Dynamic Data, Shallow Copies, and Deep Copies

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Dynamic Data, Shallow Copies, and Deep Copies
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Problem

• Consider the following program that
• creates a grade tree
• makes a copy of the tree to save
• modifies the original tree

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include stutree.h int main() Gradetree
mygrades // list of my grades
mygrades.Insert(90) // put some grades in my
list mygrades.Insert(86)
mygrades.Insert(97) Gradetree savemygrades
// save a copy of my grades at this point in
term savemygrades mygrades
mygrades.Insert(99) // add/delete more
grades mygrades.Delete(86)
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NULL
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
97
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
97
NULL
savemygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
97
savemygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
97
This is called a shallow copy Only the private
data is copied to the new class instance There
is really only one copy of the dynamic data!
savemygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
86
97
99
savemygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
97
99
savemygrades.root
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90
include stutree.h int main() Gradetree
mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
Gradetree savemygrades savemygrades
mygrades mygrades.Insert(99)
mygrades.Delete(86)
mygrades.root
97
99
savemygrades.root
Problem I really didnt get two copies of my
grades, so when I make changes to mygrades, I
really am changing both mygrades and savemygrades!
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Problem

• Consider the following program that
• creates a grade tree
• calls a function, passing the grade tree by value
• modifies the copy of the tree passed to the
  function
• returns to the main function

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include stutree.h void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades)
90
mygrades.root
86
97
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include stutree.h Void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades)
tempgrades.root
90
mygrades.root
86
97
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include stutree.h Void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades)
tempgrades.root
90
mygrades.root
86
97
99
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include stutree.h Void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades)
tempgrades.root
90
mygrades.root
97
99
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Problem Passing a class by value only gives a
shallow copy, so function actually modifies
mygrades!
include stutree.h Void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades)
tempgrades.root
90
mygrades.root
97
99
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include stutree.h Void GradeFunction
(Gradetree tempgrades) tempgrades.Insert(99)
tempgrades.Delete(86) int main()
Gradetree mygrades mygrades.Insert(90)
mygrades.Insert(86) mygrades.Insert(97)
GradeFunction(mygrades) // execution
continues here after function call
90
mygrades.root
97
99
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Solution

• We need deep copies in the two previous
  situations
• A deep copy is a copy of not only the private
  root pointer, but also a copy of all of the
  dynamic data, i.e., a copy of all nodes in the
  entire tree
• Can you think of other classes we have used that
  needed deep copies?
• any class using linked lists (files gradelnklist,
  stackll, queuell)

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Solution (continued)

• Add a function to the Gradetree class that makes
  a deep copy when the assignment operator is used
• This is a function that overloads the assignment
  operator, i.e., the sign.
• I actually already included this function in the
  Gradetree class

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Solution (continued)

• Add a function to the Gradetree class that makes
  a deep copy when the class is passed by value to
  a function
• This is a function called a copy constructor.
• The copy constructor is called when a class is
  passed by value.
• I actually already included this function in the
  Gradetree class

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stutree.h
class Gradetree public // default
constructor Gradetree () //
copy constructor - for deep copies
Gradetree (const Gradetree othergradetree)
// destructor Gradetree ()
// overload assignment for deep copies
void operator (const Gradetree
othergradetree) .
.
. private Treenode copytree
(Treenode t) // used by copy constructor and
operator void destroytree (Treenode t)
// used by destructor
. .
. Treenode root
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stutree.cpp
// copy constructor - for deep copies GradetreeG
radetree (const Gradetree othergradetree)
root copytree (othergradetree.root)
// overload assignment for deep copies void
Gradetreeoperator (const Gradetree
othergradetree) root copytree
(othergradetree.root)
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stutree.cpp
// used by copy constructor and
operator Treenode Gradetreecopytree
(Treenode t) Treenode tmp NULL // for
building copy of t if (t ! NULL)
tmp new Treenode tmp-gtgrade
t-gtgrade tmp-gtcount t-gtcount
tmp-gtleft copytree (t-gtleft)
tmp-gtright copytree (t-gtright)
return tmp
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For more information

• see section 6.5 in text pp. 360 - 370