Template in C ++

Template in C ++ is a very efficient feature provided in C++. It allows you to write a single code and use it for various inter-related functions or classes. The former is termed as function templates and the latter is termed as class template in C ++. The general syntax for a template in   C ++ is:

           template <class dataType>

           declaration; 

template  is a reserved word for defining template in C ++ and class in the heading refers to any built-in or user-defined type. dataType is the name of the data type, user-defined or built-in and is referred to as a formal parameter to the template in C ++ in the same way as variables are passed as parameters to the functions. In the heading, the word class can also be replaced by typename.

Function Template in C ++

Suppose you wrote a function in C ++ that compares two integers and returns the smaller one. And now, if you want to compare two characters or two strings, you have to write separate functions for them (function overloading). In short, you can’t use the already written function for integers for all other data types.

Template in C ++ allows you to write a single code and use it for all data types. This is the purpose what template in C ++ is designed for.

The general syntax of a function template in C ++ is:

           template <class dataType>

           function definition;

where dataType is the formal parameter to the templates which is used to specify the data type of parameters to the function, return type of the function and the data type of variables declared within the function.

Lets define a function template:

      

           template <class dataType>

           Type getLarger(Type p, Type q){

               if (p > q)

                  return p;

               else

                  return q;

           }

This function receives two variables as parameters and returns the larger one. dataType in the function definition will be replaced by the data type of the parameters when the function is called.

A point should be noted here that the above function template will work only for those data types for which the operator > is defined.

The statement 

         cout << getLarger(11, 13) << endl; 

makes a call to the function template getLarger. As 11 and 13 are integers, the dataType will be replaced by int and the resulting code would be compiled and executed.

Class Template in C ++

Similar to the function template, class template in C ++ is designed to use a single code for many related classes. Take the example of a class List. This is a class that stores a list (array) of a data type and performs various functions on that list. If we transform this class into a class template, then we can use a single code to declare List of any data type.

General syntax of a class template in C ++ is:

           template <class dataType>   

                          class declaration;
 
Class template in C ++ is generally termed as parameterized types because a specific class is generated on the basis of parameter type.

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Header File and Implementation File of A Class Template in C ++

Generally when we define a class, we place the definition of the class in header file and the definition of the member functions of the class in implementation file. The appropriate code for the object declared in main is linked with the code of the user. However, this separation of class definition and its member functions doesn’t work with class template in C ++. Passing parameters to a function has an effect on run time but passing parameters to a class template has an effect on compile time. As the parameters to a class template in C ++ are defined in user code and the compiler cannot instantiate a class template without parameters to the class, we cannot compile the implementation file independent of the user code.

Learn C Online provides you various solutions for this problem of class template in C ++:

1)     Put the class definition and the definition of the member functions in the client code

2)     Put the class definition and the definition of the member functions in the same header file

3)     Put the class definition and the definition of the member functions in separate files (as discussed above) but include a directive to the implementation file at the end of the header file

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