Got trouble understanding Pointers

[Bv202]

Hey,

I'm now learning how to use Pointers, but I'm stuck already.

int* FirstPointer = &Variable;

That will work. But when using an array, it MUST be this:
int* FirstPointer = Variable;

So no more & in the variable (array in this case) name.
So why is the & needed in normal variables and why isn't it allowed in array's?

I'm quite confused about this.

Thanks

[Duthomhas]

The array name is the address of the first item in the array.

In C and C++, the code a[ n ] is a shortcut for *(a + n).

Hope this helps.

[Bv202]

Yes, I understand that, but I still don't understand why &arrayname is wrong but &normalvarname is correct.

Thanks :)

[writetonsharma]

int Variable[100];
int* FirstPointer = &Variable[0];

now you should be happy..!!??

[Bv202]

Aaah, I understand it now.

Thanks :)

[S6k9GNh0]

Same as:

int variable[3];
int * firstPointer = &variable;
firstPointer[2] = 3;
//And since firstPointer[0] is in the exact same location as firstPointer.
firstPointer = 6; //firstPointer[0] now equals 6 and vice versa. 

[Bv202]

Ok, I think I understand it.

But now I'm stuck again.

Can anyone please explain me the use of this:
int* dynInt = new int;

I know this is a dynamic integer. I know this is useful for arrays, but what is the use with normal variables?

[Disch]

er--- that code is wrong @ computerquip

int variable[3];
int* firstPointer;

firstPointer = variable;      // OK
firstPointer = &variable[0];  // OK (does same thing)
firstPointer = &(*variable);  // OK (does same thing)

//firstPointer = &variable;   // bad (error)

firstPointer[2] = 3;          // OK (same as variable[2] = 3;
                              //      because firstPointer2 points to variable)

firstPointer = 6;             // bad -- changes firstPointer to point to
                              //      random garbage.  Might throw a compiler
                              //      warning/error depending on the strictness 

EDIT

I know this is a dynamic integer. I know this is useful for arrays, but what is the use with normal variables?

It isn't generally useful for normal variables because you can just make a normal variable instead of making a pointer. What it is useful for is dynamic object creation. IE Polymorphism and inheritance:

class Parent
{ /* stuff */ };

class ChildOne : public Parent
{ /* stuff */ };

class ChildTwo : public Parent
{ /* stuff */ };

//---------------------
Parent* MakeObj();

int main()
{
  Parent* p = MakeObj();  // get an object from MakeObj
  p->DoStuff();         // and do something with it

  delete p;
  return 0;
}

Parent* MakeObj()
{
  // get input from user
  int i;
  cin >> i;

  if(i == 0)         // return a different object, depending on what the user selected
    return new ChildOne;
  else
    return new ChildTwo;
}

-- if you didn't get into these concepts yet, don't worry about it too much. Just showing that it will be useful later on, even if it's not useful now.

[S6k9GNh0]

int main()
{
    int variable[3];
    int* firstPointer;

    firstPointer = variable;      // OK
    firstPointer = &variable[0];  // OK (does same thing)
    firstPointer = &(*variable);  // OK (does same thing)

    firstPointer[2] = 3;

    *firstPointer = 6;
}
//firstPointer = &variable; I knew this was wrong, not sure what I was thinkin' 

[Bv202]

Thank you both Computerquip and Disch

@Disch: ok, thanks. For now, I'll forget it in that case =)

[Bv202]

Ok, I'm back with yet another question :(

I've seen now how to pass pointers to a function parameter. But I often see things like:
int* FunctionName(int* par1, int par2, int par3)

I understand what int* par1 does, but I don't really understand how the return type of a function (int* FunctionName) can be a pointer.

Can anyone explain this please? Thanks

[writetonsharma]

it depends why you want a return type of int* or void* or anything..
one of them is this:

void some_function()
{
//some code ........................

//here i want to allocate memory of 10 bytes.
int *i_ptr = get_memory_int(10);

//some code ....................
}


int* get_memory_int(int size)
{
int *i_ptr = (int*)malloc(sizeof(int) * size);

return i_ptr;
}

hope this will help you understand.

[Bv202]

A bit yes, thanks :)