C++ memory leaks with unordered_map

Question

I've discovered that I have a memory leak somewhere in my application but I've been having trouble narrowing it down. I've tried using the function _CrtDumpMemoryLeaks as instructed in this example but it doesn't show file names. So, I've been trying to find the first point of memory leaks (as there seem to be quite a few according to the output) by placing calls to this dump function at various points. It seems that I am getting a lot of them straight away very early in my constructors for objects.

Upon further investigation, I seem to be getting them with definitions for std::unordered_map even to the point where having a simple main function that just declares a local variable has a memory leak.

For example the following code produces a memory leaks:

void main()
{
    _CrtDumpMemoryLeaks(); // executing this line, no memeory leaks found

    std::unordered_map<int, int> intMap;

    _CrtDumpMemoryLeaks(); // executing this line, memeory leaks found
}

I'm completely confused at this point and get the feeling that chasing this is not going to help me find the memory leak that I am originally noticed.

Any help is very much appreciated.

Answer 1

So you expect that std::unordered_map<T> doesn't hold any memory while you are clinging to an object? That seems to be an unreasonable assumption! At the very least, you should destroy the object you created:

int main()
{
    _CrtDumpMemoryLeaks(); // executing this line, no memeory leaks found

    {
        std::unordered_map<int, int> intMap;
    }

    _CrtDumpMemoryLeaks(); // executing this line, memeory leaks found
}

I don't know whether this will release all memory but it has a better chance. The standard C++ library may still keep some memory pooled in its memory allocation mechanism.

Answer 2

What's the definition of a memory leak except "memory that has been allocated and not yet freed"?

If the unordered_map constructor allocates any memory internally, then you have your "leak" right there. Try putting it in its own scope and see if you get the same results.

Answer 3

It only makes sense to call _CrtDumpMemoryLeaks when you have freed everything you've allocated.

As the other answers have stated, you have a unordered_map in scope and this object has allocated memory. Once it's gone, the allocations will go with it. So wrap it in a scope!