Const and the Copy Constructors

1
Const and the Copy Constructors
2
Object Oriented Paradigm

• Separating interface from implementation
• Constructors destructors
• Public vs private
• Not C specific

3
Const specific to C

• const keyword
• Ensures initialized value is its only value ever
• Sometimes its a promise

4
Example
5
No call to Read/Write?

• No matching call to write(Point)?
• We have function write(Point)!
• But wait
• Errors are on lines that contain P.reflectX()
• Compiler sees write(Point) as candidate, but
  just wont let you pass the result of
  P.refelctX() as a reference

6
Read Problem

• read() tries to change what is returned by
  P.reflectX()
• P.reflectX() is rvalue
• rvalues can only by on the right side of
  assignment
• (opposite are lvalues)

7
But why an rvalue?

• Result of function call or expression evaluation
  is a temporary
• Object exists temporarily until expression in
  which it occurs is evaluated, then dies
• C does not let you modify temporaries
• Does not let you use them as lvalues

8
Our Errors

• Read() tries to modify result of function call, a
  temporary - Error
• Write doesn't try to modify temporary returned by
  the function call P.reflectX()
• Why is this a problem then?

9
Compiler Doesnt Know

• Doesnt know function wont modify temporary
• So tell itusing const
• Promise to compiler function wont change
  temporary

10
Quick Fix

• Const fixed everything

11
Why is write() a friend?

• Dont need write() as friend
• Can use public accessor functions

12
What is THIS error?

• In function void write(const Point )'
• passing const Point' as this' argument of
  double PointgetX()' discards qualifiers
• passing const Point' as this' argument of
  double PointgetY()' discards qualifiers
• First off this is a pointer to object whose
  member function was called
• Called made to A.foo()
• Inside foo() this points to A
• Called made to B.foo()
• Inside foo() this points to B

13
Errors
In function void write(const Point )' passing
const Point' as this' argument of double
PointgetX()' discards qualifiers passing
const Point' as this' argument of double
PointgetY()' discards qualifiers

• In write() we have variable P
• Type const Point
• Compiler complains calling Ps (a const Point )
  getX() discards qualifiers
• const is a qualifier
• Compiler has no guarantee getx() wont modify P

14
Not Discarding Qualifiers

• We know getX() doesnt modify P
• So let the compiler knowuse const
• Syntax
• Put const immediately after parenthesized
  argument list to member function
• double getX() const return x
• double getY() const return y

15
Object Creation
16
How Many Objects Created?

• Quick answer -- 7

Default constructor! Default constructor! (2,3)
(-1,5) Copy constructor! Copy constructor!
2-arg constructor! Point dies! Point dies!
Copy constructor! 2-arg constructor! Point
dies! Point dies! (0,4) Point dies! Point
dies! Point dies!
17
How Many Objects Created?

• Quick answer -- 7

18
Where It Happen?

• Points P Q created with default
• constructors when defined
• Two calls to midpoint each create
• point they return with call to
• 2-argument constructor
• Where do 3 extra points come from?
• All created with the copy constructor
• From pass-by-value calls to midpoint write
• All of this extra work can be avoided by using
  pass by reference (with const) instead!

19
Using Const (for optimization)
20
Output
Default constructor! Default constructor! (2,3)
(-1,5) 2-arg constructor! 2-arg constructor!
(0,4) Point dies! Point dies! Point dies!
Point dies!
21
Things Get Messy
how many components? 3 Enter 3 values 9.3 8.9
3.2 21.4 2.8771e-309 8.9 3.2
22
Why?

• Sum makes copy of V calls it A
• When V.val is copied to A.val
• A.val points to same array
• When sum() finished As
• destructor is called
• Deletes A.val
• But thats V.val too!

23
Two solutions

• Avoid pass-by-value, use pass-by-reference
  instead
• Define copy constructor that makes deep copy,
  including allocating new arrays

24
Make it Go Away!

• You can make it all go away by using
  pass-by-reference in conjunction with const
• Java, for example, doesnt even have
  pass-by-value for user defined types