CODEWITHSUNDEEP
c++pointers
Abdul Ahmad
Abdul Ahmad

Reputation: 10021

confused about pointers

I just started learning c++ (I'm more of a java developer right now) and am having some confusion with using pointers... for example, the following code works

int main() {
    int x = 5;
    int * y;
    y = &x; //note this line of code
    *y = 10;
}

whereas this code does not work

int main() {
     int x = 5;
     int * y;
     y = x;
     *y = 10;
}

Can someone explain to me why getting the value "location" using y = &x works but as soon as I replace it with y = x it causes an error. If anyone knows of a good explanation on pointers please share the link :)

Thank you!

Upvotes: 0

Views: 389

Answers (3)

Gabriel L.
Gabriel L.

Reputation: 5014

Let's see how this works with pointers. 

int x = 5;

You're assigning the value 5 to x which is an int.

int *y;

You're declaring a pointer to an int.

y = &x;

Now, the address stored in y is the same as the address of x.

But, if you do this : y = x, you're assigning an integer (5 in that case) to a variable that holds addresses of integers.

Finally, you have to remember that :

  • & is the address-of operator, and can be read as "address of"
  • * is the indirection operator, and can be read as "value pointed to by"

Tutorial about pointers

Upvotes: 7

zero298
zero298

Reputation: 26867

y is a pointer to an int. x is an int. Therefore whenever you set y=x you are telling y that whenever it gets dereferenced, it should go looking at memory location 10 (or whatever x is at assignment time). And there probably isn't a memory allocation at 10. Or at least not what you are expecting. If you want to get the location of x, get a pointer to it with &x.

int main() {
     int x = 5;
     int * y;
     y = &x; // <-- NOTE THE "&" WHICH MEANS GET REFERENCE
     *y = 10;
}

Upvotes: 0

Jeff.Clark
Jeff.Clark

Reputation: 611

a pointer is a variable that holds a memory location. In your instance, the variable "x" holds the value 5, where the variable y holds a location in memory. the "&" operator will enable you to use the location in memory of the "x" variable

cout >> x; //will give you an output of 5.
cout >> &x; //will give you an output of the memory location of the variable x.

y = &x;
cout >> y; //will give you an output of the memory location of the variable x.

Upvotes: 0

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