Chapter 3 Templates

1
Chapter 3Templates
2
Objective

• In Chapter 3, we will discuss
• The concept of a template
• Function templates
• Class templates
• vector and matrix classes
• Fancy stuff
• Bugs

3
3.1 What is a Template?

• A template is a mechanism for writing routines
  that work for arbitrary types without knowing
  these types.
• For example Type independent algorithms and data
  structures
• Templates are most likely seen in generic
  algorithm implementations, i.e. sorting
  searching.

4
3.2 Function Templates

• typedef keyword for defining new type from an
  existing one.
• Ex typedef double Object
• Object a 3.5 // double a 3.5
• A Function template is a design or pattern for a
  function which allows processors to generate an
  actual function from this design.
• A function template is not an actual function,
  instead its a design or a pattern, for what
  could become an actual function.

5
Function template example Swap routine

• typedef double Object
• void swap(Object lhs,
• Object rhs)
• Object tmp lhs
• lhs rhs
• rhs tmp
• // figure 3.1
• Note swap is part of the STL

• //Object must have copy
• // constructor operator
• template ltclass Objectgt
• void Swap(Object lhs,
• Object rhs)
• Object tmp lhs
• lhs rhs
• rhs tmp
• // figure 3.2

6
Swap routine used in main function

• int main()
• int x 5, y 7
• double a 2, b 4
• Swap (x,y) // swap(int,int)
• Swap(x,y) //reuse previous instantiation
• Swap(a,b) //Swap(double, double)
• //Swap(x, b) // illegal no match
• return 0
• // figure 3.3

7
3.3 A Sorting Function Template

• template ltclass Comparablegt
• void insertionSort(vectorltComparablegt a)
• for(int p 1 p lt a.size() p)
• Comparable tmp ap
• int j
• for(j p j gt 0 tmp lt aj-1 j)
• aj a j 1
• aj tmp

8
Insertion Sort example

• The given insertionSort routine work for double,
  int, float but not char, (primitive string),
  because operator and lt are undefined.
• sorting demo(1)
• sorting demo(2)

9
3.4 Class Templates

• A class can be a template.
• Example vector is a class template
• A call by value should not be used for parameters
  of the template type use constant reference
  instead.

10
A class template example

• template ltclass Objectgt
• class MemoryCell
• public
• explicit MemoryCell(const Object initVal
  Object())
• storedValue(initVal)
• const Object read() const
• return storedValue
• void write(const Object x)
• storedValue x
• private
• Object storedValue
• // figure 3.8

11

• Calling the class template from main
• int main()
• MemoryCellltintgt m
• m.write(5)
• coutltltCell contents are ltltm.read()ltltendl
  
• return 0
• When the interface is separated from the
  implementation, all member functions should be
  implemented as template functions

12

• include MemoryCell.h
• template ltclass Objectgt
• MemoryCellltObjectgtMemoryCell
• (const Object initVal)
• storedValue(initVal)
• templateltclass Objectgt
• const Object MemoryCellltObjectgtread() const
• return storedValue
• template ltclass Objectgt
• void MemoryCellltObjectgtwrite(
• const Object x)
• storedValue x
• // figure 3.11

• templateltclass Objectgt
• class MemoryCell
• public
• explicit MemoryCell(const Object
  initVal Object())
• const Object read() const
• const write(const Object x)
• private
• Object storedValue
• // figure 3.10

Separating the interface and implementation of
the template
13

• templateltclass objectgt
• class ClassName
• public
• //public members
• private
• //private member
• // typical template interface
• template ltclass objectgt
• ReturnType
• ClassNameltobjectgtmemberName(parameterList)
  /const/
• // member body
• // typical member implementation

14
Implementing the Vector Class Template

• Provides Indexing, resizing, copying, and index
  range checking capabilities
• figure 3.14 vector.h
• figure 3.15 vector.cpp

15
3.5 Templates of Templates A Matrix Class

• Figure 3.16 A complete matrix class
• Matrix class is represented as a vector of
  vectors.
• Overload operator for mutators and accessors
• No need for destructors, copy operator and
  assignment operator, (already exist in vector)
• Note There must be at least a space between gt
  gt in the declaration otherwise compilers will
  confuse gtgt with the shift operator .
• Ex vectorltvectorltObjectgt gt array // white space
  needed

16
3.6 Fancy Templates

• Multiple template parameters
• template ltclass keyType, class valueTypegt
• Class Map
• . . .
• Ex mapltstring,intgt zipCodes
• Note class map is in STL

17
Default Template Parameters

• template ltclass keyType, class valueType
  stringgt
• Class Map
• . . .
• Mapltint, intgt m1 // keyType int, valueType
  int
• Mapltintgt m2 // keyType int, valueType string
• Default template parameters are widely used in
  STL
• Note some compilers do not support default
  template parameters

18
Default Template Parameters

• typename a new keyword can be used instead of
  class
• template lttypename Objectgt
• class memoryCell
• . . .

19
3.7 Bugs Associated with Templates

• Template STD changes frequently
• gt incompatible compilers
• Matching algorithm breaks when template involve
  (function template)
• Nested templates are not widely supported
• Static functions data members are often not
  handled correctly

20
Common mistakes

• When a class template is instantiated, all needed
  operations must be available.
• For instance, the insertion sort template needs
  to have operatorlt defined for whatever Comparable
  is instantiated. Otherwise, an error will be
  detected at link time. Clearly state what
  conditions must be satisfied by the template
  parameters
• Use either reference or constant reference for
  parameter passing. (Page 115)

21
In class exercise

• Write a function template to sort three
  Comparable objects.
• When templates are used what types of errors are
  detected when the function template is scanned?
  What errors are detected when the function is
  instantiate?
• Describe syntax for class templates.

22
Summary

• Discussion of template facilities
• Template for generic algorithms