The Destructor and the Assignment OperatorLecture 6
Section 6.3
Robb T. Koether
Hampden-Sydney College
Mon, Jan 25, 2010
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Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
Robb T. Koether (Hampden-Sydney College) The Destructor and the Assignment Operator Mon, Jan 25, 2010 2 / 30
Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
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The Destructor
The DestructorType::~Type(); // Prototype;
The destructor destroys an object, i.e., it deallocates the memoryused by the object.The destructor is not invoked explicitly.
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Purpose of the Destructor
The destructor is used to destroy an object when it passes out ofscope.A local variable passes out of scope when execution returns froma function.A variable declared within a block {} passes out of scope whenexecution leaves that block.A volatile object passes out of scope when the evaluation of theexpression in which it occurs is completed.
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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?
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Purposes of the Default Constructor
The Function operator*()Vectr operator*(double s, const Vectr& v){
return v.scalarMultiply(s);}
How many vectors get destroyed inside operator*()?
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Purposes of the Default Constructor
The Function scalarMultiply()Vectr Vectr::scalarMultiply(double s) const{
Vectr v(mSize);for (int i = 0; i < mSize; i++)
v.element[i] = s * element[i];return v;
}
How many vectors get destroyed inside scalarMultiply()?
Robb T. Koether (Hampden-Sydney College) The Destructor and the Assignment Operator Mon, Jan 25, 2010 8 / 30
Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
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The Automatic Destructor
The automatic destructorI Invokes each data member’s destructor.I Deallocates the memory used by the data members.
The automatic destructor does not deallocate memory that thedata members point to.In other words, if a data member is a pointer, then the automaticdestructor will probably create a memory leak.
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The Vectr Class
Example (The Vectr Class)The code.
I The header file vectr.h.I The implementation file vectr.cpp.I The test program Vectr Test.cpp.
The executable.
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Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
Robb T. Koether (Hampden-Sydney College) The Destructor and the Assignment Operator Mon, Jan 25, 2010 12 / 30
The this Pointer
Every (non-static) member function has a hidden parameternamed this.this is always the first parameter in such a function.this is a constant pointer to the object that invoked the memberfunction.
Type* const this
this provides us with a name for the invoking object.
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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);
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The this Pointer
Furthermore, when we write the prototype of a member functionas
Apparent PrototypeType::func(params) const;
the actual prototype is
Actual PrototypeType::func(Type const* const this, params);
In this case, this is a constant pointer to a constant object.
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Usage of the this Pointer
Inside a member function, we refer to a data member by its name,e.g. mSize.It is interpreted as this->mSize.
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Usage of the this Pointer
Inside a member function, we invoke another member function ofthe same class by the function’s name, e.g.,scalarMultiply(5).It is interpreted as this->scalarMultiply(5).
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Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
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The Assignment Operator
The Assignment OperatorType& Type::operator=(const Type&); // PrototypeObjectA = ObjectB; // Usage
The assignment operator assigns to an existing object the value ofanother existing object of the same type.The assignment operator must be a member function.It can be invoked only through the operator =.
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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;
}
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Form of the Function operator=()
The makeEmpty() and makeCopy() Functionsvoid makeEmpty();
void makeCopy(const Type& value);
makeEmpty() clears out the old value of the object.makeCopy() assigns the new value to the object.It is convenient write these two member functions and then usethem in the copy constructor, the destructor, and the assignmentoperator (and the input() function).
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The Copy Constructor
The Copy ConstructorType::Type(const Type& value){
makeCopy(value);return;
}
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The Destructor
The DestructorType::~Type(){
makeEmpty();return;
}
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The Assignment Operator
The Assignment OperatorType& Type::operator=(const Type& value){
if (this != &value){
makeEmpty();makeCopy(value);
}return *this;
}
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The input() Function
The input() Functionvoid Type::input(istream& in){
makeEmpty(); // Avoid memory leak// Read the object}
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Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
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The Automatic Assignment Operator
The automatic assignment operator uses each data member’sassignment operator to assign values to them from the otherobject.
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Multiple Assignments
The assignment operator is right-associative.The statement
a = b = c = d;
is equivalent toa = (b = (c = d));
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Multiple Assignments
What about the statements((a = b) = c) = d;
and(a = b) = (c = d);
Robb T. Koether (Hampden-Sydney College) The Destructor and the Assignment Operator Mon, Jan 25, 2010 29 / 30
Outline
1 The DestructorThe Automatic Destructor
2 The this Pointer
3 The Assignment OperatorThe Automatic Assignment Operator
3 Assignment
Robb T. Koether (Hampden-Sydney College) The Destructor and the Assignment Operator Mon, Jan 25, 2010 30 / 30
Assignment
HomeworkRead Section 6.3, pages 275 - 286.
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