Introducing Templates and Generics in C++

Introducing Templates and Generics in C++ CSC 331 20081027

Overview • Value of templates • Generic functions • Generic classes

Value of templates • Added to original C++ specification • Valuable in the creation of reusable code • Used to create generic functions and classes • In which the type of data acted upon is specified as a parameter • Allows one function or class to act on several different types of data without having to code specific versions for each type.

GENERIC FUNCTIONS (a.k.a. template functions)

Generic function

Example generic function swapargs() template <class X> void swapargs (X &a, X &b) { X temp; temp = a; a=b; b=temp; }

Example main using swapargs() int main() { int I =10, j=20; double x=10.1, y=23.3; char a=‘x’, b=‘z’; cout<<“Original i, j “<<i<<‘ ‘<<j<<‘\n’; cout<<“Original x,y “<<x<<‘ ‘<<y<<‘\n’; cout<<“Original a,b “<<a<<‘ ‘<<b<<‘\n’; swapargs(i, j); swapargs(x, y); swapargs(a, b); cout<<“Swapped i, j “<<i<<‘ ‘<<j<<‘\n’; cout<<“Swapped x,y “<<x<<‘ ‘<<y<<‘\n’; cout<<“Swapped a,b “<<a<<‘ ‘<<b<<‘\n’; return 0; }

What happens? • “template” preceding the function definition tells the compiler to create a generic (or template) function. • “X” is a placeholder for the generic type. • In main(), swapargs() is called using 3 different types of data. • This causes the compiler to generate 3 different versions of swapargs(), each for a different type. • Called “generic functions” or “specialized functions.” • Act of generating referred to as “instantiating.”

More about generic functions • More than one placeholder type may be declared for a function. • The keyword class to specify a generic type is traditional, but keyword typenamemay also be used. • Generic functions may be overridden for “explicit specialization.” • Example: void swapargs(int &a, int &b) • This “hides” the generic swapargs() only for ints. • Alternative syntax: template<> void swapargs<int> (int &a, int &b) • Generic functions may be overloaded (different parameter lists).

GENERIC CLASSES

Generic class

Generic class heading

Generic class: object creation • To create a specific instance of a generic class: • class-name <type> ob; • For previous examples: • queue<int> a, b; // creates 2 integer queues • myClass<int, double> ob1(10, 0.23);

Example generic class #include <iostream> using namespace std; template <class Type1, class Type2> class myClass { Type1 i; Type2 j; public: myClass(Type1 a, Type2 b){i=a; j=b;} void show() {cout<<i<<' '<<j<<'\n';} }; int main() { myClass<int, double>ob1(10, 0.23); myClass<char, char *>ob2('X', "This is a test"); ob1.show(); // show int, double ob2.show(); // show char, char * system("pause"); return 0; }

Introducing Templates and Generics in C++