What does "*(pointer + integer)" do in C++?

Question

I'm confused as to what a line of code does in this program:

int *temp = new int [cap];
int num = 0;

for(int i = name; i < number; i++)
{
 *(temp + count) = *(foo + i);
 num++;
}

name, number, and foo are global variables (foo being a pointer), and cap is an argument.

Specifically, I don't understand this line:

 *(temp + count) = *(foo + i);

Why are there pointers to the parentheses, and what will this do?

Answer 1

*(temp + count) = *(foo + i);

The + operators are performing pointer arithmetic. Adding an integer to a pointer value yields a new pointer incremented a specified number of objects past the original pointer. For example, if p is a pointer to arr[0], then p+2 points to arr[2].

The * operator deferences the resulting pointer, giving you the object that it points to.

In fact, the array indexing operator [] is defined in terms of pointer arithmetic, so that A[i] means *(A+i) (ignoring operator overloading). So the above line of code:

*(temp + count) = *(foo + i);

could also be written (more clearly IMHO) as:

temp[count] = foo[i];

You might want to read the comp.lang.c FAQ, particularly sections 4 (Pointers) and 6 (Arrays and Pointers). Most of the information is also applicable to C++.

However, C++ provides higher-level library interfaces that can be more robust than the low-level C equivalents. It's seldom a good idea in C++ to write code that deals directly with arrays and pointers to array elements, unless it's very low-level code and performance is critical and/or you're dealing with data structures from C code.