Using Templates

Object-Oriented Programming Using C++

Second Edition

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Objectives

• In this chapter, you will learn:

• About the usefulness of function templates

• About the structure of function templates

• How to overload function templates

• How to create function templates with multiple data types

• How to create function templates with multiple parameterized types

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Objectives

• In this chapter, you will learn:

• How to override a function template’s implicit type

• How to use multiple explicit types when you call a function template

• How to use function templates with classes

• About template classes and how to create them

• About container classes and how to create them

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Understanding the Usefulness of Function Templates

• The C++ compiler determines the function’s argument types when the function is created and compiled

• Once the function is created, the argument types remain fixed

• Overloading involves writing two or more functions with the same name but different argument lists

• It allows you to employ polymorphism, using a consistent message that acts appropriately with different objects

• A reverse() function might change the sign of a number if a numeric variable is passed to it

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Understanding the Usefulness of Function Templates

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Creating Function Templates

• In C++, you can create functions that use variable types• These function templates serve as an outline, or

template, for a group of functions that differ in the types of parameters they use

• A group of functions that generates from the same template is often called a family of functions

• In a function template, at least one argument is generic, or parameterized, meaning that one argument can stand for any number of C++ types

• C++ also allows you to define macros, which permit generic substitution

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Creating Function Templates

• Before you code a function template, you must include a template definition with the following information:– The keyword template

– A left single bracket (<)

– A list of generic types, separated with commas if more than one type is needed

– A right angle bracket (>)

• Each generic type in the list of generic types has two parts:– The keyword class

– An identifier that represents the generic type

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Creating Function Templates

• Using the keyword class in the template

definition does not necessarily mean that T

stands for a programmer-created class type, but

it may

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Using Multiple Arguments to Function Templates

• Function templates can support multiple parameters

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Using Multiple Arguments to Function Templates

• In steps listed on pages 409 and 410 of the textbook, you define a function that displays the smallest of any of three same-type arguments it receives

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Overloading Function Templates

• You overload functions when you create functions with the same name but with different argument lists

• You can overload function templates, as long as each version of the function takes different arguments, allowing the compiler to distinguish between them

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A main() Function that Uses the Overloaded invert()

Function Template

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Overloading Function Templates

• Using the steps on pages 412 and 413 of the textbook, you overload the displaySmallest() function template to accept two as well as three arguments

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Using More than One Type in a Function Template

• Like other functions, function templates can use variables of multiple types

• Suppose you want to create a function template that displays a value a given number of times

• The value could be any type, but the number of times to repeat the value is an integer

• It is perfectly legal to include some nonparameterized types in the function argument list, along with the parameterized ones

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RepeatValue Program

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Output of RepeatValue Program

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Using More than One Parameterized Type in a

Function Template

• To create a function template that employs multiple generic types, you simply use a unique type identifier for each type

• Two generic types, T and U, are defined• The first parameterized type, T, can stand for any

type• The second type, U, can stand for the same type, or

any other type• Figure 11-12 includes a demonstration main()

function that passes a variety of arguments to whichIsLarger(), and Figure 11-13 shows the results

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whichIsLarger() Function and main() Program

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Output of whichIsLarger Program

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Explicitly Specifying the Type in a Function Template

• When you call a function template, the arguments to the function dictate the types to be used

• To override a deduced type, when you call a function template you can explicitly code a type within angle brackets immediately following the function name in the function call

• You explicitly name a type by using the type name

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Program that Uses an Explicit Type for a Function’s Parameterized Type

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Explicitly Specifying the Type in a Function Template

• The main() program in Figure 11-15 calls the function template three times, using an integer, a double, and a double converted to an integer, respectively

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Using Multiple Explicit Types in a Function Template

• You can use multiple explicit types when you call a template function

• If you want the return value of a template function sometimes to be the same type as the argument, and sometimes to be a different type, write the function template with two parameterized types

• Additionally, you can exercise the option to explicitly name one or both types

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Using Multiple Explicit Types in a Function Template

• The main() program in Figure 11-17 uses the function in several ways:

– Explicitly codes int for the T type and passes an integer to implicitly assign the U type

– Explicitly codes int for the T type and passes a double to implicitly assign the U type

– Explicitly codes int for the T type and explicitly codes a double for the U type

– Explicitly codes int for both T and U

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Using Multiple Explicit Types in a Function Template

• Figure 11-18 shows the output of the program in Figure 11-17

• The explanation of the output is as follows:– When tripleVal() receives the integer 22 as an argument,

triples it, and returns the integer result , the output is 66– When tripleVal() receives the double 8.88 as an

argument, triples it to 26.64, stores the result in an integer, and returns the integer result, the output is the truncated result, 26. This is true whether tripleVal() receives 8.88 as a double implicitly or explicitly

– When 8.88 is explicitly received as an integer, it receives an 8. The output tripled value is 24

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Using Multiple Explicit Types in a Function Template

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Using Function Templates with Class Objects

• When programming in an object-oriented environment, you naturally want your function templates to work with class objects as well as with scalar variables

• Function templates work just as well with classes as they do with simple data types

• Your only additional responsibility is to ensure that any operations used within the function template have been defined for the class objects passed to the function templates

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The PhoneCall Class

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Using Function Templates with Class Objects

• Figure 11-20 shows a PhoneCall class containing private data members that store the phone number, length of call, and code

• The program in Figure 11-21 declares a PhoneCall object and provides initial values for its fields

• In the set of steps provided on pages 421 to 423 of the textbook, you create an Inventory class

• The class includes an overloaded insertion operator and an overloaded less than operator

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Using Function Templates with Class Objects

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Output of SmallestTemplate3 Program

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Using Template Classes

• Function templates allow you to create generic functions that have the same bodies but can take different data types as parameters

• In some situations classes are similar and you want to perform very similar operations with them

• If you need to create several similar classes, you might consider developing a template class, a class in which at least one type is generic or parameterized

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Using Template Classes

• The template class provides the outline for a family of similar classes

• To create a template class, you begin with the template definition, just as you do with a function template

• The class in Figure 11-24 is named Number• Its private member, theNumber, may be of any

type

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The Number Class Definition

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Creating a Complete Class Template

• Figure 11-25 shows the class definition for the Number class, along with the implementation of the Number class functions

• You can see in Figure 11-25 that the definition template<class T> also is required before the definition of the displayNumber() function, so as to identify the T in the class name, Number<T>

• The displayNumber() function always displays “Number #” just before the value of theNumber

• Figure 11-26 contains a main() function that declares three Number objects constructed using integer, double, and character argument, respectively

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Creating a Complete Class Template

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Creating a Complete Class Template

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Using Container Classes

• A container class is a template class that has been written to perform common class tasks; the purpose of container classes is to manage groups of other classes

• A common programming task that is used in a variety of applications is the construction of a linked list

• A linked list is a chain of objects, each of which consists of at least two parts—the usual components of the object itself and a pointer to another object

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Using Container Classes

• The diagram in Figure 11-28 illustrates a linked list of Students

• No matter what types of objects are linked, procedures must be developed to establish links between objects and to insert new objects into appropriate spots within the linked list

• The procedures include assigning the correct linking pointer values to the new list members

• Other common procedures are deleting a member from the list, reordering a list, and retrieving and displaying the objects from a list

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A Student Linked List

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Creating an Array Template Class

• When you create an array, you create a list of same-type objects at adjacent memory locations

• You perform many standard operations on array data, no matter what type is involved

• You can create a generic Array class with two private data members: a pointer to the beginning of the array, and an integer representing the size of the array

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The Array Class

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Creating an Array Template Class

• The Array class constructor assigns the argument’s array address to the Array class array address, and assigns the argument’s array size to the Array class member size

• The showList() function displays each element of the Array, from element 0 up through one less than size

• Figure 11-30 shows a Book class, and Figure 11-31 shows a Client class

• Neither contains anything unusual; you have created many similar classes

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The Book Class

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The Client Class

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Creating an Array Template Class

• Figure 11-32 shows a main() function that contains several types of arrays

• The program in Figure 11-32 is divided into four parts, which are:

– An array named someInts is initialized with three integers

– An array named someDoubles is initialized with four doubles

– A two-element Book array uses the Book class shown in Figure 11-30. The two Books receive values through the setBook() function

– A four-element Clients array uses the Client class shown in Figure 11-31. The Clients receive values through the setClient() function

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Program Using the Array Container Class

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Creating an Array Template Class

• You can see from Figure 11-33 that when you use the showList() function, each list appears correctly no matter how many elements the array contains

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Summary

• When the logic of several functions is similar, writing the code to overload the functions becomes tedious

• A function template is a function that serves as an outline, or template, for a group of functions that differ in the types of parameters they use

• You can overload function templates, as long as each version of the function takes different arguments, allowing the compiler to distinguish between them

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Summary

• In addition to a parameterized variable, function templates can contain multiple arguments and internal variables that are not generic

• Function templates can have multiple parameterized types; you use a unique type identifier for each type

• When you call a function template, the compiler implicitly deduces the correct types to use within the function template

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Summary

• You can use multiple explicit types when you call a function template

• Function templates work just as well with classes as they do with simple data types—as long as you define all operations for the classes you use within the function template

• A template class is a class in which at least one type is generic or parameterized

• It provides the outline for a family of similar classes

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Summary

• When you create a class template, you use the template definition prior to the class and prior to each function you write

• A container class is a template that has been written to perform common class tasks; the purpose of container classes is to manage groups of other classes

• You create container class templates to speed up the development process for applications that require similar tasks

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