This morning, in Visual Studio 2005, I tried adding a new private member variable to a class and found that it was giving me all sorts of weird segmentation faults and the like. When I went into debug mode, I found that my debugger didn’t even see the new member variable, and thus it was giving me some strange behavior.
It required a “rebuild all” in order to get my program working again (and to get the debugger to see the new member variables I had made). Why was it necessary to rebuild all? Why was just doing a regular build insufficient?
I already solved the problem, but I feel like I understanding the build process better will help me in the future. Let me know if there’s any more information you need.
Thanks in advance!
When you add or remove members of a class you change the memory layout of the object. If you don’t recompile you are breaking the ODR rule, and the segmentation faults are just the effect of that.
As to why that happens, old code might be acquiring memory for the old size, and then passing that object (without the new member) to new code that will access beyond the end of the allocated memory to access the new variable. Note that the access specifier does not affect at all, if it is private it will probably be the class member functions the ones accessing the fields.
If you did not add the field to the end, but rather to the middle of the object, the same effect will be seen while accessing those fields that are laid out by the compiler in the higher memory addresses.
The fact that you needed to use the rebuild all feature is an indication that the dependencies of your project are not correctly configured, and you should fix that as soon as possible. Having the right dependencies will force the compiler into rebuilding when needed, and will mean less useless debugging hours.