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Templates &Generic Programming

Junaed Sattar

November 12, 2008Lecture 10

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Generic Programming

Algorithms are written independentlyof data

data types are filled in during execution

functions or classes instantiated with datatypes

formally known as specialization

Also used in Metaprogramming

used by a compiler to generate temporary source

code

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Lets motivate ourselves

Number of algorithms are data-typeindependent

sorting

searching

finding n-th largest

swapping etc

Write once, use many times philosophy

dramatically reduces LOC (lines-of-code)

makes code much easier to maintain

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In C++

Templates = generic programming

Two types

function templates

special functions that can operate with generic types.

class templates

can have members that use template parameters as

types

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Function Templates

special functions using template types.

A template parameter is a special kind ofparameter used to pass a type as argument

just like regular function parameters

but pass types to a function

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Declaration Format?

format for declaring function templates with typeparameters

template function_ declaration;

template function_ declaration;

Same functionality, different keywords

use

older compilers only used format

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Example

create a template function that returns thegreater one of two objects

template myType GetMax (myType a, myType b){return (a>b?a:b);}

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Usage

int main () {

int i=5, j=6, k;long l=10, m=5, n;

k=GetMax(i,j);n=GetMax(l,m);cout

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Invalid Usage

int main () {

int i=5, j=6, k;long l=10, m=5, n;

k=GetMax(i,l);n=GetMax(j,m);}

One template type only in definition

call mixes int and long types

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Multiple template types

define template types after the templatekeyword

template myType GetMax (T1 a, T2 b){return (a>b?a:b);

}

no difference in function call

programmer must ensure sane algorithm

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Class templates

classes can have members that use templateparameters as type

template class mypair {T values [2];

public:mypair (T first, T second){values[0]=first; values[1]=second;

}};

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To use

stores two elements of any valid type

to store two integer values of type int with thevalues 115 and 36:

mypair myobject (115, 36);

to store two doubles:

mypair mydubs (3.0, 2.18);

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Non-inline definition

to define a function member outside thedeclaration of the class template, alwaysprecede that definition with the template

prefixtemplate class mypair {

T values [2];public:mypair (T first, T second){values[0]=first; values[1]=second;

}T GetMax();

};

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Continued

template T mypair::getmax (){T retval;retval = a>b? a : b;

return retval;}

int main () {mypair myobject (100, 75);

cout

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Why so many T's??

There are three T's in this declaration

first one is the template parameter.

second T refers to the type returned by thefunction

third T (the one between angle brackets)specifies that this function's template parameter

is also the class template parameter.

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Specialization

Why and what?

to define a different implementation for a templatewhen a specific type is passed as template

parameter explicitly declare a specialization of that

template

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Sample Case

A class with a sort method

sorts ints, chars, doubles, floats

also need to sort strings based on length, but the

algorithm is different not lexicographic sorting

Need to explicitly create template specializationfor the sort method when string is passed astype

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Code

template class MyContainer {

T element[100];public:

MyContainer( T *arg ){...};void Sort() {

// use your favorite sorting algorithm}

};

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Code

// class template specialization:template class MyContainer {

string element[100];

public:MyContainer (string *arg) {...}void Sort() {

// use a string-length based sort here}

};

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Non-type parameters?

templates can also have regular typedparameters, similar to those found in functions

template class mysequence {T memblock [N];

public:void setmember (int x, T value);T getmember (int x);

};

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Continued

template T mysequence::getmember (int x) {return memblock[x];

}

int main () {mysequence myints;mysequence myfloats;myints.setmember (0,100);

myfloats.setmember (3,3.1416);cout

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Next Class

STL

Exceptions and namespaces