Rule of three (C++ programming)

The rule of three (also known as the Law of The Big Three or The Big Three) is a rule of thumb in C++ that claims that if a class defines one of the following it should probably explicitly define all three^[1]:

• destructor
• copy constructor
• 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 default semantics of doing the said operation on all the members of the class.

• Destructor - Destruct all the object's members
• Copy constructor - Construct all the object's members from the equivalent members in the copy constructor's parameter
• Assignment operator - Assign all the object's members from the equivalent members in the assignment operator's parameter

The Rule of Three claims 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.^[2]

An amendment to this rule is that if Resource Acquisition Is Initialization (RAII) is used for the class members, the destructor may be left undefined (also known as The Law of The Big Two^[3]).

Because implicitly-generated constructors and assignment operators simply copy all class data members,^[4] one should define explicit copy constructors and copy assignment operators for classes that encapsulate complex data structures or have external references such as pointers, since only the pointer gets copied, not the object it points to.

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