Monday, January 10, 2011

Memory Management in C++

A memory leak is a type of programming bug that occurs when a program allocates more memory than it frees. This way, an application can run out of memory and cause the system to crash. To prevent memory leaks, you need to know when they occur most frequently and be conscientious with your use of the "new" and "delete" C++ operators.

The C++ operator "new" allocates heap memory. The "delete" operator frees heap memory. For every "new," you should use a "delete" so that you free the same memory you allocated:

1.char* str = new char [30]; // Allocate 30 bytes to house a string.

delete [] str; // Clear those 30 bytes and make str point nowhere.

2.Reallocate memory only if you've deleted. In the code below, str acquires a new address with the second allocation. The first address is lost irretrievably, and so are the 30 bytes that it pointed to. Now they're impossible to free, and you have a memory leak:

char* str = new char [30]; // Give str a memory address.
// delete [] str; // Remove the first comment marking in this line to correct.

str = new char [60]; // Give str another memory address with the first one gone forever.

delete [] str; // This deletes the 60 bytes, not just the first 30.

3.Watch those pointer assignments. Every dynamic variable (allocated memory on the heap) needs to be associated with a pointer. When a dynamic variable becomes disassociated from its pointer(s), it becomes impossible to erase. Again, this results in a memory leak:

char* str1 = new char [30];

char* str2 = new char [40];

strcpy(str1, "Memory leak");

str2 = str1; // Bad! Now the 40 bytes are impossible to free.

delete [] str2; // This deletes the 30 bytes.

delete [] str1; // Possible access violation. What a disaster!

4.Be careful with local pointers. A pointer you declare in a function is allocated on the stack, but the dynamic variable it points to is allocated on the heap. If you don't delete it, it will persist after the program exits from the function:

void Leak(int x){

char* p = new char [x];

// delete [] p; // Remove the first comment marking to correct.


5.Pay attention to the square braces after "delete." Use "delete" by itself to free a single object. Use "delete" [] with square brackets to free a heap array. Don't do something like this:

char* one = new char;

delete [] one; // Wrong

char* many = new char [30];

delete many; // Wrong!