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delete operator on smart pointers.

P: n/a
I am implementing my custom smart pointer:

template <typename T>
class MySmartPtr
{
public:
MySmartPtr(T* aPointer)
{
mPointer = aPointer;
}

inline T* operator->() const {return mPointer; }

private:
T* mPointer;
};
It could be used in this way:

MySmartPtr<Aobj = new A();
obj->Method1();
int value = obj->Method2();

The operator-overload allows accessing the A methods as they are
accessed via a common pointer.

I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
There is a way I could implement an overload of the delete pointer in
my MySmartPtr<Tor there is some workaround on this?

Thanks in advance,
ernesto

Jan 6 '07 #1
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10 Replies


P: n/a
"Ernesto Bascón" <eb******@gmail.comschrieb im Newsbeitrag
news:11*********************@s80g2000cwa.googlegro ups.com...
>I am implementing my custom smart pointer:

template <typename T>
class MySmartPtr
{
public:
MySmartPtr(T* aPointer)
{
mPointer = aPointer;
}

inline T* operator->() const {return mPointer; }

private:
T* mPointer;
};
It could be used in this way:

MySmartPtr<Aobj = new A();
obj->Method1();
int value = obj->Method2();

The operator-overload allows accessing the A methods as they are
accessed via a common pointer.

I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
There is a way I could implement an overload of the delete pointer in
my MySmartPtr<Tor there is some workaround on this?
To make your smart pointer really smart, it should also implement a default
constructor, a copy constructor and an assignment operator (actually two, a
copy assignment operator and one to assign raw pointers). If they are
implemented correctly, you could just write

obj = 0; // or NULL

to free whatever the smartpointer owned before.

HTH
Heinz
Jan 7 '07 #2

P: n/a

Ernesto Bascón wrote:
I am implementing my custom smart pointer:

template <typename T>
class MySmartPtr
{
public:
MySmartPtr(T* aPointer)
{
mPointer = aPointer;
}

inline T* operator->() const {return mPointer; }

private:
T* mPointer;
};
It could be used in this way:

MySmartPtr<Aobj = new A();
obj->Method1();
int value = obj->Method2();

The operator-overload allows accessing the A methods as they are
accessed via a common pointer.

I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
There is a way I could implement an overload of the delete pointer in
my MySmartPtr<Tor there is some workaround on this?

Thanks in advance,
ernesto
did you try debuggig and watch what object is actually in use then
delete it directly?

Jan 7 '07 #3

P: n/a

Heinz Ozwirk wrote:
>
To make your smart pointer really smart, it should also implement a default
constructor, a copy constructor and an assignment operator (actually two, a
copy assignment operator and one to assign raw pointers). If they are
implemented correctly, you could just write

obj = 0; // or NULL

to free whatever the smartpointer owned before.

HTH
Heinz
That's an elegant point!
I watched my google search carefully with my pair of glasses and I
turned out to have the same incorrect thought with others. hehe

Jan 7 '07 #4

P: n/a

Heinz Ozwirk wrote:
>
To make your smart pointer really smart, it should also implement a default
constructor, a copy constructor and an assignment operator (actually two, a
copy assignment operator and one to assign raw pointers). If they are
implemented correctly, you could just write

obj = 0; // or NULL

to free whatever the smartpointer owned before.

HTH
Heinz
That's an elegant point!
I watched my google search carefully with my pair of glasses and I
turned out to have the same incorrect thought with others. heh

Jan 7 '07 #5

P: n/a
Ernesto Bascón wrote:
MySmartPtr(T* aPointer){ mPointer = aPointer; }
Better do
MySmartPtr( T *const aPointer=0 )throw():mPointer(aPointer){ }
MySmartPtr<Aobj = new A();
It can produce sequence

MySmartPtr<A>() + MySmartPtr<A>() + MySmartPtr<A>(const MySmartPtr<A>&)
or MySmartPtr<A>::operator = (const MySmartPtr<A>&)

Better do
MySmartPtr<Aobj (new A() );
obj->Method1();
Define both
? T* operator-() ? throw(){ return mPointer; }
? T& operator* () ? throw(){ return *mPointer; }
be shure "const" instead "?" is correct for MySmartPtr behaviour
int value = obj->Method2();
I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
Try do
public:
operator T* ()throw(){ T *const tmp=mPointer; mPointer=0; return
tmp; }
~MySmartPtr()throw(){ delete mPointer; mPointer=0; }

Define (if you can not dynamic copy like this, else as need)
private:
MySmartPtr(const MySmartPtr&)throw():mPointer(0){ exit(1); }
void operator= (const MySmartPtr&)throw(){ exit(1); }

Jan 12 '07 #6

P: n/a
Grizlyk wrote:
Define (if you can not dynamic copy like this, else as need)
private:
MySmartPtr(const MySmartPtr&)throw():mPointer(0){ exit(1); }
void operator= (const MySmartPtr&)throw(){ exit(1); }
Yes, and yet
public:
T* operator= (T *const aPointer)throw() { delete mPointer;
mPointer=aPointer; }

Jan 12 '07 #7

P: n/a
Ernesto Bascón wrote:
I am implementing my custom smart pointer:

template <typename T>
class MySmartPtr
{
public:
MySmartPtr(T* aPointer)
{
mPointer = aPointer;
}

inline T* operator->() const {return mPointer; }

private:
T* mPointer;
};
It could be used in this way:

MySmartPtr<Aobj = new A();
obj->Method1();
int value = obj->Method2();

The operator-overload allows accessing the A methods as they are
accessed via a common pointer.

I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
There is a way I could implement an overload of the delete pointer in
my MySmartPtr<Tor there is some workaround on this?
I presume there are other functions omitted from your code above, and
from your description, it's not clear what semantics you are intending
to give this smart pointer (though it sounds like you want shared
ownership). Generally, smart pointers (e.g., std::auto_ptr,
std::tr1::shared_ptr, boost::scoped_ptr, Loki::SmartPtr, and the smart
pointer in FAQ 16.22) are responsible for cleaning up after themselves
at the appropriate time so that the user doesn't have worry about
deleting anything. Indeed, that's often the primary point. What is your
point?

As for overloading delete, you probably don't want to do that because
it would change the semantics of delete from operating on the object
(viz. MySmartPtr<>) to operating on the contained object. In other
words, class-specific new/delete operators are for deleting the smart
pointer class itself:

MySmartPtr<T>* pT
= new MySmartPtr<T>( 0 ); // Calls MySmartPtr<T>::operator new
delete pT; // Calls MySmartPtr<T>::operator delete

You could accomplish your goal by supplying a cast to T*, but that
allows some other dangerous code. The preferred method is to supply a
reset function:

template<class T>
void MySmartPtr<T>::Reset( T* const p = 0 )
{
delete mPointer;
mPointer = p;
}

It's best to use RAII and let the smart pointer do the dirty work. See
the FAQ mentioned above and those following, and see this chapter from
_Modern C++ Design_ on Loki's smart pointers:

http://www.informit.com/articles/pri...p?p=25264&rl=1

Cheers! --M

Jan 12 '07 #8

P: n/a
I presume there are other functions omitted from your code above, and
from your description, it's not clear what semantics you are intending
to give this smart pointer (though it sounds like you want shared
ownership). Generally, smart pointers (e.g., std::auto_ptr,
std::tr1::shared_ptr, boost::scoped_ptr, Loki::SmartPtr, and the smart
pointer in FAQ 16.22) are responsible for cleaning up after themselves
at the appropriate time so that the user doesn't have worry about
deleting anything. Indeed, that's often the primary point. What is your
point?
Primarily, I want to provide a hierarchy of smart pointers, providing
the basic implementation in something like AbstractPtr<T>; the basic
implementation should include null pointer checking on -and dangling
pointer avoiding.

I want to provide also a Ptr<T: public AbstractPtr<Timplementation
that should have the same behavior that the standard pointers, but with
the null pointer checking. This smart pointer should not have any
semantics (thus, should provide mechanisms to release the pointee
object manually).

Jan 12 '07 #9

P: n/a

Ernesto Bascón wrote:
I want to allow the final user call the delete operator as in a common
pointer when he wants to explicitly remove the pointer:

delete obj;
No you don't.

Jan 12 '07 #10

P: n/a
Ernesto Bascón wrote:
I presume there are other functions omitted from your code above, and
from your description, it's not clear what semantics you are intending
to give this smart pointer (though it sounds like you want shared
ownership). Generally, smart pointers (e.g., std::auto_ptr,
std::tr1::shared_ptr, boost::scoped_ptr, Loki::SmartPtr, and the smart
pointer in FAQ 16.22) are responsible for cleaning up after themselves
at the appropriate time so that the user doesn't have worry about
deleting anything. Indeed, that's often the primary point. What is your
point?

Primarily, I want to provide a hierarchy of smart pointers, providing
the basic implementation in something like AbstractPtr<T>; the basic
implementation should include null pointer checking on -and dangling
pointer avoiding.

I want to provide also a Ptr<T: public AbstractPtr<Timplementation
that should have the same behavior that the standard pointers, but with
the null pointer checking. This smart pointer should not have any
semantics (thus, should provide mechanisms to release the pointee
object manually).
Well, IMHO, its generally bad to have an explicit delete anywhere in
your main code. All resources are best managed by RAII and classes that
enable it (e.g., std::tr1::shared_ptr).

As for the hierarchy, I'm not quite sure why you're bothering. If
you're trying to create a variety of types of smart pointers, check out
Loki's policy-based solution.

As for a class that behaves just like a regular pointer but does
checking, how about something like:

template<class T>
class Ptr
{
T* p_;
public:
Ptr( T* const p=0 ) : p_(p) {}
operator T*() { assert( p_ ); return p_; }
T* operator->() { assert( p_ ); return p_; }
T& operator*() { assert( p_ ); return *p_; }
};

class C {};

Ptr<Cp1( new C );
delete p1;

If you had something grander in mind, I think you could achieve the
same thing (but with considerably more flexibility and code that is
already written and tested) with Loki's smart pointer by creating a
storage policy (let's call it NoDelete) identical to
Loki::DefaultSPStorage, except without the delete in its Destroy()
member function:

typedef Loki::SmartPtr<C,
Loki::NoCopy,
Loki::AllowConversion,
Loki::AssertCheck,
NoDeleteMyPtr;

MyPtr p2( new C );
delete p2;

Hope that helps.

Cheers! --M

Jan 13 '07 #11

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