Teach Yourself C++ in 21 Days
Day 19
Templates
What Are Templates?
Parameterized Types
Template Definition
Listing 19.1. A template of an Array class.
Using the Name
Implementing the Template
Listing 19.2. The implementation of the template array.
Template Functions
Templates and Friends
Non-Template Friend Classes and Functions
Listing 19.3. Non-template friend function.
General Template Friend Class or Function
Listing 19.4. Using operator ostream.
A Type-Specific Template Friend Class or Function
Using Template Items
Listing 19.5. Passing template objects
to and from functions.
Specialized Functions
Listing 19.6. Specializing template implementations.
Static Members and Templates
Listing 19.7. Using static member data and functions
with templates.
The Standard Template Library
Summary
Q&A
Workshop
Quiz
Exercises
Day 19
Templates
On Day 17, "The Preprocessor," you saw how to use macros to create various
lists using the concatenation operator. Macros have a number of problems that are
fixed by templates.
Today you will learn
What templates are and how to use them.
Why templates supply a better alternative to macros.
How to create class templates.
How to create function templates.
What Are Templates?
At the end of Week 2, you saw how to build a PartsList object and how
to use it to create a PartsCatalog. If you want to build on the PartsList
object to make a list of cats, you have a problem: PartsList only knows
about parts.
To solve this problem, you can create a List base class and derive from
it the PartsList and CatsList classes. You could then cut and paste
much of the PartsList class into the new CatsList declaration.
Next week, when you want to make a list of Car objects, you would then have
to make a new class, and again you'd cut and paste.
Needless to say, this is not a satisfactory solution. Over time, the List
class and its derived classes will have to be extended. Making sure that all the
changes are propagated to all the related classes would be a nightmare.
On Day 17, one approach to parameterizing lists was demonstrated briefly--using
macros and name concatenation. Although macros do save much of the cutting and pasting,
they have one killer disadvantage: Like everything else in the preprocessor, they
are not type-safe.
Templates offer the preferred method of creating parameterized lists in C++. They
are an integrated part of the language, they are type-safe, and they are very flexible.
Parameterized Types
Templates allow you to teach the compiler how to make a list of any type of thing,
rather than creating a set of type-specific lists--a PartsList is a list
of parts, a CatList is a list of cats. The only way in which they differ
is the type of the thing on the list. With templates, the type of the thing on the
list becomes a parameter to the definition of the class.
A common component of virtually all C++ libraries is an array class. As you saw
with Lists, it is tedious and inefficient to create one array class for
integers, another for doubles, and yet another for an array of Animals.
Templates let you declare a parameterized array class and then specify what type
of object each instance of the array will hold.
New Term: Instantiation is the
act of creating a specific type from a template. The individual classes are called
instances of the template.
Parameterized templates provide you with the ability to create a general class,
and pass types as parameters to that class, in order to build specific instances.
Template Definition
You declare a parameterized Array object (a template for an array) by
writing
1: template <class T> // declare the template and the parameter
2: class Array // the class being parameterized
3: {
4: public:
5: Array();
6: // full class declaration here
7: };
The keyword template is used at the beginning of every declaration and
definition of a template class. The parameters of the template are after the keyword
template. The parameters are the things that will change with each instance.
For example, in the array template shown previously, the type of the objects stored
in the array will change. One instance might store an array of integers, while another
might store an array of Animals.
In this example, the keyword class is used, followed by the identifier
T. The keyword class indicates that this parameter is a type. The
identifier T is used throughout the rest of the template definition to refer
to the parameterized type. One instance of this class will substitute int
everywhere T appears, and another will substitute Cat.
To declare an int and a Cat instance of the parameterized Array
class, you would write
Array<int> anIntArray;
Array<Cat> aCatArray;
The object anIntArray is of the type array of integers; the object aCatArray
is of the type array of cats. You can now use the type Array<int>
anywhere you would normally use a type--as the return value from a function, as a
parameter to a function, and so forth. Listing 19.1 provides the full declaration
of this stripped-down Array template.
NOTE: Listing 19.1 is not a complete program!
Listing 19.1. A template
of an Array class
1: Listing 19.1 A template of an array class
2: #include <iostream.h>
3: const int DefaultSize = 10;
4:
5: template <class T> // declare the template and the parameter
6: class Array // the class being parameterized
7: {
8: public:
9: // constructors
10: Array(int itsSize = DefaultSize);
11: Array(const Array &rhs);
12: ~Array() { delete [] pType; }
13:
14: // operators
15: Array& operator=(const Array&);
16: T& operator[](int offSet) { return pType[offSet]; }
17:
18: // accessors
19: int getSize() { return itsSize; }
20:
21: private:
22: T *pType;
23: int itsSize;
24: };
Output: There is no output. This is an
incomplete program.
Analysis: The definition of the template
begins on line 5, with the keyword template followed by the parameter. In
this case, the parameter is identified to be a type by the keyword class,
and the identifier T is used to represent the parameterized type.
From line 6 until the end of the template on line 24, the rest of the declaration
is like any other class declaration. The only difference is that wherever the type
of the object would normally appear, the identifier T is used instead. For
example, operator[] would be expected to return a reference to an object
in the array, and in fact it is declared to return a reference to a T.
When an instance of an integer array is declared, the operator= that
is provided to that array will return a reference to an integer. When an instance
of an Animal array is declared, the operator= provided to the Animal
array will return a reference to an Animal.
Using the Name
Within the class declaration, the word Array may be used without further
qualification. Elsewhere in the program, this class will be referred to as Array<T>.
For example, if you do not write the constructor within the class declaration, you
must write
template <class T>
Array<T>::Array(int size):
itsSize = size
{
pType = new T[size];
for (int i = 0; i<size; i++)
pType[i] = 0;
}
The declaration on the first line of this code fragment is required to identify
the type (class T). The template name is Array<T>, and the
function name is Array(int size).
The remainder of the function is exactly the same as it would be for a non-template
function. It is a common and preferred method to get the class and its functions
working as a simple declaration before turning it into a template.
Implementing the
Template
The full implementation of the Array template class requires implementation
of the copy constructor, operator=, and so forth. Listing 19.2 provides
a simple driver program to exercise this template class.
NOTE: Some older compilers do not support templates. Templates
are, however, part of the emerging C++ standard. All major compiler vendors have
committed to supporting templates in their next release, if they have not already
done so. If you have an older compiler, you won't be able to compile and run the
exercises in this chapter. It's still a good idea to read through the entire chapter,
however, and return to this material when you upgrade your compiler.
Listing 19.2. The implementation
of the template array.
1: #include <iostream.h>
2:
3: const int DefaultSize = 10;
4:
5: // declare a simple Animal class so that we can
6: // create an array of animals
7:
8: class Animal
9: {
10: public:
11: Animal(int);
12: Animal();
13: ~Animal() {}
14: int GetWeight() const { return itsWeight; }
15: void Display() const { cout << itsWeight; }
16: private:
17: int itsWeight;
18: };
19:
20: Animal::Animal(int weight):
21: itsWeight(weight)
22: {}
23:
24: Animal::Animal():
25: itsWeight(0)
26: {}
27:
28:
29: template <class T> // declare the template and the parameter
30: class Array // the class being parameterized
31: {
32: public:
33: // constructors
34: Array(int itsSize = DefaultSize);
35: Array(const Array &rhs);
36: ~Array() { delete [] pType; }
37:
38: // operators
39: Array& operator=(const Array&);
40: T& operator[](int offSet) { return pType[offSet]; }
41: const T& operator[](int offSet) const
42: { return pType[offSet]; }
43: // accessors
44: int GetSize() const { return itsSize; }
45:
46: private:
47: T *pType;
48: int itsSize;
49: };
50:
51: // implementations follow...
52:
53: // implement the Constructor
54: template <class T>
55: Array<T>::Array(int size = DefaultSize):
56: itsSize(size)
57: {
58: pType = new T[size];
59: for (int i = 0; i<size; i++)
60: pType[i] = 0;
61: }
62:
63: // copy constructor
64: template <class T>
65: Array<T>::Array(const Array &rhs)
66: {
67: itsSize = rhs.GetSize();
68: pType = new T[itsSize];
69: for (int i = 0; i<itsSize; i++)
70: pType[i] = rhs[i];
71: }
72:
73: // operator=
74: template <class T>
75: Array<T>& Array<T>::operator=(const Array &rhs)
76: {
77: if (this == &rhs)
78: return *this;
79: delete [] pType;
80: itsSize = rhs.GetSize();
81: pType = new T[itsSize];
82: for (int i = 0; i<itsSize; i++)
83: pType[i] = rhs[i];
84: return *this;
85: }
86:
87: // driver program
88: int main()
89: {
90: Array<int> theArray; // an array of integers
91: Array<Animal> theZoo; // an array of Animals
92: Animal *pAnimal;
93:
94: // fill the arrays
95: for (int i = 0; i < theArray.GetSize(); i++)
96: {
97: theArray[i] = i*2;
98: pAnimal = new Animal(i*3);
99: theZoo[i] = *pAnimal;
100: delete pAnimal;
101: }
102: // print the contents of the arrays
103: for (int j = 0; j < theArray.GetSize(); j++)
104: {
105: cout << "theArray[" << j << "]:\t";
106: cout << theArray[j] << "\t\t";
107: cout << "theZoo[" << j << "]:\t";
108: theZoo[j].Display();
109: cout << endl;
110: }
111:
112: for (int k = 0; k < theArray.GetSize(); k++)
113: delete &theZoo[j];
114: return 0;
115: }
Output: theArray[0]: 0 theZoo[0]: 0
theArray[1]: 2 theZoo[1]: 3
theArray[2]: 4 theZoo[2]: 6
theArray[3]: 6 theZoo[3]: 9
theArray[4]: 8 theZoo[4]: 12
theArray[5]: 10 theZoo[5]: 15
theArray[6]: 12 theZoo[6]: 18
theArray[7]: 14 theZoo[7]: 21
theArray[8]: 16 theZoo[8]: 24
theArray[9]: 18 theZoo[9]: 27
Analysis: Lines 8 to 26 provide a stripped-down
Animal class, created here so that there are objects of a user-defined type
to add to the array.
Line 29 declares that what follows is a template, and that the parameter to the template
is a type, designated as T. The Array class has two constructors
as shown, the first of which takes a size and defaults to the constant integer DefaultSize.
The assignment and offset operators are declared, with the latter declaring both
a const and a non-const variant. The only accessor provided is
GetSize(), which returns the size of the array.
One can certainly imagine a fuller interface, and, for any serious Array
program, what has been supplied here would be inadequate. At a minimum, operators
to remove elements, to expand the array, to pack the array, and so forth would be
required.
The private data consists of the size of the array and a pointer to the actual
in-memory array of objects.
Template Functions
If you want to pass an array object to a function, you must pass a particular
instance of the array, not a template. Therefore, if SomeFunction() takes
an integer array as a parameter, you may write
void SomeFunction(Array<int>&); // ok
but you may not write
void SomeFunction(Array<T>&); // error!
because there is no way to know what a T& is. You also may not write
void SomeFunction(Array &); // error!
because there is no class Array--only the template and the instances.
To accomplish the more general approach, you must declare a template function.
template <class T>
void MyTemplateFunction(Array<T>&); // ok
Here the function MyTemplateFunction() is declared to be a template function
by the declaration on the top line. Note that template functions can have any name,
just as other functions can.
Template functions can also take instances of the template, in addition to the
parameterized form. The following is an example:
template <class T>
void MyOtherFunction(Array<T>&, Array<int>&); // ok
Note that this function takes two arrays: a parameterized array and an array of
integers. The former can be an array of any object, but the latter is always an array
of integers.
Templates and Friends
Template classes can declare three types of friends:
A non-template friend class or function.
A general template friend class or function.
A type-specific template friend class or function.
Non-Template Friend
Classes and Functions
It is possible to declare any class or function to be a friend to your template
class. Each instance of the class will treat the friend properly, as if the declaration
of friendship had been made in that particular instance. Listing 19.3 adds a trivial
friend function, Intrude(), to the template definition of the Array
class, and the driver program invokes Intrude(). Because it is a friend,
Intrude() can then access the private data of the Array. Because
this is not a template function, it can only be called on Arrays of int.
NOTE: To use Listing 19.3, copy lines
1-26 of Listing 19.2 after line 1 of this listing, and then copy lines 51-86 of Listing
19.2 after line 37 of this listing.
Listing 19.3. Non-template
friend function.
1: // Listing 19.3 - Type specific friend functions in templates
2:
3: template <class T> // declare the template and the parameter
4: class Array // the class being parameterized
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType; }
11:
12: // operators
13: Array& operator=(const Array&);
14: T& operator[](int offSet) { return pType[offSet]; }
15: const T& operator[](int offSet) const
16: { return pType[offSet]; }
17: // accessors
18: int GetSize() const { return itsSize; }
19:
20: // friend function
21: friend void Intrude(Array<int>);
22:
23: private:
24: T *pType;
25: int itsSize;
26: };
27:
28: // friend function. Not a template, can only be used
29: // with int arrays! Intrudes into private data.
30: void Intrude(Array<int> theArray)
31: {
32: cout << "\n*** Intrude ***\n";
33: for (int i = 0; i < theArray.itsSize; i++)
34: cout << "i: " << theArray.pType[i] << endl;
35: cout << "\n";
36: }
37:
38: // driver program
39: int main()
40: {
41: Array<int> theArray; // an array of integers
42: Array<Animal> theZoo; // an array of Animals
43: Animal *pAnimal;
44:
45: // fill the arrays
46: for (int i = 0; i < theArray.GetSize(); i++)
47: {
48: theArray[i] = i*2;
49: pAnimal = new Animal(i*3);
50: theZoo[i] = *pAnimal;
51: }
52:
53: int j, k;
54: for (j = 0; j < theArray.GetSize(); j++)
55: {
56: cout << "theZoo[" << j << "]:\t";
57: theZoo[j].Display();
58: cout << endl;
59: }
60: cout << "Now use the friend function to ";
61: cout << "find the members of Array<int>";
62: Intrude(theArray);
63:
63: // return the allocated memory before the arrays are destroyed.
64: for (k = 0; k < theArray.GetSize(); k++)
65: delete &theZoo[j];
66:
67: cout << "\n\nDone.\n";
68: return 0;
69: }
Output: theZoo[0]: 0
theZoo[1]: 3
theZoo[2]: 6
theZoo[3]: 9
theZoo[4]: 12
theZoo[5]: 15
theZoo[6]: 18
theZoo[7]: 21
theZoo[8]: 24
theZoo[9]: 27
Now use the friend function to find the members of Array<int>
*** Intrude ***
i: 0
i: 2
i: 4
i: 6
i: 8
i: 10
i: 12
i: 14
i: 16
i: 18
Done.
Analysis: The declaration of the Array
template has been extended to include the friend function Intrude(). This
declares that every instance of an array will consider Intrude() to be a
friend function; thus, Intrude() will have access to the private member
data and functions of the array instance.
On line 33, Intrude() accesses itsSize directly, and on line 34
it accesses pType directly. This trivial use of these members was unnecessary
because the Array class provides public accessors for this data, but it
serves to demonstrate how friend functions can be declared with templates.
General Template
Friend Class or Function
It would be helpful to add a display operator to the Array class. One
approach would be to declare a display operator for each possible type of Array,
but this would undermine the whole point of having made Array a template.
What is needed is an insertion operator that works for any possible type of Array.
ostream& operator<< (ostream& Array<T>&);
To make this work, we need to declare operator<< to be a template
function.
template <class T> ostream& operator<< (ostream&, Array<T>&)
Now that operator<< is a template function, you need only to provide
an implementation. Listing 19.4 shows the Array template extended to include
this declaration and provides the implementation for the operator<<.
NOTE: To compile this listing, copy lines 8-26
of Listing 19.2 and insert them between lines 3 and 4. Also copy lines 51-86 of Listing
19.2 and insert them between lines 37 and 38.
Listing 19.4. Using
operator ostream.
1: #include <iostream.h>
2:
3: const int DefaultSize = 10;
4:
5: template <class T> // declare the template and the parameter
6: class Array // the class being parameterized
7: {
8: public:
9: // constructors
10: Array(int itsSize = DefaultSize);
11: Array(const Array &rhs);
12: ~Array() { delete [] pType; }
13:
14: // operators
15: Array& operator=(const Array&);
16: T& operator[](int offSet) { return pType[offSet]; }
17: const T& operator[](int offSet) const
18: { return pType[offSet]; }
19: // accessors
20: int GetSize() const { return itsSize; }
21:
22: friend ostream& operator<< (ostream&, Array<T>&);
23:
24: private:
25: T *pType;
26: int itsSize;
27: };
28:
29: template <class T>
30: ostream& operator<< (ostream& output, Array<T>& theArray)
31: {
32: for (int i = 0; i<theArray.GetSize(); i++)
33: output << "[" << i << "] " << theArray[i] << endl; return output;
34: }
35:
36: enum BOOL { FALSE, TRUE};
37:
38: int main()
39: {
40: BOOL Stop = FALSE; // flag for looping
41: int offset, value;
42: Array<int> theArray;
43:
44: while (!Stop)
45: {
46: cout << "Enter an offset (0-9) ";
47: cout << "and a value. (-1 to stop): " ;
47: cin >> offset >> value;
48:
49: if (offset < 0)
50: break;
51:
52: if (offset > 9)
53: {
54: cout << "***Please use values between 0 and 9.***\n";
55: continue;
56: }
57:
58: theArray[offset] = value;
59: }
60:
61: cout << "\nHere's the entire array:\n";
62: cout << theArray << endl;
63: return 0;
64: }
Output: Enter an offset (0-9) and a value. (-1 to stop): 1 10
Enter an offset (0-9) and a value. (-1 to stop): 2 20
Enter an offset (0-9) and a value. (-1 to stop): 3 30
Enter an offset (0-9) and a value. (-1 to stop): 4 40
Enter an offset (0-9) and a value. (-1 to stop): 5 50
Enter an offset (0-9) and a value. (-1 to stop): 6 60
Enter an offset (0-9) and a value. (-1 to stop): 7 70
Enter an offset (0-9) and a value. (-1 to stop): 8 80
Enter an offset (0-9) and a value. (-1 to stop): 9 90
Enter an offset (0-9) and a value. (-1 to stop): 10 10
***Please use values between 0 and 9.***
Enter an offset (0-9) and a value. (-1 to stop): -1 -1
Here's the entire array:
[0] 0
[1] 10
[2] 20
[3] 30
[4] 40
[5] 50
[6] 60
[7] 70
[8] 80
[9] 90
Analysis: On line 22, the function
template operator<<() is declared to be a friend of the Array
class template. Because operator<<() is implemented as a template
function, every instance of this parameterized array type will automatically have
an operator<<(). The implementation for this operator starts on line
29. Every member of an array is called in turn. This only works if there is an operator<<
defined for every type of object stored in the array.
A Type-Specific
Template Friend Class or Function
Although the insertion operator shown in Listing 19.4 works, it is still not quite
what is needed. Because the declaration of the friend operator on line 29 declares
a template, it will work for any instance of Array and any insertion operator
taking an array of any type.
The insertion operator template shown in Listing 19.4 makes all instances of the
insertion operator<< a friend of any instance of Array, whether
the instance of the insertion operator is an integer, an Animal, or a Car.
It makes no sense, however, for an Animal insertion operator to be a friend
to the insertion operator for an integer array.
What is needed is for the insertion operator for an array of int to be
a friend to the Array of int class, and for the insertion operator
of an array of Animals to be a friend to the Array of animals instance.
To accomplish this, modify the declaration of the insertion operator on line 29
of Listing 19.4, and remove the words template <class T>. That is,
change line 30 to read
friend ostream& operator<< (ostream&, Array<T>&);
This will use the type (T) declared in the template of Array.
Thus, the operator<< for an integer will only work with an array of
integers, and so forth.
Using Template Items
You can treat template items as you would any other type. You can pass them as
parameters, either by reference or by value, and you can return them as the return
values of functions, also by value or by reference. Listing 19.5 demonstrates how
to pass template objects.
Listing 19.5. Passing
template objects to and from functions.
1: #include <iostream.h>
2:
3: const int DefaultSize = 10;
4:
5: // A trivial class for adding to arrays
6: class Animal
7: {
8: public:
9: // constructors
10: Animal(int);
11: Animal();
12: ~Animal();
13:
14: // accessors
15: int GetWeight() const { return itsWeight; }
16: void SetWeight(int theWeight) { itsWeight = theWeight; }
17:
18: // friend operators
19: friend ostream& operator<< (ostream&, const Animal&);
20:
21: private:
22: int itsWeight;
23: };
24:
25: // extraction operator for printing animals
26: ostream& operator<<
27: (ostream& theStream, const Animal& theAnimal)
28 {
29: theStream << theAnimal.GetWeight();
30: return theStream;
31: }
32:
33: Animal::Animal(int weight):
34: itsWeight(weight)
35: {
36: // cout << "Animal(int)\n";
37: }
38:
39: Animal::Animal():
40: itsWeight(0)
41: {
42: // cout << "Animal()\n";
43: }
44:
45: Animal::~Animal()
46: {
47: // cout << "Destroyed an animal...\n";
48: }
49:
50: template <class T> // declare the template and the parameter
51: class Array // the class being parameterized
52: {
53: public:
54: Array(int itsSize = DefaultSize);
55: Array(const Array &rhs);
56: ~Array() { delete [] pType; }
57:
58: Array& operator=(const Array&);
59: T& operator[](int offSet) { return pType[offSet]; }
60: const T& operator[](int offSet) const
61: { return pType[offSet]; }
62: int GetSize() const { return itsSize; }
63
64: // friend function
65: friend ostream& operator<< (ostream&, const Array<T>&);
66:
67: private:
68: T *pType;
69: int itsSize;
70: };
71:
70: template <class T>
72: ostream& operator<< (ostream& output, const Array<T>& theArray)
73: {
74: for (int i = 0; i<theArray.GetSize(); i++)
75: output << "[" << i << "] " << theArray[i] << endl;
76: return output;
77: }
78:
79: void IntFillFunction(Array<int>& theArray);
80: void AnimalFillFunction(Array<Animal>& theArray);
81: enum BOOL {FALSE, TRUE};
82:
84: int main()
85: {
86: Array<int> intArray;
87: Array<Animal> animalArray;
88: IntFillFunction(intArray);
87: AnimalFillFunction(animalArray);
89: cout << "intArray...\n" << intArray;
90: cout << "\nanimalArray...\n" << animalArray << endl;
91: return 0;
92: }
93:
94: void IntFillFunction(Array<int>& theArray)
95: {
96: BOOL Stop = FALSE;
97: int offset, value;
98: while (!Stop)
99: {
100: cout << "Enter an offset (0-9) ";
101: cout << "and a value. (-1 to stop): " ;
102: cin >> offset >> value;
103: if (offset < 0)
104: break;
105: if (offset > 9)
106: {
107: cout << "***Please use values between 0 and 9.***\n";
108: continue;
109: }
110: theArray[offset] = value;
111: }
112: }
113:
114:
115: void AnimalFillFunction(Array<Animal>& theArray)
116: {
117: Animal * pAnimal;
118: for (int i = 0; i<theArray.GetSize(); i++)
119: {
120: pAnimal = new Animal;
121: pAnimal->SetWeight(i*100);
122: theArray[i] = *pAnimal;
123: delete pAnimal; // a copy was put in the array
124: }
125: }
Output: Enter an offset (0-9) and a value. (-1 to stop): 1 10
Enter an offset (0-9) and a value. (-1 to stop): 2 20
Enter an offset (0-9) and a value. (-1 to stop): 3 30
Enter an offset (0-9) and a value. (-1 to stop): 4 40
Enter an offset (0-9) and a value. (-1 to stop): 5 50
Enter an offset (0-9) and a value. (-1 to stop): 6 60
Enter an offset (0-9) and a value. (-1 to stop): 7 70
Enter an offset (0-9) and a value. (-1 to stop): 8 80
Enter an offset (0-9) and a value. (-1 to stop): 9 90
Enter an offset (0-9) and a value. (-1 to stop): 10 10
***Please use values between 0 and 9.***
Enter an offset (0-9) and a value. (-1 to stop): -1 -1
intArray:...
[0] 0
[1] 10
[2] 20
[3] 30
[4] 40
[5] 50
[6] 60
[7] 70
[8] 80
[9] 90
animalArray:...
[0] 0
[1] 100
[2] 200
[3] 300
[4] 400
[5] 500
[6] 600
[7] 700
[8] 800
[9] 900
Analysis: Most of the Array
class implementation is left out to save space. The Animal class is declared
on lines 6-23. Although this is a stripped-down and simplified class, it does provide
its own insertion operator (<<) to allow the printing of Animals.
Printing simply prints the current weight of the Animal.
Note that Animal has a default constructor. This is necessary because, when
you add an object to an array, the object's default constructor is used to create
the object. This creates some difficulties, as you'll see.
On line 79, the function IntFillFunction() is declared. The prototype
indicates that this function takes an integer array. Note that this is not a template
function. IntFillFunction() expects only one type of an array--an integer
array. Similarly, on line 80, AnimalFillFunction() is declared to take an
Array of Animal.
The implementations for these functions are different from one another, because
filling an array of integers does not have to be accomplished in the same way as
filling an array of Animals.
Specialized Functions
If you uncomment the print statements in Animal's constructors
and destructor in Listing 19.5, you'll find there are unanticipated extra constructions
and destructions of Animals.
When an object is added to an array, the object's default constructor is called.
The Array constructor, however, goes on to assign 0 to the value
of each member of the array, as shown on lines 59 and 60 of Listing 19.2.
When you write someAnimal = (Animal) 0;, you call the default operator=
for Animal. This causes a temporary Animal object to be created,
using the constructor, which takes an integer (zero). That temporary is used as the
right-hand side of the operator= and then is destroyed.
This is an unfortunate waste of time, because the Animal object was already
properly initialized. However, you can't remove this line, because integers are not
automatically initialized to a value of 0. The solution is to teach the
template not to use this constructor for Animals, but to use a special Animal
constructor.
You can provide an explicit implementation for the Animal class, as indicated
in Listing 19.6.
Listing 19.6. Specializing
template implementations.
1: #include <iostream.h>
2:
3: const int DefaultSize = 3;
4:
5: // A trivial class for adding to arrays
6: class Animal
7: {
8: public:
9: // constructors
10: Animal(int);
11: Animal();
12: ~Animal();
13:
14: // accessors
15: int GetWeight() const { return itsWeight; }
16: void SetWeight(int theWeight) { itsWeight = theWeight; }
17:
18: // friend operators
19: friend ostream& operator<< (ostream&, const Animal&);
20:
21: private:
22: int itsWeight;
23: };
24:
25: // extraction operator for printing animals
26: ostream& operator<<
27: (ostream& theStream, const Animal& theAnimal)
28: {
29: theStream << theAnimal.GetWeight();
30: return theStream;
31: }
32:
33: Animal::Animal(int weight):
34: itsWeight(weight)
35: {
36: cout << "animal(int) ";
37: }
38:
39: Animal::Animal():
40: itsWeight(0)
41: {
42: cout << "animal() ";
43: }
44:
45: Animal::~Animal()
46: {
47: cout << "Destroyed an animal...";
48: }
49:
50: template <class T> // declare the template and the parameter
51: class Array // the class being parameterized
52: {
53: public:
54: Array(int itsSize = DefaultSize);
55: Array(const Array &rhs);
56: ~Array() { delete [] pType; }
57:
58: // operators
59: Array& operator=(const Array&);
60: T& operator[](int offSet) { return pType[offSet]; }
61: const T& operator[](int offSet) const
62: { return pType[offSet]; }
62:
63: // accessors
64: int GetSize() const { return itsSize; }
65:
66: // friend function
67: friend ostream& operator<< (ostream&, const Array<T>&);
68:
69: private:
70: T *pType;
71: int itsSize;
72: };
73:
74: template <class T>
75: Array<T>::Array(int size = DefaultSize):
76: itsSize(size)
77: {
78: pType = new T[size];
79: for (int i = 0; i<size; i++)
80: pType[i] = (T)0;
81: }
82:
83: template <class T>
84: Array<T>& Array<T>::operator=(const Array &rhs)
85: {
86: if (this == &rhs)
87: return *this;
88: delete [] pType;
89: itsSize = rhs.GetSize();
90: pType = new T[itsSize];
91: for (int i = 0; i<itsSize; i++)
92: pType[i] = rhs[i];
93: return *this;
94: }
95: template <class T>
96: Array<T>::Array(const Array &rhs)
97: {
98: itsSize = rhs.GetSize();
99: pType = new T[itsSize];
100: for (int i = 0; i<itsSize; i++)
101: pType[i] = rhs[i];
102: }
103:
104:
105: template <class T>
106: ostream& operator<< (ostream& output, const Array<T>& theArray)
107: {
108: for (int i = 0; i<theArray.GetSize(); i++)
109: output << "[" << i << "] " << theArray[i] << endl;
110: return output;
111: }
112:
113:
114: Array<Animal>::Array(int AnimalArraySize):
115: itsSize(AnimalArraySize)
116: {
117: pType = new Animal[AnimalArraySize];
118: }
119:
120:
121: void IntFillFunction(Array<int>& theArray);
122: void AnimalFillFunction(Array<Animal>& theArray);
123: enum BOOL {FALSE, TRUE};
124:
125: int main()
126: {
127: Array<int> intArray;
128: Array<Animal> animalArray;
129: IntFillFunction(intArray);
130: AnimalFillFunction(animalArray);
131: cout << "intArray...\n" << intArray;
132: cout << "\nanimalArray...\n" << animalArray << endl;
133: return 0;
134: }
135:
136: void IntFillFunction(Array<int>& theArray)
137: {
138: BOOL Stop = FALSE;
139: int offset, value;
140: while (!Stop)
141: {
142: cout << "Enter an offset (0-9) and a value. ";
143: cout << "(-1 to stop): " ;
143: cin >> offset >> value;
144: if (offset < 0)
145: break;
146: if (offset > 9)
147: {
148: cout << "***Please use values between 0 and 9.***\n";
149: continue;
150: }
151: theArray[offset] = value;
152: }
153: }
154:
155:
156: void AnimalFillFunction(Array<Animal>& theArray)
157: {
158: Animal * pAnimal;
159: for (int i = 0; i<theArray.GetSize(); i++)
160: {
161: pAnimal = new Animal(i*10);
162: theArray[i] = *pAnimal;
163: delete pAnimal;
164: }
165: }
NOTE: Line numbers have been added to
the output to make analysis easier. Line numbers will not appear in your output.
Output: 1: animal() animal() animal() Enter an offset (0-9) and a value. (-1 to stop): 0 0
2: Enter an offset (0-9) and a value. (-1 to stop): 1 1
3: Enter an offset (0-9) and a value. (-1 to stop): 2 2
4: Enter an offset (0-9) and a value. (-1 to stop): 3 3
5: Enter an offset (0-9) and a value. (-1 to stop): -1 -1
6: animal(int) Destroyed an animal...animal(int) Destroyed an animal...animal(int) Destroyed an animal...initArray...
7: [0] 0
8: [1] 1
9: [2] 2
10:
11: animal array...
12: [0] 0
13: [1] 10
14: [2] 20
15:
16: Destroyed an animal...Destroyed an animal...Destroyed an animal...
17:
<<< Second run >>>
18: animal(int) Destroyed an animal...
19: animal(int) Destroyed an animal...
20: animal(int) Destroyed an animal...
21: Enter an offset (0-9) and a value. (-1 to stop): 0 0
22: Enter an offset (0-9) and a value. (-1 to stop): 1 1
23: Enter an offset (0-9) and a value. (-1 to stop): 2 2
24: Enter an offset (0-9) and a value. (-1 to stop): 3 3
25: animal(int)
26: Destroyed an animal...
27: animal(int)
28: Destroyed an animal...
29: animal(int)
30: Destroyed an animal...
31: initArray...
32: [0] 0
33: [1] 1
34: [2] 2
35:
36: animal array...
37: [0] 0
38: [1] 10
39: [2] 20
40:
41: Destroyed an animal...
42: Destroyed an animal...
43: Destroyed an animal...
Analysis: Listing 19.6 reproduces both
classes in their entirety, so that you can see the creation and destruction of temporary
Animal objects. The value of DefaultSize has been reduced to 3
to simplify the output.
The Animal constructors and destructors on lines 33-48 each print a statement
indicating when they are called.
On lines 74-81, the template behavior of an Array constructor is declared.
On lines 114-118, the specialized constructor for an Array of Animals
is demonstrated. Note that in this special constructor, the default constructor is
allowed to set the initial value for each Animal, and no explicit assignment
is done.
The first time this program is run, the first set of output is shown. Line 1 of
the output shows the three default constructors called by creating the array. The
user enters four numbers, and these are entered into the integer array.
Execution jumps to AnimalFillFunction(). Here a temporary Animal
object is created on the heap on line 161, and its value is used to modify the Animal
object in the array on line 162. On line 163, the temporary Animal is destroyed.
This is repeated for each member of the array and is reflected in the output on line
6.
At the end of the program, the arrays are destroyed, and when their destructors
are called, all their objects are destroyed as well. This is reflected in the output
on line 16.
For the second set of output (lines 18-43), the special implementation of the
array of character constructor, shown on lines 114-118 of the program, is commented
out. When the program is run again, the template constructor, shown on lines
74-81 of the program, is run when the Animal array is constructed.
This causes temporary Animal objects to be called for each member of
the array on lines 79 and 80 of the program, and is reflected in the output on lines
18 to 20 of the output.
In all other respects, the output for the two runs is identical, as you would
expect.
Static Members and
Templates
A template can declare static data members. Each instantiation of the template
then has its own set of static data, one per class type. That is, if you add a static
member to the Array class (for example, a counter of how many arrays have
been created), you will have one such member per type: one for all the arrays of
Animals, and another for all the arrays of integers. Listing 19.7 adds a
static member and a static function to the Array class.
Listing 19.7. Using
static member data and functions with templates.
1: #include <iostream.h>
2:
3: template <class T> // declare the template and the parameter
4: class Array // the class being parameterized
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType; itsNumberArrays--; }
11:
12: // operators
13: Array& operator=(const Array&);
14: T& operator[](int offSet) { return pType[offSet]; }
15: const T& operator[](int offSet) const
16: { return pType[offSet]; }
17: // accessors
18: int GetSize() const { return itsSize; }
19: static int GetNumberArrays() { return itsNumberArrays; }
20:
21: // friend function
22: friend ostream& operator<< (ostream&, const Array<T>&);
23:
24: private:
25: T *pType;
26: int itsSize;
27: static int itsNumberArrays;
28: };
29:
30: template <class T>
31: int Array<T>::itsNumberArrays = 0;
32:
33: template <class T>
34: Array<T>::Array(int size = DefaultSize):
35: itsSize(size)
36: {
37: pType = new T[size];
38: for (int i = 0; i<size; i++)
39: pType[i] = (T)0;
40: itsNumberArrays++;
41: }
42:
43: template <class T>
44: Array<T>& Array<T>::operator=(const Array &rhs)
45: {
46: if (this == &rhs)
47: return *this;
48: delete [] pType;
49: itsSize = rhs.GetSize();
50: pType = new T[itsSize];
51: for (int i = 0; i<itsSize; i++)
52: pType[i] = rhs[i];
53: }
54:
55: template <class T>
56: Array<T>::Array(const Array &rhs)
57: {
58: itsSize = rhs.GetSize();
59: pType = new T[itsSize];
60: for (int i = 0; i<itsSize; i++)
61: pType[i] = rhs[i];
62: itsNumberArrays++;
63: }
64:
65:
66: template <class T>
67: ostream& operator<< (ostream& output, const Array<T>& theArray)
68: {
69: for (int i = 0; i<theArray.GetSize(); i++)
70: output << "[" << i << "] " << theArray[i] << endl;
71: return output;
72: }
73:
74:
75: Array<Animal>::Array(int AnimalArraySize):
76: itsSize(AnimalArraySize)
77: {
78: pType = new T[AnimalArraySize];
79: itsNumberArrays++;
80: }
81:
82: int main()
83: {
84:
85: cout << Array<int>::GetNumberArrays() << " integer arrays\n";
86: cout << Array<Animal>::GetNumberArrays();
87 cout << " animal arrays\n\n";
88: Array<int> intArray;
89: Array<Animal> animalArray;
90:
91: cout << intArray.GetNumberArrays() << " integer arrays\n";
92: cout << animalArray.GetNumberArrays();
93: cout << " animal arrays\n\n";
93:
94: Array<int> *pIntArray = new Array<int>;
95:
96: cout << Array<int>::GetNumberArrays() << " integer arrays\n";
97: cout << Array<Animal>::GetNumberArrays();
98: cout << " animal arrays\n\n";
98:
99: delete pIntArray;
100:
101: cout << Array<int>::GetNumberArrays() << " integer arrays\n";
102: cout << Array<Animal>::GetNumberArrays();
103: cout << " animal arrays\n\n";
103: return 0;
104: }
Output: 0 integer arrays
0 animal arrays
1 integer arrays
1 animal arrays
2 integer arrays
1 animal arrays
1 integer arrays
1 animal arrays
Analysis: The declaration of the Animal
class has been left out to save space. The Array class has added the static
variable itsNumberArrays on line 27, and because this data is private, the
static public accessor GetNumberArrays() was added on line 19.
Initialization of the static data is accomplished with a full template qualification,
as shown on lines 30 and 31. The constructors of Array and the destructor
are each modified to keep track of how many arrays exist at any moment.
Accessing the static members is exactly like accessing the static members of any
class: You can do so with an existing object, as shown on lines 91 and 92, or by
using the full class specification, as shown on lines 85 and 86. Note that you must
use a specific type of array when accessing the static data. There is one variable
for each type.
DO use statics with templates as needed. DO specialize template behavior
by overriding template functions by type. DO use the parameters to template
functions to narrow their instances to be type-safe.
The Standard Template
Library
A new development in C++ is the adoption of the Standard Template Library (STL).
All the major compiler vendors now offer the STL as part of their compilers. STL
is a library of template-based container classes, including vectors, lists, queues,
and stacks. The STL also includes a number of common algorithms, including sorting
and searching.
The goal of the STL is to give you an alternative to reinventing the wheel for
these common requirements. The STL is tested and debugged, offers high performance,
and is free. Most important, the STL is reusable; once you understand how to use
an STL container, you can use it in all your programs without reinventing it.
Summary
Today you learned how to create and use templates. Templates are a built-in facility
of C++, used to create parameterized types--types that change their behavior based
on parameters passed in at creation. They are a way to reuse code safely and effectively.
The definition of the template determines the parameterized type. Each instance
of the template is an actual object, which can be used like any other object--as
a parameter to a function, as a return value, and so forth.
Template classes can declare three types of friend functions: non-template, general
template, and type-specific template. A template can declare static data members,
in which case each instance of the template has its own set of static data.
If you need to specialize behavior for some template functions based on the actual
type, you can override a template function with a particular type. This works for
member functions as well.
Q&A
Q. Why use templates when macros will do?
A. Templates are type-safe and built into the language.
Q. What is the difference between the parameterized type of a template function
and the parameters to a normal function?
A. A regular function (non-template) takes parameters on which it may take
action. A template function allows you to parameterize the type of a particular parameter
to the function. That is, you can pass an Array of Type to a function, and
then have the Type determined by the template instance.
Q. When do you use templates and when do you use inheritance?
A. Use templates when all the behavior, or virtually all the behavior, is
unchanged, except in regard to the type of the item on which your class acts. If
you find yourself copying a class and changing only the type of one or more of its
members, it may be time to consider using a template.
Q. When do you use general template friend classes?
A. When every instance, regardless of type, should be a friend to this class
or function.
Q. When do you use type-specific template friend classes or functions?
A. When you want to establish a one-to-one relationship between two classes.
For example, array<int> should match iterator<int>,
but not iterator<Animal>.
Workshop
The Workshop provides quiz questions to help you solidify your understanding of
the material covered, and exercises to provide you with experience in using what
you've learned. Try to answer the quiz and exercise questions before checking the
answers in Appendix D, and make sure you understand the answers before continuing
to the next chapter.
Quiz
1. What is the difference between a template and a macro?
2. What is the difference between the parameter in a template and the parameter
in a function?
3. What is the difference between a type-specific template friend class and a
general template friend class?
4. Is it possible to provide special behavior for one instance of a template
but not for other instances?
5. How many static variables are created if you put one static member into a
template class definition?
Exercises
1. Create a template based on this List class:
class List
{
private:
public:
List():head(0),tail(0),theCount(0) {}
virtual ~List();
void insert( int value );
void append( int value );
int is_present( int value ) const;
int is_empty() const { return head == 0; }
int count() const { return theCount; }
private:
class ListCell
{
public:
ListCell(int value, ListCell *cell =
):val(value),next(cell){}
int val;
ListCell *next;
};
ListCell *head;
ListCell *tail;
int theCount;
};
2. Write the implementation for the List class (non-template)
version.
3. Write the template version of the implementations.
4. Declare three list objects: a list of Strings, a list of Cats,
and a list of ints.
5. BUG BUSTERS: What is wrong with the following code? (Assume the List
template is defined and Cat is the class defined earlier in the book.)
List<Cat> Cat_List;
Cat Felix;
CatList.append( Felix );
cout << "Felix is " <<
( Cat_List.is_present( Felix ) ) ? "" : "not " << "present\n";
HINT (this is tough): What makes Cat different from int?
6. Declare friend operator== for List.
7. Implement friend operator== for List.
8. Does operator== have the same problem as in Exercise 5?
9. Implement a template function for swap, which exchanges two variables.
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