Setting member variables before constructor call

[syntheticpp@yahoo.com]

Is the program at the end a correct C++ program?

I ask because the output is:

B::operator A&()
A::A()
B::B()
B::~B()
A::~A()

that means the casting operator which sets the
member variable B::p is called before the constructor
of B.


Here the program:


#include <iostream>

using std::cout;

struct A
{
A(){ cout <<"A::A()\n"; }
~A(){ cout <<"A::~A()\n" ; }
};

struct B
{
B() // : p(0) // this kills the desruction of p
{ cout <<"B::B()\n"; }

~B()
{
cout <<"B::~B()\n" ;
delete p;
}

A* p;

operator A&()
{
cout <<"B::operato r A&()\n";
p = new A;
return *p;
}
};

struct C
{
A& a;
B b;

C() : a(b) {}
};

int main()
{
C* c = new C;
delete c;

#if defined(__BORLA NDC__) || defined(_MSC_VE R)
system("PAUSE") ;
#endif

}

[Victor Bazarov]

Re: Setting member variables before constructor call

syntheticpp@yah oo.com wrote:[color=blue]
> Is the program at the end a correct C++ program?[/color]

A member function is called for an object whose lifetime hasn't started
yet because it's not been constructed. The storage the object occupies
is already create at that time. A pointer to that storage can be used,
but that use is limited. Dereferencing that pointer to access any member
of the object causes the program to have undefined behaviour. In your
case you attempt to assign the 'p' member inside B::operator A&(). Due
to that, your program has undefined behaviour.

It is a correct C++ program whose behaviour is undefined.
[color=blue]
> I ask because the output is:
>
> B::operator A&()
> A::A()
> B::B()
> B::~B()
> A::~A()
>
> that means the casting operator which sets the
> member variable B::p is called before the constructor
> of B.
>
>
> Here the program:
>
>
> #include <iostream>
>
> using std::cout;
>
> struct A
> {
> A(){ cout <<"A::A()\n"; }
> ~A(){ cout <<"A::~A()\n" ; }
> };
>
> struct B
> {
> B() // : p(0) // this kills the desruction of p
> { cout <<"B::B()\n"; }
>
> ~B()
> {
> cout <<"B::~B()\n" ;
> delete p;
> }
>
> A* p;
>
> operator A&()
> {
> cout <<"B::operato r A&()\n";
> p = new A;[/color]
^^^^
This is the cause of the undefined behaviour.
[color=blue]
> return *p;
> }
> };
>
> struct C
> {
> A& a;
> B b;
>
> C() : a(b) {}[/color]

It is generally a BAD IDEA(tm) to use uninitialised or only partially
constructed objects in the constructor initialisation list.
[color=blue]
> };
>
> int main()
> {
> C* c = new C;
> delete c;
>
> #if defined(__BORLA NDC__) || defined(_MSC_VE R)
> system("PAUSE") ;
> #endif
>
> }
>[/color]

V

[syntheticpp@yahoo.com]

Re: Setting member variables before constructor call

I've assumed such a answer.
One way to circumvent the undefined behaviour is to use inheritance,
but is there a way to initialize the refernce without using
inheritance?
Peter

[syntheticpp@yahoo.com]

Re: Setting member variables before constructor call


Victor Bazarov schrieb:[color=blue]
> A member function is called for an object whose lifetime hasn't started
> yet because it's not been constructed. The storage the object occupies
> is already create at that time.[/color]

But why is it then dangerous to use this storage?

[Victor Bazarov]

Re: Setting member variables before constructor call

syntheticpp@yah oo.com wrote:[color=blue]
> I've assumed such a answer.
> One way to circumvent the undefined behaviour is to use inheritance,
> but is there a way to initialize the refernce without using
> inheritance?[/color]

I think you need to rearrange the members. Your 'b' will have been
already initialised at the time when you initialise 'a'.

V

[Victor Bazarov]

Re: Setting member variables before constructor call

syntheticpp@yah oo.com wrote:[color=blue]
> Victor Bazarov schrieb:
>[color=green]
>>A member function is called for an object whose lifetime hasn't started
>>yet because it's not been constructed. The storage the object occupies
>>is already create at that time.[/color]
>
>
> But why is it then dangerous to use this storage?[/color]

"Use" is a very vague term. What do you mean by "use"? It's allowed to
pass the pointer to that storage elsewhere, and even store that pointer
in another object, for example, since the storage is not going away. I
don't want to quote the entire subclause of the standard, if you want to
see what is disallowed, please look at "3.8 Object Lifetime".

If you'd like a speculation on the subject of possible problems you can
run into, then imagine that you're trying to access a member of a class
that adjusts its own layout when initialising. There is no layout at all
when an object is still under construction, even if the compiler will let
you access a member, it's possible that the address (or the offset) it
will use is incorrect since the initialisation proper hasn't yet been
completed. So, you'll write something in the wrong place where it either
gets overridden or worse where your action overrides some other value,
which the program needs. That's just one example.

V

[syntheticpp@yahoo.com]

Re: Setting member variables before constructor call

It depends on the declaration order:

B b;
A& a;

solves the problem.