The Destructor and the Assignment Operator Lecture 8 Thu, Feb 8, 2007

Four Fundamental Member Functions The Destructor The destructor destroys an object, i.e., it deallocates the memory used by the object. Prototype: Usage: The destructor is not invoked directly. Type::~Type(); 2/22/2019 Four Fundamental Member Functions

Example: The Vector Class vectr.h vectr.cpp VectrTest.cpp 2/22/2019 Four Fundamental Member Functions

Purpose of the Destructor The destructor is used to destroy an object when it passes out of scope. A local variable passes out of scope when execution returns from a function. A variable declared within a block {} passes out of scope when execution leaves that block. A volatile object passes out of scope when the evaluation of the expression in which it occurs is completed. 2/22/2019 Four Fundamental Member Functions

The Automatic Destructor Invokes each data member’s destructor. Deallocates the memory used by the data members. The automatic destructor does not deallocate memory that the data members point to. 2/22/2019 Four Fundamental Member Functions

Four Fundamental Member Functions The this Pointer Every (non-static) member function has a hidden parameter named this. this is always the first parameter in such a function. this is a constant pointer to the object (Type* const this) that invoked the member function. this provides us with a name for the invoking object. 2/22/2019 Four Fundamental Member Functions

Four Fundamental Member Functions The this Pointer When we write the prototype of a member function as the actual prototype is this is a constant pointer to an object. Type::Function(Parmeters); Type::Function(Type* const this, Parmeters); 2/22/2019 Four Fundamental Member Functions

Four Fundamental Member Functions The this Pointer Furthermore, when we write the prototype of a member function as the actual prototype is this is a constant pointer to a constant object. Type::Function(…) const; Type::Function(Type const* const this, …); 2/22/2019 Four Fundamental Member Functions

Usage of the this Pointer Inside a member function, we refer to a data member as This is interpreted as member-name this->member-name 2/22/2019 Four Fundamental Member Functions

Usage of the this Pointer Inside a member function, we invoke another member function by writing This is interpreted as function(parameters); this->function(parameters); 2/22/2019 Four Fundamental Member Functions

The Assignment Operator The assignment operator assigns to an existing object the value of another existing object of the same type. Prototype: Usage: The assignment operator must be a member function. Type& Type::operator=(const Type&); ObjectA = ObjectB; 2/22/2019 Four Fundamental Member Functions

Purpose of the Assignment Operator The assignment operator is use to assign the value of one object to another object in an assignment statement. 2/22/2019 Four Fundamental Member Functions

Form of the Function operator=() Type& Type::operator=(const Type& value) { if (this != &value) // Clear out the old value // Assign the new value } return *this; 2/22/2019 Four Fundamental Member Functions

Form of the Function operator=() Typically we write two member functions makeEmpty() makeCopy() Type& Type::operator=(const Type& value) { if (this != &value) makeEmpty(); // Clear out old value makeCopy(value); // Copy new value } return *this; 2/22/2019 Four Fundamental Member Functions

The Copy Constructor and the Destructor Then we can also write the copy constructor and destructor more easily. Type::Type(const Type& x) { makeCopy(x); return; } Type::~Type() makeEmpty(); 2/22/2019 Four Fundamental Member Functions

The Automatic Assignment Operator Uses each data member’s assignment operator to assign values to them from the other object. 2/22/2019 Four Fundamental Member Functions

Four Fundamental Member Functions Multiple Assignments The assignment operator is right-associative. The statement is equivalent to a = b = c; a = (b = c); 2/22/2019 Four Fundamental Member Functions

Four Fundamental Member Functions Multiple Assignments What about the statement (a = b) = c; 2/22/2019 Four Fundamental Member Functions