Templates

1
Templates

• Generic Programming
• ECE 297

2
Templates

• A way to write code once that works for many
  different types of variables
• float, int, char, string, List, Vector,
• Reference Problem Solving in C by Savitch,
  Chapter 17

3
Template Functions
4
Types Functions

• All parameters and local variables in a function
  must have a type
• void swap_val (int a, int b)
• int temp
• temp a
• a b
• b temp
• void swap_val (double a, double b)
• double temp
• temp a
• a b
• b temp

OK. Function overloading
5
Function Templates

• Allow you to write abstract algorithms
• Work for many types
• templateltclass VariableTypegt
• void swap_val (VariableType a, VariableType b)
• VariableType temp
• temp a
• a b
• b temp

template prefix defines a type parameter
Can use that type parameter instead of a
regular type In the function
6
Using a Function Template

• templateltclass VariableTypegt
• void swap_val (VariableType a, VariableType b)
• VariableType temp
• temp a
• a b
• b temp
• int main ()
• int i1 5, i2 10
• swap_val (i1, i2)
• // Compiler will create swap_valltintgt
• float f1 1e6, f2 2e6
• swap_val (f1, f2)
• // Compiler will create swap_valltfloatgt

Template definition must come before template use
7
Using a Function Template
swap_val.h
templateltclass VariableTypegt void swap_val
(VariableType a, VariableType b)
VariableType temp temp a a b b
temp
main.cpp

• include swap_val.h
• int main ()
• int i1 5, i2 10
• swap_val (i1, i2) // swap_valltintgt created
  
• float f1 1e6, f2 2e6
• swap_val (f1, f2) // swap_valltfloatgt
  created

8
Restrictions on Swap
templateltclass VariableTypegt void swap_val
(VariableType a, VariableType b)
VariableType temp temp a a b b
temp
?

• swap_val (int, int)
• swap_val (char, char)
• swap_val (List, List)
• swap_val (int, float)
• Works for any one type that has proper (deep
  copy) operator

?
?
?
9
Generic Minimum

• templateltclass VariableTypegt
• VariableType minimum (VariableType a,
  VariableType b)
• if (a lt b)
• return (a)
• else
• return (b)
• minimum (int, int)
• minimum (float, float)
• minimum (int20, int20)
• minimum (List, List)
• minimum (string, string)
• Works for any type that defines operatorlt
  (reasonably)

?
?
?
?
?
10
Generic Quicksort

• void QuickSort (int A, int left, int right)
• int PivotIndex SelectAndShuffle (A, left,
  right)
• if (PivotIndex gt left) //
  Basis
• QuickSort (A, left, PivotIndex-1) //
  Recursion
• if (PivotIndex lt right) //
  Basis
• QuickSort (A, PivotIndex1, right) //
  Recursion
• templateltclass Tbasegt
• void QuickSort (Tbase A, int left, int right)
• int PivotIndex SelectAndShuffle (A, left,
  right)
• if (PivotIndex gt left) //
  Basis
• QuickSort (A, left, PivotIndex-1) //
  Recursion
• if (PivotIndex lt right) //
  Basis
• QuickSort (A, PivotIndex1, right) //
  Recursion

11
Generic Quicksort

• int SelectAndShuffle (int A, int left, int
  right)
• int i, PivotIndex
• PivotIndex left
• for (i left1 i lt right i)
• if (Ai lt Aleft)
• PivotIndex PivotIndex 1
• swap_val (Ai, APivotIndex)
• swap_val (Aleft, APivotIndex)
• return (PivotIndex)

12
Generic Quicksort

• templateltclass Tbasegt
• int SelectAndShuffle (Tbase A, int left, int
  right)
• int i, PivotIndex
• PivotIndex left
• for (i left1 i lt right i)
• if (Ai lt Aleft)
• PivotIndex PivotIndex 1
• swap_val (Ai, APivotIndex)
• swap_val (Aleft, APivotIndex)
• return (PivotIndex)

13
Template Classes
14
Template Classes

• Can make templated (type-generic) classes
• Member functions
• Member data
• Most useful for container classes
• Classes that implement basic data structures
• List, Vector, Tree (Map),

15
Example Pair of Data

• Often useful to store two pieces of data together
• key, value in ECE 244 lab5 (binary tree for DNS)
  used together
• string name, int ip (244 lab 5)
• Could want string name, string data
• or int key, float data
• Make a class that can store two arbitrary pieces
  of data
• first (any type)
• second (any type)

16
Generic Pair Class

• templateltclass Tfirst, class Tsecondgt
• class MyPair
• public
• MyPair (Tfirst _first, Tsecond _second)
• void setFirst (Tfirst _first)
• void setSecond (Tsecond _second)
• Tfirst getFirst ()
• Tsecond getSecond ()
• private
• Tfirst first
• Tsecond second

In MyPair.h
17
Generic Pair Class

• include MyPair.h
• int main ()
• MyPairltint,floatgt pair1(1, 2.5)
• cout ltlt pair1.getFirst() ltlt ltlt
  pair1.getSecond()
• ...

In main.cpp
Creates a variable named pair1 class (type) is
MyPairltint, floatgt
pair1
int first 1float second 2.5
18
Generic Pair Class

• include MyPair.h
• int main ()
• MyPairltint,floatgt pair1(1, 2.5)
• cout ltlt pair1.getFirst() ltlt ltlt
  pair1.getSecond()
• MyPairltstring,doublegt tscore ("Your test
  score", 7.5)
• cout ltlt tscore.getFirst() ltlt " "
  ltlt tscore.getSecond() ltlt endl
• tscore.setFirst ("Revised test score")
• tscore.setSecond (9.5)
• cout ltlt tscore.getFirst() ltlt " " ltlt
  tscore.getSecond() ltlt endl

In main.cpp
Creates a variable named tscore class (type) is
MyPairltstring, doublegt
tscore
string first Your test scoredouble second
7.5
19
Generic Pair Class

• include MyPair.h
• int main ()
• MyPairltint,floatgt pair1(1, 2.5)
• cout ltlt pair1.getFirst() ltlt ltlt
  pair1.getSecond()
• MyPairltstring,doublegt tscore ("Your test
  score", 7.5)
• cout ltlt tscore.getFirst() ltlt " "
  ltlt tscore.getSecond() ltlt endl
• tscore.setFirst ("Revised test score")
• tscore.setSecond (9.5)
• cout ltlt tscore.getFirst() ltlt " " ltlt
  tscore.getSecond() ltlt endl

In main.cpp
Program output
1 2.5 Your test score 7.5 Revised test score 9.5
20
Generic Pair Class
Template prefix tells compiler this is a
templated function
In MyPair.h

• templateltclass Tfirst, class Tsecondgt
• MyPairltTfirst,TsecondgtMyPair (Tfirst _first,
• Tsecond _second)
  
• first _first
• second _second
• templateltclass Tfirst, class Tsecondgt
• void MyPairltTfirst,TsecondgtsetFirst (Tfirst
  _first)
• first _first
• // Rest of member function defintitions

Class name is MyPairltTfirst, Tsecondgt
21
Function Definitions Where?
In MyPair.h
templateltclass Tfirst, class Tsecondgt MyPairltTfirs
t,TsecondgtMyPair (Tfirst _first,
Tsecond _second) first
_first second _second ... // Rest of
class member function definitions
include MyPair.h int main ()
MyPairltint,floatgt pair1(1, 2.5)
In main.cpp

• Usually bad style to define all member functions
  in header file
• But with templates, compiler must be able to
  generate class as it sees the specific type to
  build
• ? Needs to see function definitions as it
  compiles class instantiations
• E.g. MyPairltint, floatgt in main.cpp
• Need to generate instructions (.o) for MyPair,
  with a Tfirst int, and Tsecond float
• Easiest to achieve by putting whole class
  implementation in header file

22
Writing Templates

• Write code for a specific type first
• Debug, test
• Convert to template
• Can get complex / subtle syntax errors

23
STL Generic Algorithms

• Using the Standard Template Libraries

24
STL

• Writing templates ? fairly hard
• Using templates easy productive
• Standard Template Library
• Fast, well-tested implementations of useful
  algorithms and data structures

25
STL References
Basic Reference Problem Solving in C by
Savitch, Chapter 18
Detailed reference cplusplus.com
Really nitty gritty details C Primer by
Lippman, Lajoie and Moo
26
STL

• Writing templates ? fairly hard
• Using templates easy productive
• Standard Template Library
• Fast, well-tested implementations of useful
  algorithms and data structures

27
STL Generic Algorithms
Useful algorithms that work with many types in
ltalgorithmgt
templateltclass Tgt T max (T a, T b) if (a lt
b) return (b) else return (a)
ltalgorithmgt
include ltalgorithmgt using namespace std int
main ( ) double x 1.1, y 1.2 double
z max (x, y) int j 2, k 3 int i
max (j, k) z max (i, x) z max
(x, 2) z max (x, 2.0)
main.cpp
// OK ? max (int, int)
// Wont compile
// Wont compile
// OK! max (double, double)
28
STL Generic Algorithms

• Also in ltalgorithmgt min (), sort ()
• Work for any one type that defines lt and
  assignment (operator)
• and lots more see cplusplus.com
• But I only use the very basic ones

29
STL Container Classes
30
STL Container Classes

• Most useful part of STL
• Data structures that just contain some other
  data type / class
• E.g. vector, linked list, binary tree of some
  type
• Can use to store any type of data (templated)
• Avoids a lot of repetitive coding of linked
  lists, binary trees, etc.

31
Vector

• Example motivation
• Want to read integers from cin to an array until
  EOF
• gt 1 -20 3 31 55 ltCtrlDgt
• Then pass the array on to the rest of the program
  to process, print, examine,
• Problem how big an array should we allocate?
• int array new int??
• Dont know until after the input is read!
• Could code up a linked list, load it, count
  elements, then allocate array, copy data from
  linked list, delete linked list

32
Vector

• Wouldnt it be great to have something just like
  an array that could grow as needed?

include ltvectorgt include ltiostreamgt using
namespace std vectorltintgt get_input ( )
vectorltintgt vec // vector of ints,
initially empty int val cin gtgt val
while ( !cin.fail() ) vec.push_back
(val) // Add new value to end of vector
cin gtgt val return (vec) //
Return the whole vector
main.cpp
33
Vector Can Use Like Array
include ltvectorgt include ltiostreamgt using
namespace std . . . int main ()
vectorltintgt in_vals in_vals get_input ()
for (int i 0 i lt in_vals.size() i)
cout ltlt in_valsi ltlt cout ltlt endl
for (int i in_vals.size() 1 i gt 0 i--)
cout ltlt in_valsi ltlt
main.cpp
vectors know how many elements they contain.
Valid data from index 0 to size()-1
Fast, O(1), random access to any entry
How would I print out the vector in reverse order?
Input 1 -20 3 31 55 ltCtrlDgt Output 1 -20 3 31
55 55 31 3 -20 1
34
Slightly Cleaner Version
int main () vectorltintgt in_vals in_vals
get_input () for (int i 0 i lt
in_vals.size() i) cout ltlt in_valsi ltlt

Compiler will give a type mismatch warning
(.size() is unsigned int). Harmless, but I prefer
to have no warnings.
Will make warning go away
for (int i 0 i lt (int) in_vals.size() i)
cout ltlt in_valsi ltlt
i now unsigned int ? watch out for gt 0 tests
C purists way
for (vectorltintgtsize_type i 0 i lt
in_vals.size() i) cout ltlt in_valsi ltlt

35
How Does It Work?
vectorltintgt get_input ( ) vectorltintgt vec
// vector of ints, initially empty int
val cin gtgt val while ( !cin.fail() )
vec.push_back (val) // Add new value to end
of vector cin gtgt val return (vec)
// Return the whole vector
1
vec
1
-20
int array int size int capacity
1
2
3
4
5
0
2
1
-20
4
8
1
3
31
1
-20
3
31
55
Input
1
-20
3
31
55
36
Vector Key Properties

• Efficient to add elements at end (push_back)
• Because when there isnt enough space it grows
  the storage quite a bit (usually 2x)
• Means even for a large number N of push_back(),
  we get only a few array copies
• O(1) on average
• Efficient random access to data
• Because internally the vector stores the data in
  an array ? operator can be fast
• O(1)

37
Handy Constructors
include ltvectorgt using namespace std int main
() vectorltintgt vec1 // default
constructor size 0 vectorltintgt vec2(10) //
size 10, but values unknown vectorltintgt
vec3(10,-1) // size 10, all values -1
38
Assignment Destruction
include ltvectorgt using namespace std int main
() int a new int10 int b new
int10 vectorltintgt v1(10), v2(10) a0
2 v10 2 . . . a b // OK?
v1 v2 // OK? delete a delete
b delete v1 // Should I?
No! Shallow copy
Yes! Proper deep copy defined in ltvectorgt
No! ltvectorgt defines a destructor cleans up its
internal array
39
operator
include ltvectorgt using namespace std int main
() vectorltintgt v1 v1.push_back(-3)
v10 -2 // OK? v11 -2 // OK?
v1.push_back(-3) v11 -2 // OK?
Yes, entry 0 exists
No, cant create values with
Yes, can create values with push_back()
Yes, entry 1 exists now

• Really bad ? out of bounds array access and
  memory corruption (maybe seg fault, maybe worse!)
• Most compilers have an option to make vector
  check the index value is between 0 and
  size()-1 and print an error (g
  -D_GLIBCXX_DEBUG)
• Slows program down ? we have turned this on only
  for the debug configuration of your milestone1
  NetBeans project

40
Shrinking a vector
include ltvectorgt using namespace std int main
() vectorltintgt v1 v1.push_back(1)
v1.push_back(2) for (int i 0 i lt
in_vals.size() i) cout ltlt in_valsi ltlt
v1.pop_back() // Removes one entry
from end of vector for (int i 0 i lt
in_vals.size() i) cout ltlt in_valsi ltlt

Output 1 2
Output 1
41
Vectors should be familiar

• vectorltchargt
• almost same as ltstringgt
• Difference string defines some extra utility
  functions
• Otherwise a string really is a vectorltchargt
• Full reference on vector member functions at
  http//www.cplusplus.com/reference/vector/vector/