C++ object created with new, destroyed with free(); How bad is this?

Yes it does matter.

For memory obtained using new you must use delete.

For memory obtained using malloc you must use free.

new and malloc may use different data structures internally to keep track of what and where it has allocated memory. So in order to free memory, you have to call that corresponding function that knows about those data structures. It is however generally a bad idea to mix these two types of memory allocation in a piece of code.

If you call free(), the destructor doesn't get called.

Also, there's no guarantee that new and free operate on the same heap.

You can also override new and delete to operate specially on a particular class. If you do so, but call free() instead of the custom delete, then you miss whatever special behavior you had written into delete. (But you probably wouldn't be asking this question if you had done that, because you'd know what behaviors you were missing..)

Packet has a destructor, even if you haven't explicitly declared one. It has a default destructor. The default destructor probably doesn't actually do much, but you can't count on that being the case. It's up to the compiler what it does.

new and malloc also may have wildly different implementations. For example, delete is always called in a context where it has perfect information about the size of the data structure it's deleting at compile time. free does not have this luxury. It's possible that the allocator that new is using may not store the bytes at the beginning of the memory area stating how many bytes it occupies. This would lead free to do entirely the wrong thing and crash your program when freeing something allocated with new.

Personally, if getting people to do the right thing or fixing the code yourself is completely impossible, I would declare my own global operator new that called malloc so then free would definitely not crash, even though it would still not call the destructor and be generally really ugly.