21.1. Pointer Pathology

21.1. Pointer Pathology
	Chapter 21. Memory Access

[ fromfile: ptrpathology.xml id: ptrpathology ]

In Section 1.15, we introduced pointers and demonstrated some of the basics of working with them.
We now look at two short code examples to demonstrate some of the weird and dangerous things that can happen when pointers are not handled correctly.
Example 21.1 shows a few of the many ways one can declare pointers.

Example 21.1. src/pointers/pathology/pathologydecls1.cpp

[ . . . . ]

int main() {
    int a, b, c;  
    int* d, e, f; 
    int *g, *h;   
    int* i, * j;  

    return 0;
}

	As expected, this line creates three ints.
	This line creates one pointer to an int and two ints. (!)
	This line creates two pointers to int.
	This line also creates two pointers to int.

To declare several pointer variables in one line, you must place an asterisk to the left of each pointer variable.
That is why we recommend having a separate declaration (on a separate line) for each pointer.
Example 21.2 contains three groups of statements.

Example 21.2. src/pointers/pathology/pathologydecls2.cpp

[ . . . . ]
int main() {
    int myint = 5;
    int* ptr1 = &myint;
    cout << "*ptr1 = " << *ptr1 << endl;
    int anotherint = 6;
//  *ptr1 = &anotherint;   
    
    int* ptr2;             
    cout << "*ptr2 = " << *ptr2 << endl;
    *ptr2 = anotherint;    

    int yetanotherint = 7;
    int* ptr3;
    ptr3 = &yetanotherint; 
    cout << "*ptr3 = " << *ptr3 << endl;
    *ptr1 = *ptr2;         
    cout << "*ptr1 = " << *ptr1 << endl;

    return 0;
}
[ . . . . ]

	Error, invalid conversion from int* to int.
	Uninitialized pointer.
	Unpredictable results.
	Regular assignment.
	Dangerous assignment!

This code illustrates some serious pointer issues, the worst of which are not detected by the compiler.

The first is simply a type mismatch.

src/pointers/pathology> g++ pathologydecls2.cpp
pathologydecls.cpp: In function `int main()':
pathologydecls.cpp:17: error: invalid conversion from `int*' to `int'
src/pointers/pathology>

After commenting out the invalid conversion, we try again.

*ptr1 = 5
*ptr2 = -1218777888
*ptr3 = 7
*ptr1 = 6
Segmentation fault

Dereferencing the uninitialized pointer ptr2 gives unpredictable (i.e., undefined) results.

Dereferencing uninitialized pointers for read purposes is bad enough, but then we wrote to it.
This is a form of memory corruption, which can cause problems later in the program's execution.
The Segmentation fault was caused by the memory corruption that occurred when we dereferenced ptr2.


Chapter 21. Memory Access		21.2. Further Pointer Pathology with Heap Memory