class - In C++ I Cannot Grasp Pointers and Classes

Question

I'm fresh out of college and have been working in C++ for some time now. I understand all the basics of C++ and use them, but I'm having a hard time grasping more advanced topics like pointers and classes. I've read some books and tutorials and I understand the examples in them, but then when I look at some advanced real life examples I cannot figure them out. This is killing me because I feel like its keeping me from bring my C++ programming to the next level. Did anybody else have this problem? If so, how did you break through it? Does anyone know of any books or tutorials that really describe pointers and class concepts well? or maybe some example code with good descriptive comments using advanced pointers and class techniques? any help would be greatly appreciated.

Answer 1

Understanding Pointers in C/C++

Before one can understand how pointers work, it is necessary to understand how variables are stored and accessed in programs. Every variable has 2 parts to it - (1) the memory address where the data is stored and (2) the value of the data stored.

The memory address is often referred to as the lvalue of a variable, and the value of the data stored is referred to as the rvalue (l and r meaning left and right).

Consider the statement:

int x = 10;

Internally, the program associates a memory address with the variable x. In this case, let's assume that the program assigns x to reside at the address 1001 (not a realistic address, but chosen for simplicity). Therefore, the lvalue (memory address) of x is 1001, and the rvalue (data value) of x is 10.

The rvalue is accessed by simply using the variable “x”. In order to access the lvalue, the “address of” operator (‘&’) is needed. The expression ‘&x’ is read as "the address of x".

Expression          Value
----------------------------------
x                   10
&x                  1001

The value stored in x can be changed at any time (e.g. x = 20), but the address of x (&x) can never be changed.

A pointer is simply a variable that can be used to modify another variable. It does this by having a memory address for its rvalue. That is, it points to another location in memory.

Creating a pointer to “x” is done as follows:

int* xptr = &x;

The “int*” tells the compiler that we are creating a pointer to an integer value. The “= &x” part tells the compiler that we are assigning the address of x to the rvalue of xptr. Thus, we are telling the compiler that xptr “points to” x.

Assuming that xptr is assigned to a memory address of 1002, then the program’s memory might look like this:

Variable    lvalue    rvalue
--------------------------------------------
x           1001      10   
xptr        1002      1001

The next piece of the puzzle is the "indirection operator" (‘*’), which is used as follows:

int y = *xptr;

The indirection operator tells the program to interpret the rvalue of xptr as a memory address rather than a data value. That is, the program looks for the data value (10) stored at the address provided by xptr (1001).

Putting it all together:

Expression      Value
--------------------------------------------
x                   10
&x                  1001
xptr                1001
&xptr               1002
*xptr               10

Now that the concepts have been explained, here is some code to demonstrate the power of pointers:

int x = 10;
int *xptr = &x;

printf("x = %d
", x);
printf("&x = %d
", &x);        
printf("xptr = %d
", xptr);
printf("*xptr = %d
", *xptr);

*xptr = 20;

printf("x = %d
", x);
printf("*xptr = %d
", *xptr);

For output you would see (Note: the memory address will be different each time):

x = 10
&x = 3537176
xptr = 3537176
*xptr = 10
x = 20
*xptr = 20

Notice how assigning a value to ‘*xptr’ changed the value of ‘x’. This is because ‘*xptr’ and ‘x’ refer to the same location in memory, as evidenced by ‘&x’ and ‘xptr’ having the same value.