How do I use a pointer to an object from one function in another function?

Question

I have a pointer to an object which i will be using in one method. but i need to use the same pointer again in another method how can i achieve this without declaring as global object. This is part of my dynamic biding achievement Shape is the parent class and the Rectangle is the child class.

int main(){
switch (choice){
case 1:
 create();
 break;
case 2:
 process();
 break;
 }
}

create(){
Shape *shape[3];
shape[0]=&objRectangle;
}

process(){
for(int i=0;i<3;i++){
shape->print; // I want to acchieve like this.
}

Now i cant do this cause, the shape object is gone once it exits the create process. Please assist.

Please post code that even approximately compiles. Otherwise it is hard to help you. — pmr, Oct 23 '12 at 19:01
@pmr he wants the `shape` array to outlive the scope in which it is declared; he just needs `vector` or `new[]`. — Seth Carnegie, Oct 23 '12 at 19:01
@SethCarnegie But apparently he doesn't even know how to iterate over an array. It would be crazy to show him `new`. — pmr, Oct 23 '12 at 19:05
You really shouldn't be messing with pointers until you at least understand function arguments. — David Schwartz, Oct 23 '12 at 19:11
@SethCarnegie Appearantly the OP doesn't have any clue about the basics! — πάντα ῥεῖ, Oct 23 '12 at 19:19
you should definetely do something like: `Shape * shape = new Shape[3]`. But, What you really want to do probably is `Shape ** shape = new Shape*[3]`. — Acorbe, Oct 23 '12 at 19:22
@Acorbe No! The OP should use a `std::vector` or `std::array` instead of messing around with memory management additionally! — πάντα ῥεῖ, Oct 23 '12 at 19:29
@g-makulik, sure and stack allocate it in main or something. — Acorbe, Oct 23 '12 at 19:32
what if I create the shape object in the main() function and pass the address to the methods, so they will be able to acess the object. will it work ? if yes how can i acheieve this — rasul1719435, Oct 24 '12 at 06:12

Jan Včelák · Answer 1 · 2012-10-23T19:47:59.443

1

Local variables are created on the call stack. Which means that these variables are always destroyed when the function finishes.

It is unclear how do you retrieve the Rectangle object and how do you handle the array of Shape object pointers from the code you pasted. But you probably need two things:

Your Rectangle must not be on the stack, but it must be allocated on the heap.
You need access to the shape array on all places, where you want to access it.

I would suggest you to create a class which will wrap your functions and have Shape objects instantiated and removed dynamically using new and delete keywords. I believe this is the easiest way. Something like this should work:

/* interface */

class Example
{
     Shape *shape[3];

     public:

     Example();
     ~Example();

     void create();
     void process();
};

/* implementation */

Example::Example() {}

Example::~Example()
{
    delete shape[0];
}

void Example::create()
{
    shape[0] = new Rectangle();
}

void Example::process()
{
    shape[0]->print();
}

/* main */

int main()
{
    Example example();
    example.create();
    example.process();
}

edited Oct 23 '12 at 19:47

answered Oct 23 '12 at 19:31

Jan Včelák

2,459
1
21
19

This solution is not right either. `example` is stack allocated, and should be initialized using `Example example`. – Acorbe Oct 23 '12 at 19:42
Sorry, I submitted the answer by mistake before it was finished. I believe it is correct now. Sure, `example` is allocated on the stack. I think the problem was with `Rectangle` object. – Jan Včelák Oct 23 '12 at 19:50
@JanVčelák Cant i use by referrencem & to access the items. – rasul1719435 Oct 24 '12 at 06:14
@rasul1719435 Only if the object exists at the time when you want to access it. You need to understand what is the difference between stack and heap. And where is the variable allocated in C++ depending on the place where the variable is defined. Start with it. ;-) – Jan Včelák Oct 24 '12 at 09:14

Acorbe · Accepted Answer · 2012-10-24T09:13:16.877

I would suggest, as others do, to let the library manage the memory for you.

In order to be able to use dynamic binding and std::vector you should start allocating (in your main) your vector as

    std::vector<Shape*> shape(3);

Doing so you can access your dynamically bound vector entries as

    shape[0]->process();

The bad thing is that you still have to manage the memory pointed by vector entries (they are just C pointers, in fact). Hence, why don't you consider doing

    std::vector< std::tr1::shared_ptr< Shape > > shape(3);

?

Doing this way, the smart pointer std::tr1::shared_ptr will free the memory for you when the pointed Shape object goes out of scope.

Moreover, in this setting, you should allocate Shape-type objects as

     shape[0] = std::tr1::shared_ptr< Shape >(new Rectangle);

to properly create the smart pointer you need.

Finally, the vector shape should be passed by reference (or const reference) to functions using it.

Thanks, i will give a try to this method. – rasul1719435 Oct 24 '12 at 11:05 — rasul1719435, Oct 24 '12 at 11:05

How do I use a pointer to an object from one function in another function?

2 Answers2