c++ - What is the Rule of Four (and a half)? -


for handling object copying, rule of thumb rule of three. c++11, move semantics thing, instead it's rule of five. however, in discussions around here , on internet, i've seen references rule of 4 (and half), combination of rule of 5 , copy-and-swap idiom.

so rule of 4 (and half)? functions need implemented, , should each function's body like? function half? there disadvantages or warnings approach, compared rule of five?

here's reference implementation resembles current code. if incorrect, correct implementation like?

//i understand in example, use `std::unique_ptr`. //just assume it's more complex resource. #include <utility>  class foo { public:     //we must have default constructor can swap during copy construction.     //it need not useful, should swappable , deconstructable.     //it can private, if it's not valid state object.     foo() : resource(nullptr) {}      //normal constructor, acquire resource     foo(int value) : resource(new int(value)) {}      //copy constructor     foo(foo const& other) {         //copy resource here.         resource = new int(*other.resource);     }      //move constructor     //delegates default constructor put in safe state.     foo(foo&& other) : foo() {         swap(other);     }      //assignment     foo& operator=(foo other) {         swap(other);         return *this;     }      //destructor     ~foo() {         //free resource here.         //we must handle default state can appear copy ctor.         //(the if not technically needed here. `delete nullptr` safe.)         if (resource != nullptr) delete resource;     }      //swap     void swap(foo& other) {         using std::swap;          //swap resource between instances here.         swap(resource, other.resource);     }      //swap adl     friend void swap(foo& left, foo& right) {         left.swap(right);     }  private:     int* resource; };

so rule of 4 (and half)?

“the rule of big 4 (and half)" states if implement 1 of

  • the copy constructor
  • the assignment operator
  • the move constructor
  • the destructor
  • the swap function

then must have policy others.

which functions need implemented, , should each function's body like?

  • default constructor (which private)
  • copy constructor (here have real code handle resource)

  • move constructor (using default constructor , swap) :

    s(s&& s) : s{} { swap(*this, s); }

  • assignment operator (using constructor , swap)

    s& operator=(s s) { swap(*this, s); }

  • destructor (deep copy of resource)

  • friend swap (doesn't have default implementation :/ should want swap each member). 1 important contrary swap member method: std::swap uses move (or copy) constructor, lead infinite recursion.

which function half?

from previous article:

"to implement copy-swap idiom resource management class must implement swap() function perform member-by-member swap (there’s “…(and half)”)"

so swap method.

are there disadvantages or warnings approach, compared rule of five?

the warning wrote write correct swap avoid infinite recursion.


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