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c++classpointersinstantiation
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Reputation: 823

C++ instantiation via pointer

Here is my code : 

#include <iostream>
using namespace std;
class form{
public:
    form(int c ){
        code = c;
    }
    int code;
};
void createForm(form* f,int c){
    f = new Form(c);
}
int main(){
    form* f1;
    form* f2;
    createForm(f1,1111);
    createForm(f2,2222);
    cout<<f1->code<<endl;
    cout<<f2->code<<endl;
    return 0;
}

As a result,I didn't see anything printed out. I know that f1 & f2 are not created actually. So I'm wondering if I can instantiate a class like this? If yes, then how ?

Upvotes: 0

Views: 575

Answers (2)

Marius Bancila
Marius Bancila

Reputation: 16328

You need to pass the pointer by reference.

void createForm(form* &f,int c){
    f = new Form(c);
}

However, I would rather change the createForm function to look like this:

form* createForm(int c) {
   return new Form(c);
}

Upvotes: 5

Luchian Grigore
Luchian Grigore

Reputation: 258618

void createForm(form* f,int c){
    f = new Form(c);
}

will create an object and assign its address to the local f - which is a copy of the original f1 and f2 withing that function's scope.

The original pointers are left unchanged, so the following cout<< statements lead to undefined behavior.

What you're trying to achieve can be accomplished by passing the pointer by reference:

createForm(form*& f,int c)

You also forgot to call delete, so now you have a memory leak.

Even better - don't use pointers at all.

Upvotes: 6

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