posted: Fri Aug 22, 2008 1:02 pm Post subject: Re: Why union cannot have member having user defined constru | |
On 2008-08-21 20:50:40 -0400, ashwin <ashwindows@gmail.com> said:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
[snip]
Why this restriction ?
|
Because members of unions are default-initialized. If you have a type
that needs some other form of initialization it won't get properly
initialized inside a union.
This has been changed for C++0x, where the rule will be that by default
members of a union will not be initialized.
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)
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| |
| | | peter koch larsen | |
| | Posted: Fri Aug 22, 2008 1:04 pm Post subject: Re: Why union cannot have member having user defined constru | |
| | | |
On 22 Aug., 08:50, ashwin <ashwind...@gmail.com> wrote:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
#include<iostream
using namespace std;
class A
{
public:
//union 'Union' : member 'iObj' has user-defined constructor or
//non-trivial default constructor ??
A():
i(2)
{
cout<<"A::A()"<<endl;
}
private:
int i;
};
union Union
{
int Val;
A Obj;
};
int main()
{
Union un;
cout<<sizeof(un)<<endl;
return 0;
}
Why this restriction ?
|
How would you implement it? Every object constructed must be
deconstructed, and how would you know what type is currently in the
union? While it is possible to implement, it would probably change to
much code for compatibility reasons.
You could either use a struct or look at boosts solution for unions:
while the first is a hack, the second solution gives you all the
advantages of a C union and adds typesafety.
/Peter
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| |
| | | Martin Bonner | |
| | Posted: Fri Aug 22, 2008 2:28 pm Post subject: Re: Why union cannot have member having user defined constru | |
| | | |
On Aug 22, 7:50 am, ashwin <ashwind...@gmail.com> wrote:
| Quote: | In the below code, "un" cannot be created as it has "Obj" member,
which has user defined constructor.
#include<iostream
using namespace std;
class A
{
public:
//union 'Union' : member 'iObj' has user-defined constructor or
//non-trivial default constructor ??
A():
d(2.0)
{
cout<<"A::A()"<<endl;
}
private:
double d; |
// Changed member to double for exposition.
| Quote: | };
union Union
{
int Val;
A Obj;
};
int main()
{
Union un;
|
At this point, does un contain a int (Val) or an A (containg a
double)?
In fact, it contains *neither* (so neither can be constructed). You
have to assign to either un.Val or un.Obj before you are allowed to
use the member. Furthermore, you are not allowed to use a member
unless the most recent assignment was to that member (*).
| Quote: |
cout<<sizeof(un)<<endl;
return 0;
}
Why this restriction ?
See above |
(*) Yes, yes, I know. There are exceptions concerning POD-structs
with common initial sequences of members.
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