Free Memory Occupied by Std List, Vector, Map etc

Question

Coming from a C# background, I have only vaguest idea on memory management on C++-- all I know is that I would have to free the memory manually. As a result my C++ code is written in such a way that objects of the type std::vector, std::list, std::map are freely instantiated, used, but not freed.

I didn't realize this point until I am almost done with my programs, now my code is consisted of the following kinds of patterns:

struct Point_2
{
    double x;
    double y;
};

struct Point_3
{
    double x;
    double y;
    double z;
};

list<list<Point_2>> Computation::ComputationJob
    (list<Point_3>pts3D, vector<Point_2>vectors)
{
    map<Point_2, double> pt2DMap=ConstructPointMap(pts3D);
    vector<Point_2> vectorList = ConstructVectors(vectors);
    list<list<Point_2>> faceList2D=ConstructPoints(vectorList , pt2DMap);
    return faceList2D;
}

My question is, must I free every.single.one of the list usage ( in the above example, this means that I would have to free pt2DMap, vectorList and faceList2D)? That would be very tedious! I might just as well rewrite my Computation class so that it is less prone to memory leak.

Any idea how to fix this?

Answer 1

No: if objects are not allocated with new, they need not be freed/deleted explicitly. When they go out of scope, they are deallocated automatically. When that happens, the destructor is called, which should deallocate all objects that they refer to. (This is called Resource Acquisition Is Initialization, or RAII, and standard classes such as std::list and std::vector follow this pattern.)

If you do use new, then you should either use a smart pointer (scoped_ptr) or explicitly call delete. The best place to call delete is in a destructor (for reasons of exception safety), though smart pointers should be preferred whenever possible.