(C++) Memory leak

When a program dynamically creates new instances of classes without freeing it in the end.

A common beginner's error is forgetting to call delete

{
  MyClass * pMyClass = new MyClass;
  //Forget to write 'delete pMyClass'
}

You can prevent this memory leak using auto_ptr's (and auto_ptr's do more then only this):

include <memory>


{
  std::auto_ptr<MyClass>pMyClass(new MyClass);
  //No need to write 'delete pMyClass'
}

Another common beginner's error is to generate an array dynamically, without calling delete[]. They either forget it in total, or use the 'plain' delete:

{
  double * pArray = new double[1000];
  //Forgot to call 'delete[] pArray'
}
{
  double * pArray = new double[1000];
  delete pArray; //MEMORY LEAK!!!
  //Forgot to call 'delete[] pArray'
}
{
  double * pArray = new double[1000];
  delete[] pArray; //Correct!
}

To prevent this error, use a vector instead.

Generate a memory leak

To see what errors might occur:

int main()
{
  while (1)
  {
    int * i = new int[10000000];
  }
}

Visual Studio

To prevent memory leaks in Visual Studio, when working in console application, add the lines below to your code. When working with the ?MFC it does this automatically.

include <crtdbg.h>

int main()
{
  void* leak = malloc(1234);
  _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
}

To find out the allocation that causes the leak, elaborate with:

int __cdecl MyAllocHook(
  int                    //nAllocType,
  void   *               //pvData,
  size_t                 //nSize,
  int                    //nBlockUse,
  long lRequest,
  const unsigned char *  //szFileName,
  int                    //nLine
  )
{
  //Set a conditional breakpoint to break on the leaking 'lRequest'
  return true; // Allow the memory operation to proceed
}
void* leak()
{
	return malloc(1234);
}
int main()
{
  _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
  _CrtSetAllocHook( MyAllocHook );
  leak();
}

'Memory leak' links

External links

How Bjarne Stroustrup deals with memory leaks: http://www.research.att.com/~bs/bs_faq2.html#memory-leaks

last edited (May 6, 2007) by bilderbikkel, Number of views: 2571, Current Rev: 8 (Diff)

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