FAQ 30.06 What is the Law of the Big Three?

If a class needs any of the Big Three, it needs them all.

This doesn't mean that every class should have all three of the Big Three. On the contrary, the Big Three are needed only in a relatively small percentage of classes. That is one of the reasons this is such an insidious error. Programmers see these infrastructure routines in only some of their classes, so they don't remember the critical Law of the Big Three.

This law first appeared in 1991 in the comp.lang.c++ FAQ, and it seems to be rediscovered every six months or so. Violations almost always lead to incorrect behavior and often lead to disasters.

In particular, violations of the Law of the Big Three often corrupt the heap. This usually means that the program does not crash until much later in the program's execution (and simple test programs may not crash at all). By the time the programmer goes in with a debugger, the root cause is hard to identify and the heap has so many things wrong with it that it's difficult to trace what's going wrong.