delete a void * pointer ?

[mandatory]

hi,

i have a pointer to a class, which is dynamically allocated with new.

myClass *mC = new myClass();


What if i at a later point in my code i have this:

void *pointer;

pointer = (void*)myClass;
...
....
delete pointer;


Will this work at all ? if you say it does, what if i have a destructor on
the class, i guess the compiler cant recognize it as the class type right -
but lets say its only a datasegment with X bytes in a struct, will it work
then ?

[Victor Bazarov]

Re: delete a void * pointer ?

mandatory wrote:[color=blue]
> i have a pointer to a class, which is dynamically allocated with new.
>
> myClass *mC = new myClass();
>
>
> What if i at a later point in my code i have this:
>
> void *pointer;
>
> pointer = (void*)myClass;[/color]

Get used to combining this into a single statement:

void *pointer = myClass;

it's much easier to read (and no cast is necessary, BTW).
[color=blue]
> ..
> ...
> delete pointer;
>
>
> Will this work at all ?[/color]

No.
[color=blue]
> if you say it does, what if i have a destructor on
> the class, i guess the compiler cant recognize it as the class type right -
> but lets say its only a datasegment with X bytes in a struct, will it work
> then ?[/color]

No.

V

[Jonathan Turkanis]

Re: delete a void * pointer ?

Victor Bazarov wrote:
[color=blue][color=green]
>> What if i at a later point in my code i have this:[/color][/color]
[color=blue]
> void *pointer = myClass;[/color]
[color=blue][color=green]
>> delete pointer;
>>
>>
>> Will this work at all ?[/color]
>
> No.[/color]

I think it will work unless myClass has a non-trivial destructor or deallocation
function, according to 5.3.5/5.

Jonathan

[Ioannis Vranos]

Re: delete a void * pointer ?

Jonathan Turkanis wrote:
[color=blue]
> I think it will work unless myClass has a non-trivial destructor or deallocation
> function, according to 5.3.5/5.[/color]

I think the type is deduced by the argument passed, and I guess it is
undefined. Also MINGW GCC 3.4.2 says:


C:\c>g++ temp.cpp -o temp.exe
temp.cpp: In function `int main()':
temp.cpp:9: warning: deleting `void*' is undefined

C:\c>



Also consider the non-virtual destructor situation:


#include <iostream>

class BaseClass
{
public:
~BaseClass() { std::cout<<"~Ba seClass() called.\n"; }
};


class DerivedClass: public BaseClass
{
public:
~DerivedClass() { std::cout<<"~De rivedClass() called.\n"; }
};


int main()
{
DerivedClass *p1= new DerivedClass, *p2= new DerivedClass,
*p3= new DerivedClass;

BaseClass *bp= p2;

void *vp= p3;

delete p1;

delete bp;

delete vp;
}


C:\c>temp
~DerivedClass() called.
~BaseClass() called.
~BaseClass() called.

C:\c>



and the virtual destructor situation:


#include <iostream>

class BaseClass
{
public:
virtual ~BaseClass() { std::cout<<"~Ba seClass() called.\n"; }
};


class DerivedClass: public BaseClass
{
public:
~DerivedClass() { std::cout<<"~De rivedClass() called.\n"; }
};


int main()
{
using namespace std;

DerivedClass *p1= new DerivedClass, *p2= new DerivedClass,
*p3= new DerivedClass;

BaseClass *bp= p2;

void *vp= p3;

cout<<"delete p1:\n";
delete p1;

cout<<"\n\ndele te bp:\n";
delete bp;

cout<<"\n\ndele te vp:\n";
delete vp;
}


C:\c>temp
delete p1:
~DerivedClass() called.
~BaseClass() called.


delete bp:
~DerivedClass() called.
~BaseClass() called.


delete vp:

C:\c>



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Ioannis Vranos]

Re: delete a void * pointer ?

Ioannis Vranos wrote:[color=blue]
>
> Jonathan Turkanis wrote:
>[color=green]
>> I think it will work unless myClass has a non-trivial destructor or
>> deallocation
>> function, according to 5.3.5/5.[/color]
>
>
> I think the type is deduced by the argument passed, and I guess it is
> undefined. Also MINGW GCC 3.4.2 says:
>
>
> C:\c>g++ temp.cpp -o temp.exe
> temp.cpp: In function `int main()':
> temp.cpp:9: warning: deleting `void*' is undefined
>
> C:\c>
>
>
>
> Also consider the non-virtual destructor situation:
>
>
> #include <iostream>
>
> class BaseClass
> {
> public:
> ~BaseClass() { std::cout<<"~Ba seClass() called.\n"; }
> };
>
>
> class DerivedClass: public BaseClass
> {
> public:
> ~DerivedClass() { std::cout<<"~De rivedClass() called.\n"; }
> };
>
>
> int main()
> {
> DerivedClass *p1= new DerivedClass, *p2= new DerivedClass,
> *p3= new DerivedClass;
>
> BaseClass *bp= p2;
>
> void *vp= p3;
>
> delete p1;
>
> delete bp;
>
> delete vp;
> }
>
>
> C:\c>temp
> ~DerivedClass() called.
> ~BaseClass() called.
> ~BaseClass() called.[/color]
<-- delete vp; is the space here.[color=blue]
> C:\c>
>
>
>
> and the virtual destructor situation:
>
>
> #include <iostream>
>
> class BaseClass
> {
> public:
> virtual ~BaseClass() { std::cout<<"~Ba seClass() called.\n"; }
> };
>
>
> class DerivedClass: public BaseClass
> {
> public:
> ~DerivedClass() { std::cout<<"~De rivedClass() called.\n"; }
> };
>
>
> int main()
> {
> using namespace std;
>
> DerivedClass *p1= new DerivedClass, *p2= new DerivedClass,
> *p3= new DerivedClass;
>
> BaseClass *bp= p2;
>
> void *vp= p3;
>
> cout<<"delete p1:\n";
> delete p1;
>
> cout<<"\n\ndele te bp:\n";
> delete bp;
>
> cout<<"\n\ndele te vp:\n";
> delete vp;
> }
>
>
> C:\c>temp
> delete p1:
> ~DerivedClass() called.
> ~BaseClass() called.
>
>
> delete bp:
> ~DerivedClass() called.
> ~BaseClass() called.
>
>
> delete vp:
>
> C:\c>[/color]



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Jonathan Turkanis]

Re: delete a void * pointer ?

Ioannis Vranos wrote:[color=blue]
> Jonathan Turkanis wrote:
>[color=green]
>> I think it will work unless myClass has a non-trivial destructor or
>> deallocation function, according to 5.3.5/5.[/color]
>
> I think the type is deduced by the argument passed, and I guess it is
> undefined. Also MINGW GCC 3.4.2 says:
>
>
> C:\c>g++ temp.cpp -o temp.exe
> temp.cpp: In function `int main()':
> temp.cpp:9: warning: deleting `void*' is undefined[/color]

I guess I should have reread 5.3.5/5 before I cited it. It applies to incomplete
class types.

Jonathan