21.2. Further Pointer Pathology with Heap Memory

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

[ fromfile: ptrpathology2.xml id: ptrpathologystorage ]

One of the richest sources of runtime errors is the production of memory leaks.
A memory leak is produced when a program causes memory to be allocated and then loses track of that memory so that it can neither be accessed nor deleted.

The operators new and delete give the C++ programmer increased power and increased responsibility.

Following is some sample code that illustrates a memory leak.

 int* ip = new int;        // allocate space for an int
 int* jp = new int(13);    // allocate and initialize
 cout << ip  << '\t'  <<  jp  << endl;

Figure 21.1. Initial Values of Memory

  After executing the following line of code, memory looks like Figure 21.2. 
 jp = new int(3);   // reassign the pointer - MEMORY LEAK!!

Figure 21.2. Memory After Leak

In Example 21.3, we delete the pointer jp twice.

Example 21.3. src/pointers/pathology/pathologydemo1.cpp

#include <iostream>
using namespace std;

int main() {
    int* jp = new int(13); 
    cout << jp << '\t' << *jp << endl;
    delete jp;
    delete jp;             
    jp = new int(3);       
    cout << jp << '\t' << *jp << endl;
    jp = new int(10);      
    cout << jp << '\t' << *jp << endl;
    int* kp = new int(17);
    cout << kp << '\t' << *kp << endl;
    return 0;
}
Output:

OOP> g++ pathologydemo1.cpp
OOP> ./a.out
0x8049e08       13
0x8049e08       3
0x8049e08       10
Segmentation fault
OOP>

	Allocate and initialize.
	Error: pointer already deleted.
	Reassign the pointer, memory leak.
	Reassign the pointer, memory leak.

The second deletion is a serious error but the compiler did not catch it.
That error corrupted the heap, made any further memory allocation impossible, and made the behavior of the program beyond that point undefined.
For example, notice that when we attempted to produce a memory leak by reassigning the pointer jp we did not get any new memory.
When we attempted to introduce another pointer variable we got a segmentation fault.
This is all undefined behavior and may be different on another platform or with another compiler.