182 Technique 31: Extending a Template Class

LISTING 31-1 (continued)

{

printf(“TemplateAsMember\n”); _fooEntry.Print();

}

};

//Case 2: Using the base template as a base class

class TemplateAsBase : public Base<Foo>

{

public:

TemplateAsBase(void)

: Base<Foo>( “TemplateAsBase”, NULL )

{

}

TemplateAsBase(const char *name, Foo *pFoo)

: Base<Foo>( name, pFoo )

{

}

virtual ~TemplateAsBase(void)

{

}

void Print()

{

printf(“TemplateAsBase:\n”);

Base<Foo>::Print();

}

};

//Case 3: Using the base template as a base class

//for another templated class

template < class A, class B >

class TemplateAsBaseTemplate : public Base<A>

{

private:

B *_anotherPointer; public:

TemplateAsBaseTemplate( void )

:Base<Foo>( “TemplateAsBaseTemplate”, NULL )

{

_anotherPointer = NULL;

}

TemplateAsBaseTemplate( A* anA, B* aB )

:Base<Foo>( “TemplateAsBaseTemplate”, anA )

{

_anotherPointer = aB;

}

B* getBPointer()

{

return _anotherPointer;

}

void Print()

{

Base<A>::Print();

if ( _anotherPointer ) _anotherPointer->Print();

else

printf(“Another pointer is NULL\n”);

}

};

class AnotherBase

{

private: int x;

public:

AnotherBase()

{

x = 0;

}

AnotherBase( int i )

{

x = i;

}

virtual ~AnotherBase(void)

{

}

void Print()

{

printf(“AnotherBase: x = %d\n”, x );

}

};

In Listing 31-1, the code shows how each possible case is addressed and used. We have implemented two “normal” classes, called Foo and AnotherBase, which are used as template arguments to designate the template classes.

Testing the Template Classes

To check whether the code is really working, we need to implement a test driver. The following steps do so for the code in Listing 31-1:

Testing the Template Classes 183

1. In the code editor of your choice, reopen the source file for the code you just created.

In this example, the file is named ch31.cpp, although you can use whatever you choose.

2. Append the code from Listing 31-2 to your file.

Better yet, copy the code from the source file on this book’s companion Web site.

LISTING 31-2: THE TEST DRIVER FOR THE TEMPLATED CLASS

EXAMPLE

int main()

{

printf(“Creating base\n”); Base<Foo> fooBase;

printf(“Creating template as member\n”); TemplateAsMember tempMem; printf(“Creating template as base\n”); TemplateAsBase tempBase; printf(“Creating template as base template\n”); TemplateAsBaseTemplate<Foo,AnotherBase> tempAsBT;

fooBase.Print();

tempMem.Print();

tempBase.Print();

tempAsBT.Print();

return 0;

}

3. Save the source file in your code editor and close the code editor.

4. Compile the source code with the compiler of your choice, on the operating system of your choice.

When the program is run, if you have done everything properly, you should see the following output in the shell window:

Creating base

Void constructor called Creating template as member Creating base with name [TemplateAsMember] Creating template as base

Creating base with name [TemplateAsBase]

Creating template as base template
Creating base with name
[TemplateAsBaseTemplate]
Base:
Name = Nothing
Pointer =		1
Pointer is NULL
TemplateAsMember

Base:

Name = TemplateAsMember

Pointer =

Pointer is NULL

TemplateAsBase:

Base:

Name = TemplateAsBase

Pointer =

Pointer is NULL

Base:

Name = TemplateAsBaseTemplate

Pointer =

Pointer is NULL

Another pointer is NULL

The output from this program shows us that each of the various template instantiations works. As you can see, in each case (see, for example, the line marked 1), the constructor was called and the various member variables assigned proper default values. Looking at the examples, it should be clear that each of the various methods arrives at the same conclusion.

Concrete classes that have been made into templates as a specific form of a class are best suited for extension. This is to say, if you have a template class that accepts a particular type of class for its argument, you are better off extending your template class by creating a form of it as a specific class — and then deriving from that specific class. The reason for this is more human than technical: People usually don’t think in terms of templates so much as in terms of class names.

184 Technique 31: Extending a Template Class

Using Non-class Template

Arguments

Looking at the four choices in extending base classes, you will probably notice a few things that suggest particular approaches to the process.

To utilize methods in a base class being used as a template, you must specify which version of the class the template is to use. The reason is that you could conceivably have a class that inherited from multiple classes of the same template, with different types associated with it. This is a good thing because it allows you to segregate specific functionality into separate classes.

Another thing worth noticing is that you may create a templated class that does not require a class as its argument. For example, we could create a template with a numeric argument; the following steps show you how:

1. In the code editor of your choice, create a new file.

In this example, the file is named ch31a.cpp, although you can use whatever you choose.

2. Type the code from Listing 31-3 into your file.

LISTING 31-3: A TEMPLATE CLASS WITH A NON-CLASS

ARGUMENT

template <class A> class LessThanTen

{

A _element;

public:

LessThanTen( A entry )

{

set ( entry );

}

LessThanTen( const LessThanTen& aCopy )

{
set(	aCopy._element);
}			3
void set(	A value )
{

if ( value > 0 && value < 10 )		2
_element = value;

}

A get()

{

return _element;

}

};

This code works for a class argument, so long as that class can be compared to an integer. It can also be used for a non-class argument, such as an integer, long integer, or even a floating point value.

With this class shown in Listing 31-3, there is no reason that the template argument should be a class. In fact, it would be implied that a numeric element was used, since the value which is passed into the set method is compared to an integral value of 10 (see the line marked 2).

3. Add the following code to your source file to test the integer class. This code will test the template class we just defined above.

int main(void)
{
LessThanTen<int> ten(3);			5
printf(“The	value is %d\n”,
ten.get() );
ten.set( 23	);		4
printf(“The	value is now %d\n”,

ten.get() ); return 0;

}

4. Save the source file in your code editor and then close the code editor.

5. Compile the source code with the compiler of your choice, on the operating system of your choice.

When the program is run, if you have done everything properly, you should see the following output in the shell window:

Using Non-class Template Arguments

185

$ ./a.exe

The value is 3 The value is now 3

As you can see from the output, the program indeed does properly create a new class that is a templated version of the LessThanTen class, using an integer as its template argument. The resulting class contains a method called set that takes an integer argument (shown at 3) that must be between 0 and 10. Since our value (see 4) is not within that range, it is not assigned, and we see that the print statement following the assignment still contains the value 3.

Notice that the code works with an integer argument (see the line marked with 5), even though the template specifies a class argument. For C++, integers, floating-point numbers, and the like can be considered first-class (that is, a basic type) arguments for the purpose of templates. In fact (in this case at least), any argument can be used, so long as the code includes comparison operators greater-than

(>) and less-than (<) to ensure that the set method works properly.

The ability to use either classes or basic types as template arguments makes the template construct extremely powerful in C++. Because you can write a single class that manages number, character strings, or class values and have it work seamlessly, you save enormous amounts of time and duplicated work.