vector of pointers

**chesschi** · 04-06-2011

Please read the following snippets:

Code:

class B
{
public:
B() {}
~B() {}
};

class A
{
public:
A()
{
B* b1 = new B();
B* b2 = b1;
B* b3 = new B();
B* b4 = b2;
m_list.push_back(b1);
m_list.push_back(b2);
m_list.push_back(b3);
m_list.push_back(b4);}

~A()
{
for (std::vector<B*>::size_type i = 0; i < m_list.size(); i++)
{
if (m_list[i] != 0)
{
delete m_list[i];//crash when deleting b2
m_list[i] = 0;
}
}}

private:
std::vector<B*> m_list;
};

void main()
{
A * a = new A();
delete a;
}

It will crash when I attempt to delete b2 in the destructor. But I need to have a vector storing one object but is pointed by two or more vectors (like b1 and b2). I knew it is not the proper way to implement like this. Please could someone give any hint how to implement this without crashing?

**kmdv** · 04-06-2011

Each pointer must be deleted only once. B1 B2 and B4 are the same (they point to the exact memory location).

**laserlight** · 04-06-2011

Use a std::vector<std::tr1::shared_ptr<B> > instead.

**chesschi** · 04-06-2011

Originally Posted by laserlight

Use a std::vector<std::tr1::shared_ptr<B> > instead.

TR1 is fairly new and it may not be supported in linux. Please could you suggest other approaches apart from using shared pointer? Thanks.

**kmdv** · 04-06-2011

Originally Posted by chesschi

TR1 is fairly new and it may not be supported in linux. Please could you suggest other methods apart from using shared pointer? Thanks.

It is supported. shared_ptr - Boost 1.46.1

Methods to what? I have already pointed out what is wrong with the code - and THIS was the point of this thread.

**chesschi** · 04-06-2011

Originally Posted by kmdv

It is supported. shared_ptr - Boost 1.46.1

Methods to what? I have already pointed out what is wrong with the code - and THIS was the point of this thread.

I knew the reason why it crashes. The question I was asking is
"what is the proper way to handle the situation of two or more pointers pointing to the same object and being stored in the same container like vector"
The container only copies the pointers and so they no longer share the same address.

I am using a pretty old compiler and am keen to have my code compatible with different platforms and so I ask for alternative approach.

Sorry for any misleading questions.

**laserlight** · 04-06-2011

Well, short of writing your own smart pointer class, a possible solution is to use another vector to store the objects, then in this vector of pointers, you never use delete on any of the pointers. You only insert and erase elements of the vector.

When you want to remove an element from that other vector, you first remove all of the pointers to that element in this vector of pointers.

**kmdv** · 04-06-2011

Originally Posted by chesschi

I am using a pretty old compiler and am keen to have my code compatible with different platforms and so I ask for alternative approach.

If you use an old compiler, don't expect your application to be cross-platform.
Smart pointers are the way to go.

**C_ntua** · 04-06-2011

Originally Posted by chesschi

I knew the reason why it crashes. The question I was asking is
"what is the proper way to handle the situation of two or more pointers pointing to the same object and being stored in the same container like vector"
The container only copies the pointers and so they no longer share the same address.

I am using a pretty old compiler and am keen to have my code compatible with different platforms and so I ask for alternative approach.

Sorry for any misleading questions.

The other way is generally to change the logic of the code. So first to ask "why do you need to store a duplicate pointer"? Can you make a check before you store or before you delete a pointer? (is this performance wise efficient for you?)
The point is that you should first think of your initial design to see if this can be avoided.

Try building your own smart pointer. Its an interesting project. Apart from that there is no other automatic/generic way to solve this, since what you are asking is having a smart pointer, at least the reference counting part.

**kmdv** · 04-07-2011

If you are about to build you own smart pointer, do it only for educational purposes. If you do not have enough knowledge, you will run into a load of problems. I have implemented a set of smart pointers because of my special needs. What is implemented in boost will be enough, if not too much, for you.

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