Rule of three (C++ programming)

Rule of three

The rule of three (also known as the law of the big three or the big three) is a rule of thumb in C++ (prior to C++11) that claims that if a class defines any of the following then it should probably explicitly define all three:{{cite book | url-access = limited

• destructor
• copy constructor
• copy assignment operator

These three functions are special member functions. If one of these functions is used without first being declared by the programmer it will be implicitly implemented by the compiler with the following default semantics:

• Destructor – call the destructors of all the object's class-type members
• Copy constructor – construct all the object's members from the corresponding members of the copy constructor's argument, calling the copy constructors of the object's class-type members, and doing a plain assignment of all non-class type (e.g., int or pointer) data members
• Copy assignment operator – assign all the object's members from the corresponding members of the assignment operator's argument, calling the copy assignment operators of the object's class-type members, and doing a plain assignment of all non-class type (e.g. int or pointer) data members.

The rule of three states that if one of these had to be defined by the programmer, it means that the compiler-generated version does not fit the needs of the class in one case and it will probably not fit in the other cases either. The term "Rule of three" was coined by Marshall Cline in 1991.

An amendment to this rule is that if the class is designed in such a way that resource acquisition is initialization (RAII) is used for all its (nontrivial) members, the destructor may be left undefined (also known as The Law of The Big Two{{cite web

Because implicitly-generated constructors and assignment operators simply copy all class data members ("shallow copy"),{{cite book

Rule of five

With the advent of C++11 the rule of three can be broadened to the rule of five (also known as "the rule of the big five") as C++11 implements move semantics,{{cite web

• destructor
• copy constructor
• copy assignment operator
• move constructor
• move assignment operator Situations exist where classes may need destructors, but cannot sensibly implement copy and move constructors and copy and move assignment operators. This happens, for example, when the base class does not support these latter Big Four members, but the derived class's constructor allocates memory for its own use. In C++11, this can be simplified by explicitly specifying the five members as default.

References

References

1. Koenig, Andrew. (2001-06-01). "C++ Made Easier: The Rule of Three".
2. "C++11: The Rule of the Big Five".
3. "The rule of three/five/zero". cppreference.com.