The memory cound not be "read". - random crash

Question

I have a problem with a random crash caused by CSceneNode* pRoot = nodes[0]; // The real problem is = nodes[0]; Crash message is: The instruction at "0x0059d383" referenced memory at "0x00000000". The memory cound not be "read". I do not see the problem, could please help me out ?

In Save.cpp

void CNESave::SaveLocation(CNELocation* pLoc)
// Other code

    CSceneNode* scene = pLoc->GetScene();
    vector<CSceneNode*> nodes;          
    scene->GetNodes(GetNodesByPartOfName, nodes, &string("_Ldynamic"));
    CSceneNode* pRoot = nodes[0];   //This is the problem causing random crashes!
        // If I change it (just for testing) to CSceneNode* pRoot = scene 
        // them it isn't crashing.
        // So CSceneNode* pRoot is OK and the problematic part is = nodes[0];

In SceneNode.cpp

bool GetNodesByPartOfName(CSceneNode* node, const void *data)
{
    string *str = (string *)data;
    return node->GetName().find(*str)!=npos;
}

void CSceneNode::GetNodes( GetNodesFunc gf, vector<CSceneNode*> &nodes, 
   const void *data)
{
    if (gf(this, data)) nodes.push_back(this);
    SceneNodeIterator begin=FirstChild(), end=LastChild();
    for (;begin!=end;begin++) ((CSceneNode*)*begin)->GetNodes(gf, nodes, data);
}

CSceneNode* CSceneNode::CreateNew() 
{
    return new CSceneNode();
}

// a lot of other code

In SceneNode.h

class EXPORTDECL CSceneNode;
typedef bool (*GetNodesFunc)(CSceneNode *node, const void *data);
EXPORTDECL bool GetNodesByPartOfName(CSceneNode* node, const void *data);
typedef vector<CSceneNode*>::iterator SceneNodeIterator;
class EXPORTDECL CSceneNode : public CTreeNode<CSceneNode, true>  
{
public:
//construction & destruction
    CSceneNode();
    virtual ~CSceneNode();
    virtual CSceneNode *CreateNew();
// a lot of other code

---

Solved. Thank you guys very much.

Answer 1

try using :

CSceneNode* pRoot = nodes.at(0);

If you get an exception, then you know something is wrong. Maybe your filter is too restrictive or you don't have child so nothing is returned.

You shouldn't use nodes[i] if you aren't 100% sure that the i index is valid.

Answer 2

If the result of node->GetName() does not contain the string _Ldynamic, this will crash.

Because in this case GetNodesByPartOfName(..) will return false and GetNodes(..) will not execute nodes.push_back(this) and you will be left with an empty vector and later will try to access the first element of this empty vector after returning from GetNodes(..).

Answer 3

What you want to do is make sure the vector isn't empty:

CSceneNode *const pRoot = nodes.empty() ? NULL : nodes[0];
if (pRoot != NULL)
{
    // Do something...
}