Memory leak in C++

In C++, memory leak is a situation where the memory allocated for a particular task remains allocated even after it is no longer needed. This leads to the wastage of memory because it is unavailable for other tasks till the end of the program.

Why memory leak occurs in C++?

In C++, there is no automatic garbage collection. It means that any memory that is dynamically allocated by the programmer needs to be freed after its usage manually by the programmer. If the programmer forgets to free this memory, it will not be deallocated till the program lives and will be unavailable to other processes. This is called memory leak.

Example:

#include <stdlib.h>

void f() {
  
    // Allocate memory
    int* ptr = new int[10];

    // Return without freeing ptr
    return;
}

int main() {
    // do some tasks
}

Memory for an array of 10 integers is allocated using new, but the memory is never freed. This results in a memory leak since the allocated memory is no longer accessible but remains occupied.

Consequences of Memory Leakage

Different problems occur when memory leakage happens:

• Reduced Performance: Leaked memory is not available for other parts of your program or other programs running on the computer. Over time, if leaks are significant, program and even the whole system can slow down due to lack of available memory.
• Program Crashes: If program keeps leaking memory, it might eventually use up all the available RAM. When this happens, the program might become unstable, start behaving erratically, or crash entirely.
• Resource Depletion: Memory is a limited resource and memory leaks can lead to resource depletion.
• Long-Running Programs Suffer Most: Memory leaks are often more of a problem in programs that run for a long time (like servers, daemons, etc.). Short-running programs might leak memory for little time, but long running program will keep that memory for a long period of time.

How to Find Memory Leaks?

Detecting memory leaks in C++ can be challenging due to the lack of automatic memory management. However, several strategies and tools can help detect and prevent memory leaks. There are basically two methods:

• Manual Method: It is basically going through the whole code and finding the memory which is not deallocated using corresponding delete operation.
• Using Tools: Tools like Valgrind, AddressSanitizer, etc. makes it easier to find the sources or memory leak without reviewing the whole code.

To know more methods to find memory leaks in C++, refer to the article - Detect Memory Leaks in C++

How to Avoid Memory Leaks?

The most basic method is to avoid memory leak is by careful usage of new and delete. For every time you allocate memory (e.g., with new), make sure there's a corresponding delete to free it when it's no longer needed.

There are also some methods that helps in reducing memory leaks:

• Smart Pointers: Use smart pointers wherever you can and avoid using raw pointers. They automatically manage memory and deallocate it when objects go out of scope.
• Constructor and Destructor: If you have complex memory allocation (like 2d array), wrap your resource in a class and then manually deallocate the memory using destructor which will be automatically called when the object goes out of scope.
• Use Detection Tools: Use memory leak detection tools such as Valgrind to look for memory leaks and handle them.