Default move constructor vs. Default copy constructor vs. Default assignment operator

Question

Why does C++ compiler have more restriction on automatically generated move constructors than on automatically generated copy constructor or assignment operator ?

Automa

Answer 1

When C++ was created, it was decided that default constructor, copy-constructor, assignment-operator and destructor would be generated automatically (unless provided). Why ? Because C++ compilers should be able to compile (most) C code with identical semantics, and that's how struct work in C.

However, it was later noticed that whenever a user writes a custom destructor, she probably needs to write a custom copy-constructor/assignment-operator too; this is known as the Rule of Big Three. With hindsight, we can see that it could have been specified that the generated copy-constructor/assignment-operator/destructor would have only been generated if none of the 3 were user-provided, and it would have helped catch a lot of bugs... and still retain backward compatibility with C.

Therefore, as C++11 came around, it was decided that this time things would be done right: the new move-constructor and move-assignment-operator would only be generated automatically if it was clear that the user was not doing anything "special" with the class. Anything "special" being defined as redefining move/copy/destruction behavior.

To help with the case were people would be doing something special but still wanted "automatically generated" special methods, the = default sugar-coating was added as well.

Unfortunately, for backward compatibility reasons, the C++ committee could not go back in time and change the rules of automatic generation for copy; I wish they had deprecated it to pave the way for the next version of the Standard, but I doubt they will. it is however deprecated (see §12.8/7 for the copy constructor for example, courtesy of @Nevin).