item5.md

Item 5： Know what functions C++ silently writes and calls.

Compilers will declare their own versions of a copy constructor, a copy assignment operator and a destructor, and even the contructor if you don't declare them yourself. All of them will be both puiblic and inline. Thus, writing C++ class Empty{}; is equivalent to:

class Empty {
public: 
  Empty(){...}                             // default constructor
  Empty(const Empty& rhs) {...}            // copy constructor
  ~Empty(){...}                            // destructor
  
  Empty& operator=(const Empty& rhs) {...} //copy assignment operator
};

These functions are generated only when needed, but its pretty easy to cause each function to be generated:

Empty e1;        // default constructor;
                 // destructor

Empty e2(e1);    // copy constructor 
e2 = e1;         // copy assignment operator

• Default constructor and destructor: give compilers a place to put "behind the scenes" code such as invocation of constructors/destructors of base classes and non-static data members. The generated destructor is non-virtual unless it's for a class inheriting from a base class than itself declares a virtual destructor (in which case the function's virtualness comes from the base class).

• copy constructor & copy assignment operator: compiler-generated version of them simply copy each non-static data member of the source object over to the target object. Copy constructor is called when a new object is created from an existing object, as a copy of the existing object. Assignment operator is called when an already initialized object is assigned a new value from another existing object.

• • Note: when member variables have reference or const type, default generated copy assignment opeartor will not work. C++ does not provide a way to make a reference refer to a different object, and const member cannot be changed. Compilers also reject implicity copy assignment operators in derived classes that inherit from base classes declaring the copy assignment operator private (can't invoke member functions).

- Things to Remember

• Compilers may implicitly generate a class's default constructor, copy constructor, copy assignment opeartor, and destructor.