CMPUT 201: Lab 8

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CMPUT 201 Lab 8
Templates
November 14th, 2002
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Function Templates

• Often program methods implement some general
  algorithm that applies to any data type (e.g.
  swap values, sort a list, compare values)
• We would like to define one generic function
  that handles all variable types (primitive types
  or user-defined classes)
• Template functions allow us to do this in C
• This saves us from declaring many functions
  that are identical in logic but differ only in
  their types

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Function Template Syntax

• Template functions must be preceded by a
  template prefix, which has the following form
• templateltclass Tgt
• This tells the compiler the function that
  follows is a template and T is a type parameter
• T can be any word except a keyword
• In the body of the function, T is used like
  any other type

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Function Template Example

• Consider the example below
• template ltclass Tgt
• int findCreature(T list, int size, char
  name) int location 0
• while ( (location lt size) strcmp(
  listlocation.getName(), name) )
• location
• return location

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Function Template Example
int main() int l Hobbit10 hobbits
Elf10 elves ... // calls the definition
of findCreature // that uses hobbits l
findCreature(hobbits, 10, Samwise) // calls
the definition of findCreature // that uses
elves l findCreature(elves, 10, Legolas)
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Function Template Implementations

• The preceding code suggests there is a
  separate version of findCreature for each
  variable type that is defined in that file
• The compiler does not generate a separate
  version for every possible data type, it only
  generates versions for types that are used
• When the compiler comes across a call to
  findCreature, it looks at the parameter type and
  generates a method definition for this type,
  assuming no such definition already exists

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Template Function vs. Virtual Functions

• Template function selection is done at compile
  time whereas virtual function selection is done
  at run time
• The compiler uses the static type of the
  parameters in the template function invocation to
  determine which method definition to call
• Template functions are a convenience mechanism
  offered by the compiler

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Class Templates

• Template classes allow us to define generic
  classes that specify a certain structure and
  purpose regardless of the data types they hold
• Common examples are lists, arrays, stacks,
  queues, etc
• Such classes are also called container classes
• Template classes save us from declaring many
  classes that are identical in structure and
  purpose but differ only in their instance
  variable data types

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Class Template Example (Part I)

• Consider the example below
• templateltclass Tgt
• class creatureProfile
• public
• creatureProfile(T aCreature)
• creatureProfile(T aCreature, int
  amountOfGold)
• private
• T creature int gold

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Class Template Example (Part I)
templateltclass Tgt creatureProfileltTgtcreatureProf
ile( T aCreature) creature
aCreature templateltclass Tgt creatureProfileltTgt
creatureProfile( T aCreature, int
amountOfGold) creature aCreature gold
amountOfGold
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Class Template Details

• Syntax is the same as that of function
  templates (a template prefix must precede the
  class and all its function definitions)
• All of a class templates member functions are
  function templates
• You declare objects of a class template by
  providing a name and a parameter type (much like
  you declare an array)
• The compiler will produce a class with that
  corresponding parameter type

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Class Template Example (Part II)
int main() Hobbit h1(Frodo) Dwarf
d1(Gimli) // create an instance of the //
creatureProfile class that has a hobbit // data
type and give the hobbit 25 gold
creatureProfilelthobbitgt FrodoPfl(h1, 25) //
create an instance of the // creatureProfile
class that has an elf // data type
creatureProfileltelfgt GimliPfl(d1)
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Templates and Inheritance

• Inheritance of class templates works the same
  way as normal inheritance
• template ltclass Tgt
• class magicalCreatureProfile public
  creatureProfileltTgt public
• magicalCreatureProfile(T aCreature,
  SpellBook spellsItKnows)
• private
• SpellBook spells

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Templates and Inheritance
templateltclass Tgt magicCreatureProfileltTgtmagicCr
eatureProfile( T aCreature, SpellBook
spellsItKnows) creatureProfileltTgt(aCreature)
spells new SpellBook() spells
spellsItKnows.deepCopy()