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

2. 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();
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Four Fundamental Member Functions

3. Example: The Vector Class
vectr.h
vectr.cpp
VectrTest.cpp
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Four Fundamental Member Functions

4. 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.
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5. 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.
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6. 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.
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Four Fundamental Member Functions

7. 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);
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Four Fundamental Member Functions

8. 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, …);
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9. 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
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10. 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);
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11. 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;
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12. Purpose of the Assignment Operator
The assignment operator is use to assign the value of one object to another object in an assignment statement.
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13. 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;
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14. 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;
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15. 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();
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16. The Automatic Assignment Operator
Uses each data member’s assignment operator to assign values to them from the other object.
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17. Four Fundamental Member Functions
Multiple Assignments
The assignment operator is right-associative.
The statement
is equivalent to
a = b = c;
a = (b = c);
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18. Four Fundamental Member Functions
Multiple Assignments
What about the statement
(a = b) = c;
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