Rule of 3/5/0/6

There are rules, fairly firm guidelines, regarding what we do with the default special member functions.

• http://en.cppreference.com/w/cpp/language/rule_of_three

• https://en.wikipedia.org/wiki/Rule_of_three_(C%2B%2B_programming)

The numbers are three, five, and zero, because the default constructor is often treated differently but not always ...

... so the rule of six is mentioned sometimes too:

• https://accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf

The rule of three ...

The rule of three says if you define one of destructor, assignment operator, or copy operator, you should define all three.

• Why?

• If you need to define one it’s unlikely everything will be consistent with the others by default.

See

http://en.cppreference.com/w/cpp/language/rule_of_three

The rule of five (or seven)

The rule of five is the C++11 extension of the rule of three to take into account the need to typically define the move constructor and move assignment too.

• These are sometimes referred to as copy control and, if we don’t provide them, the compiler does.

• Why seven? Include a non-default constructor and the default constructor.

• If you didn’t need moving for whatever reason the rule of three applies.

• The rule of zero is for the case where you don’t need copying or moving.