Question

I have a relatively large class that I’m working with and it’s all worked fine so far (note: I didn’t actually write the class, I’m just adding some functionality). However, after declaring one more string in the header file, everything now crashes (I get a memory access error). If I erase that string and rebuild, everything works fine.

I’m not actually doing ANYTHING with that string….just the act of declaring it is causing some weird memory error.

I can’t explain in much more detail than this, since it would be a waste to try to explain every function. What kind of things should I look for here to find the problem? What might cause this weird behavior?

The error itself is:

Unhandled exception at 0x65fd17fd (msvcp80d.dll) in myFile.exe: 0xC0000005: Access violation writing location 0xcdcdcdcd.

Basically all that changed in the .h file was:

StringType string1;

Turned into:

StringType string1;
StringType string2;

StringType is an extension of basic_string

Answer 1

You’ve allocated some memory on the heap and failed to initialize it.

0xcd is a fill pattern used by the debug heap: before dynamically allocated memory is given to your program, it is filled with that pattern to help you find uninitialized variables.

As for why changing the class definition affects the outcome, you may be doing incorrect pointer arithmetic, accessing something beyond the end of a dynamically allocated object, or one of any number of other things that no longer manifests as a bug when you have a larger object. You could also be violating the one-definition rule if some of the source was built using the old definition and some of the source is built with the new definition.

There are many things that can cause this kind of problem: your best bet is to break in the debugger when it happens, and trace backwards to see where the error originated (sometimes this can be lots of fun; I had to trace an uninitialized variable across a network connection once).