TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Destructor and the AssignmentOperatorLecture 7

Section 6.3

Robb T. Koether

Hampden-Sydney College

Fri, Jan 30, 2009

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Outline

1 The Destructor

2 The this Pointer

3 The Assignment Operator

4 Assignment

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Destructor

The DestructorType::~Type(); // Prototype;

The destructor destroys an object, i.e., it deallocatesthe memory used by the object.The destructor is not invoked explicitly.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Purpose of the Destructor

The destructor is used to destroy an object when itpasses out of scope.A local variable passes out of scope when executionreturns from a function.A variable declared within a block {} passes out ofscope when execution leaves that block.A volatile object passes out of scope when theevaluation of the expression in which it occurs iscompleted.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Purposes of the Default Constructor

The Destructorint main(){

Vectr v(5, 123);{

Vectr u = 5*v;}return 0;

}

How many vectors get destroyed and exactly when?

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Automatic Destructor

The automatic destructorInvokes each data member’s destructor.Deallocates the memory used by the data members.

The automatic destructor does not deallocate memorythat the data members point to.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The this Pointer

Every (non-static) member function has a hiddenparameter 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.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The this Pointer

When we write the prototype of a member function as

Apparent PrototypeType::func(params);

the actual prototype is

Actual PrototypeType::func(Type* const this, params);

this is a constant pointer to an object.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The this Pointer

Furthermore, when we write the prototype of a memberfunction as

Apparent PrototypeType::func(params) const;

the actual prototype is

Actual PrototypeType::func(Type const* const this, params);

this is a constant pointer to a constant object.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Usage of the this Pointer

Inside a member function, we refer to a data memberas member.This is interpreted as this->member.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Usage of the this Pointer

Inside a member function, we invoke another memberfunction of the same class by writing func(params).This is interpreted as this->func(params).

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Assignment Operator

The Assignment OperatorType& Type::operator=(const Type&); // ProtoObjectA = ObjectB; // Usage

The assignment operator assigns to an existing objectthe value of another existing object of the same type.The assignment operator must be a member function.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Purpose of the Assignment Operator

The assignment operator is use to assign the value ofone object to another object in an assignmentstatement.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Form of the Function operator=()

The Assignment OperatorType& Type::operator=(const Type& value){

if (this != &value){// Clear out the old value// Assign the new value}return *this;

}

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Form of the Function operator=()

The makeEmpty() and makeCopy() Functionsvoid makeEmpty();Clears out the old value of the object.void makeCopy(const Type& value);Assign the new value to the object.

It is convenient write these two member functions andthen use them in the copy constructor, the destructor,and the assignment operator.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Copy Constructor

The Copy ConstructorType::Type(const Type& value){

makeCopy(value);return;

}

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Destructor

The DestructorType::~Type(){

makeEmpty();return;

}

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Assignment Operator

The Assignment OperatorType& Type::operator=(const Type& value){

if (this != &value){

makeEmpty();makeCopy(value);

}return *this;

}

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

The Automatic Assignment Operator

The automatic assignment operator uses each datamember’s assignment operator to assign values tothem from the other object.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Multiple Assignments

The assignment operator is right-associative.The statement

a = b = c = d;

is equivalent toa = (b = (c = d));

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Multiple Assignments

What about the statements((a = b) = c) = d;

and(a = b) = (c = d);

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Example

Example (The Vectr Class)Download vectr.h.Download vectr.cpp.Download and run VectrTest.cpp.

TheDestructor

and theAssignment

Operator

Robb T.Koether

TheDestructor

The thisPointer

TheAssignmentOperator

Assignment

Assignment

HomeworkRead Section 6.3, pages 275 - 286.