Thread: push_back(), assertion fails, MSVC

I'm rather stumped. Using MSVC.NET, my code compiles fine, but when I run it, I get a "Debug Assertion Failed! .... File: dgbdel.cpp Line: 52 Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)" dialog box when it tries to push_back(cSwitch). There's too much code to post everything, so I will attempt to post what I think is relevant.

Code:

class cPort
{
	private:
		int connectionType;
		int connectionIndex;
	public:
		BOOL connectedToSwitch();
		BOOL setConnection(int index, int objectType);
		int index();
		cPort();
};

class cSwitch
{
	private:
		cNetwork *network;
		int indexOfThis;
	protected:
		void afunction();
	public:
		void anotherfunction();
		cPort *port;
		friend class cSwitch;
		cSwitch(int index,cNetwork *networkPointer);
		~cSwitch();
};

class cNetwork
{
	public:
		vector<cSwitch> switches;
		int AddSwitch();
		friend class cSwitch;
};

I think the implementation details of the classes shouldn't matter for this. When running this code,

Code:

cNetwork networkA;  //works fine
cSwitch temp(networkA.switches.size(),&networkA); //works fine
networkA.switches.push_back(temp);

It makes it past the two lines marked //works fine, but gives the error I listed above on the third line. Changing the code to

Code:

vector <cSwitch> networkAswitches;
cSwitch temp(networkA.switches.size(),&networkA);
networkAswitches.push_back(temp);

had no effect; it still couldn't "push_back". When I break, I'm taken to the line

Code:

  _ASSERTE(_BLOCK_TYPE_IS_VALID(pHead->nBlockUse));

in dbgdel.cpp (in function void operator delete( void *pUserData)), and stepping out, it appears that it was called by

Code:

_CRTIMP int __cdecl _CrtIsValidHeapPointer(
        const void * pUserData
        )

in dbgheap.c. Clearly, I did not write these two functions (and I can not step out any further). Have I done something wrong, or is this a strange bug in MSVC? Thank you.

Try adding a copy constructor to your cSwitch class. I'm guessing something is double-deleting one of those pointers.

[edit] Also, notice in this piece of code that you're using a pointer to a local variable. I hope you're not saving it in the private member network, since that variable will be out of scope as soon as the function is over.

Code:

cNetwork networkA;  //works fine
cSwitch temp(networkA.switches.size(),&networkA); //works fine
networkA.switches.push_back(temp);

Those functions in dbgdel.cpp are memory management functions, for allocation/deallocation of memory. You're getting that assertion error, but probably when running a release version of your program, that means a segmentation fault (or general protection fault in windows). Check if the pointers you keep are being well managed, and not deleted before any usage.

To expand a little: AFAIK, the error you're getting is coming from MSVC's automatic memory checking code that's inserted when compiling in debug mode. You'll come across a similar error whenever you write over memory that isn't yours; as has been mentioned, this can be caused by stuff like double deletion, array index out of bounds, etc.

In fact, this is the reason that I try to avoid placing concrete objects in STL containers; too often, you'll end up with problems where a class requires a copy constructor performing a deep copy (kinda annoying), or worse, requires the addition of reference counting (very annoying, easy to bug up). As a result, I tend to store pointers instead of objects, or even better, reference-counting smart pointers (I use a smart-pointer class I wrote some time ago, otherwise Boost is probably a good option), and dynamically allocate the objects.

Originally Posted by pianorain

Try adding a copy constructor to your cSwitch class. I'm guessing something is double-deleting one of those pointers.

[edit] Also, notice in this piece of code that you're using a pointer to a local variable. I hope you're not saving it in the private member network, since that variable will be out of scope as soon as the function is over.

The copy constructor did the trick - thanks! I am indeed saving that pointer in network, but the cSwitch class is only used within the cNetwork class (normally ... I know I used it outside in the last little bit of code in my post), so when cNetwork goes out of scope, cSwitch is going away too, so it should be okay.

Originally Posted by sean_mackrory

confuted

*gasp* Could it be?

It seems highly probable, doesn't it?