ptr_vector - _CrtDumpMemoryLeaks() - memory leaks even though destructor is called

Question

Hi.

I'm working on a game engine and in an earlier question it was suggested that I start using boost::ptr_vector to maintain a list of pointers.

The basic idea is to have several State's, each State has a SceneGraph. Each state has several resources that they initialize, and then stuff its own SceneGraph. The SceneGraph has a boost::ptr_vector that it stuffs the resource pointers in.

Here's the relevant code:

Resource creation and addition to the SceneGraph in TestState

backgroundImage = new DEBUG_NEW Fenris::Node2D::Image(std::string("backgroundImage"), std::string("../media/img/background.jpg"));
sceneGraph->addNode(backgroundImage, Fenris::Core::STRATUM_BACK);

SceneGraph

boost::ptr_vector<SceneGraphNode> backStratumList;

// The add() method

void addNode(SceneGraphNode *pNode, STRATUM_TYPE stratumType) {
    switch(stratumType) {
    case STRATUM_BACK:
        backStratumList.push_back(pNode);
        break;
    case STRATUM_NORMAL:
        normalStratumList.push_back(pNode);
        break;
    case STRATUM_FOREGROUND:
        foregroundStratumList.push_back(pNode);
        break;
    }
}

Edited main.cpp with relevant lines

PlatformGame::State::TestState *testState = new DEBUG_NEW PlatformGame::State::TestState(std::string("testState"));

// Clean up the previously registered state (deletes its sceneGraph -- verified that the destructor is in fact called via debugger)
delete testState;

// Dump memleak report if we're running in debug mode
#ifdef _DEBUG
    _CrtDumpMemoryLeaks();
#endif

I'm using _CrtDumpMemoryLeaks() to output a memory leak log report. The log report tells me I have a memory leak;

Detected memory leaks!
Dumping objects ->
{174} normal block at 0x00A56630, 32 bytes long.
 Data: <../media/img/bac> 2E 2E 2F 6D 65 64 69 61 2F 69 6D 67 2F 62 61 63 
{173} normal block at 0x00A565A0, 8 bytes long.
 Data: < c      > A8 63 A5 00 00 00 00 00 
Object dump complete.

Is _CrtDumpMemoryLeaks() having trouble with boost::ptr_vector or have I done something wrong? The debugger tells me that State does invoke its destructor (which has; delete sceneGraph) and I've also verified that the SceneGraph's destructor is also invoked.

Any help is greatly appreciated, I'd love to see an empty memory leak report :-)

Answer 1

A few possibilities:

• Your Node2D::Image class has string/char pointers and does not delete the strings/char buffer it is pointing to.

• Your switch case in the AddNode method is missing a default. It is possible that you lose any reference to pNode!

• The type is SceneGraphNode in the boost::ptr_vector but you insert Image which I presume is a subclass. Is the destructor virtual?

I would suggest you try putting a breakpoint in the Image class destructor and see if that is called.

(Caveat: been a while since I did C++!)

Answer 2

It sure doesn't look like a vector you're leaking. Note that the string is readable, that's at least one hint.

If you can get the number between the curly braces stable ("{173}") then you can get a breakpoint when the memory is allocated. Put this in your main() function:

_crtBreakAlloc = 173;

Use #include <crtdbg.h> if necessary. Repeat for 174 to find the other one.