Pointers to Pointers in C++

Question

Hello I'm new to c++ and I'm trying to figure out how to use pointers to change the same object.

If I have a vector of pointers for example

std::vector<myclass*> top;

and lets say the top[0] = NULL I want to use another pointer to change it

myclass* other = top[0];
other = new myclass();

So that when I access top[0] it will be pointed to the new object created? Sorry a little confusing but that's the idea.

Answer 1

If, for example, you had a vector of int :

std::vector<int> vec(10);

and then did something like

int other = vec[0];
other = 5;

I think most people would understand that the assignment other = 5; would change the value of the variable other only, not the value of vec[0] .

Now lets take your code:

myclass* other = top[0];
other = new myclass();

It's exactly the same thing here: The assignment other = new myclass(); only changes the variable other and where it points, it doesn't change top[0] .

What I think you want is to use other as a reference to the value in top[0] , which you can do by using references:

myclass*& other = top[0];
other = new myclass();

Now other will be a reference to the value in top[0] , so the assignment will change top[0] .

And for completeness sake, and keeping with the pointer-to-pointer thing in the title, you could of course solve it through pointers to pointers:

myclass** other = &top[0];
*other = new myclass();

With the above other will be a pointer to top[0] , and you need to use the dereference operator * (as in *other ) in the assignment to change the value at top[0] . I really recommend the references though.

Answer 2

You've got things a bit backward:

myclass *other = new myclass();
top[0] = other;

EDIT:

You need to take the address of the vector element:

std::vector<myclass*> top(5);
myclass *other = new myclass();
other->x = 4;
top[0] = other;
printf("top[0].x=%d\n", top[0]->x);
myclass **p = &top[0];
(*p)->x = 5;
printf("top[0].x=%d\n", top[0]->x);

Output:

top[0].x=4
top[0].x=5