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Introduction to Indexed Properties

 

A Property Can Be Indexed

 

Introduction

In Lesson 25, we learned how to create an array, how to assign values to its elements, and how to get the value of each element. Here is an example:

using namespace System;

int main()
{
    array<double> ^ Numbers = gcnew array<double>(5);

    Numbers[0] = 927.93;
    Numbers[1] = 45.155;
    Numbers[2] = 2.37094;
    Numbers[3] = 73475.25;
    Numbers[4] = 186.72;

    for(int i = 0; i < Numbers->Length; i++)
        Console::WriteLine(L"Number {0}: {1}", i+1, Numbers[i]);

    Console::WriteLine();
    return 0;
}

This would produce:

Number 1: 927,93
Number 2: 45,155
Number 3: 2,37094
Number 4: 73475,25
Number 5: 186,72

Press any key to continue . . .

In the same way, if we declared an array as a member variable of a class, to access the elements of that member, we had to use an instance of the class, followed by the period operator, followed by the member variable applied with the square brackets. Instead of accessing each element through its member variable, you can create a type of property referred to as an indexer.

Practical Learning: Introducing Indexed Properties

  1. Start Microsoft Visual C++ 2005
  2. To start a new, on the main menu, click File -> New -> Project...
  3. In the Project Types, make sure Visual C++ is selected.
    In the Templates list, click CLR Empty Project
  4. Set the Name to PropertyRental1
  5. To create a new class, on the main menu, click Project -> Add Class...
  6. In the Categories, select C++.
    In the Templates list, click C++ Class and click Add
  7. Set the Name to CRentalProperty and click Finish Enter
  8. Change the file as follows:
     
    #pragma once
    
    using namespace System;
    
    public ref class CRentalProperty
    {
    public:
        long propCode;
        String ^ cond;
        short beds;
        double baths;
        double val;
    
    public:
        property long PropertyCode
        {
            long get() { return propCode; }
            void set(long value) { propCode = value; }
        }
    
        property String ^ PropertyCondition
        {
            String ^ get() { return cond; }
            void set(String ^ value) { cond = value; }
        }
    
        property short Bedrooms
        {
            short get() { return beds; }
            void set(short value) { beds = value; }
        }
    
        property double Bathrooms
        {
            double get() { return (baths <= 0) ? 0.00 : baths; }
            void set(double value) { baths = value; }
        }
    
        property double MonthlyRent
        {
            double get() { return (val <= 0) ? 0.00 : val; }
            void set(double value) { val = value; }
        }
    
    	CRentalProperty(void);
    };
  9. Access the RentalProperty.cpp file and change the constructor as follows:
     
    #include "RentalProperty.h"
    
    CRentalProperty::CRentalProperty(void)
    {
        Random ^ rnd = gcnew Random;
        propCode = rnd->Next(100000, 999999);
        cond = L"Unknown";
        beds = 0;
        baths = 0.0;
        val = 0.00;
    }
  10. To create a new class, on the main menu, click Project -> Add Class...
  11. In the Categories, make sure C++ is selected.
    In the Templates list, click C++ Class and click Finish
  12. Set the Name to CPropertyListing and click Finish
  13. Change the file as follows:
     
    #pragma once
    
    #include "RentalProperty.h"
    
    public ref class CPropertyListing
    {
    public:
        array<CRentalProperty ^> ^ props;
    
    public:
    	CPropertyListing(void);
    };
  14. Access the PropertyListing.cpp file and change the constructor as follows:
     
    #include "PropertyListing.h"
    
    CPropertyListing::CPropertyListing(void)
    {
        Random ^ rnd = gcnew Random;
        props = gcnew array<CRentalProperty ^>(40);
    
        // Create a few properties ready to be rented
        props[0] = gcnew CRentalProperty;
        props[0]->PropertyCode = rnd->Next(100000, 999999);
        props[0]->PropertyCondition = L"Excellent";
        props[0]->Bedrooms = 5;
        props[0]->Bathrooms = 3.5;
        props[0]->MonthlyRent = 2650;
    
        props[1] = gcnew CRentalProperty;
        props[1]->PropertyCode = rnd->Next(100000, 999999);
        props[1]->PropertyCondition = L"Excellent";
        props[1]->Bedrooms = 3;
        props[1]->Bathrooms = 2.5;
        props[1]->MonthlyRent = 1750;
    
        props[2] = gcnew CRentalProperty;
        props[2]->PropertyCode = rnd->Next(100000, 999999);
        props[2]->PropertyCondition = L"Good";
        props[2]->Bedrooms = 4;
        props[2]->Bathrooms = 2.5;
        props[2]->MonthlyRent = 2450;
    
        props[3] = gcnew CRentalProperty;
        props[3]->PropertyCode = rnd->Next(100000, 999999);
        props[3]->PropertyCondition = L"Excellent";
        props[3]->Bedrooms = 1;
        props[3]->Bathrooms = 1.0;
        props[3]->MonthlyRent = 880;
    
        props[4] = gcnew CRentalProperty;
        props[4]->PropertyCode = rnd->Next(100000, 999999);
        props[4]->PropertyCondition = L"Excellent";
        props[4]->Bedrooms = 3;
        props[4]->Bathrooms = 2.5;
        props[4]->MonthlyRent = 1880;
    
        props[5] = gcnew CRentalProperty;
        props[5]->PropertyCode = rnd->Next(100000, 999999);
        props[5]->PropertyCondition = L"Good";
        props[5]->Bedrooms = 2;
        props[5]->Bathrooms = 1.0;
        props[5]->MonthlyRent = 1050;
    
        // Since we don't yet have a complete list of properties
        // Create some empty ones
        for (int i = 6; i < 40; i++)
        {
            props[i] = gcnew CRentalProperty;
        }
    }
  15. To create a new file, on the main menu, click Project -> Add New Item...
  16. In the Categories, click Code
  17. In the Templates list, click C++ File (.cpp)
  18. Set the Name to Exercise and click Add
  19. Access the Program.cs file and change it as follows:
     
    using namespace System;
    
    #include "PropertyListing.h"
    
    int main()
    {
        CPropertyListing ^ properties = gcnew CPropertyListing;
        CRentalProperty ^ prop = gcnew CRentalProperty;
    
        for(int i = 0; i < 6; i++)
        {
            Console::WriteLine(L"{0}.----------------------------------",
    			i + 1);
            Console::WriteLine(L"Property #:   {0}",
    			  properties->props[i]->PropertyCode);
            Console::WriteLine(L"Condition:    {0}",
    			  properties->props[i]->PropertyCondition);
            Console::WriteLine(L"Bedrooms:     {0}",
    			  properties->props[i]->Bedrooms);
            Console::WriteLine(L"Bathrooms:    {0}",
    			  properties->props[i]->Bathrooms);
            Console::WriteLine(L"Market Value: {0}\n",
    		properties->props[i]->MonthlyRent.ToString("C"));
        }
        Console::WriteLine(L"======================================");
    
        return 0; 
    }
  20. Press Ctrl + F5 to execute the application. This would produce:
     
    1.----------------------------------
    Property #:   920119
    Condition:    Excellent
    Bedrooms:     5
    Bathrooms:    3.5
    Market Value: $2,650.00
    
    2.----------------------------------
    Property #:   587917
    Condition:    Excellent
    Bedrooms:     3
    Bathrooms:    2.5
    Market Value: $1,750.00
    
    3.----------------------------------
    Property #:   904376
    Condition:    Good
    Bedrooms:     4
    Bathrooms:    2.5
    Market Value: $2,450.00
    
    4.----------------------------------
    Property #:   421662
    Condition:    Excellent
    Bedrooms:     1
    Bathrooms:    1
    Market Value: $880.00
    
    5.----------------------------------
    Property #:   305196
    Condition:    Excellent
    Bedrooms:     3
    Bathrooms:    2.5
    Market Value: $1,880.00
    
    6.----------------------------------
    Property #:   503836
    Condition:    Good
    Bedrooms:     2
    Bathrooms:    1
    Market Value: $1,050.00
    
    ======================================
    Press any key to continue . . .
  21. Close the DOS window

An Indexed Property

An indexer, also called an indexed property, is a class's property that allows you to access a member variable of a class using the features of an array. To create an indexed property, start the class like any other. In the body of the class, create a member variable that is an array and initialize it appropriately. Here is an example:

public ref class CNumber
{
    array<double> ^ Numbers;

public:
    CNumber()
    {
	Numbers = gcnew array<double>(5);
    }
};

Then, in the body of the class, create a property named default with its accessor(s). The default property must be the same type as the member variable it will refer to. The property must take a parameter as an array. This means that it must have square brackets. Inside of the brackets, include the parameter type you will use as index to access the members of the array.

Traditionally, and as we have seen so far, you usually access the members of an array using an integer-based index. Therefore, you can use an int type as the index of the array but the index' parameter must not have a name. This would be done as follows:

public ref class CNumber
{
    array<double> ^ Numbers = gcnew double[5];

public:
    property double default[int]
    {
    }
}

If you want the property to be read-only, include only a get accessor.

When creating the get accessor, you must pass a parameter to it. The parameter must be the same type you passed to the default property, except that, as mentioned above, the parameter passed to default must not have a name but the parameter passed to get must have a name. When implementing the get accessor, it must return an element of the array member variable the property refers to, using the parameter passed to get. This would be done as follows:

public ref class CNumber
{
    array<double> ^ Numbers;

public:
    CNumber()
    {
	Numbers = gcnew array<double>(5);
    }

    property double default[int]
    {
	double get(int i) { return Numbers[i]; }
    }
};

Once you have created the indexed property, the class can be used. To start, you can declare a handle of the class. To access its arrayed member variable, you can apply the square brackets directly to it. Here is an example:

using namespace System;

public ref class CNumber
{
    array<double> ^ Numbers;

public:
    CNumber()
    {
	Numbers = gcnew array<double>(5);
        Numbers[0] = 927.93;
        Numbers[1] = 45.155;
        Numbers[2] = 2.37094;
        Numbers[3] = 73475.25;
        Numbers[4] = 186.72;
    }

    property double default[int]
    {
	double get(int i) { return Numbers[i]; }
    }
};

int main()
{
    CNumber ^ nbr = gcnew CNumber;

    for(int i = 0; i < 5; i++)
        Console::WriteLine(L"Number {0}: {1}", i + 1, nbr[i]);

    Console::WriteLine();
    return 0;
}

Indexed Properties of Other Primitive Types

In the above example, we created a property that produced double-precision values. When creating an indexed property, you will decide what type of value the property must produce or the type it can have. As opposed to an int or a double, you can also create a property that takes or produces a string. Here is an example of a string-based indexed property:

using namespace System;

public ref class CPhilosopher
{
    array<String ^> ^ phil;

public:
    property String ^ default[int]
    {
        String ^ get(int i) { return phil[i]; }
    }

    CPhilosopher()
    {
	phil = gcnew array<String ^>(8);

        phil[0] = L"Aristotle";
        phil[1] = L"Emmanuel Kant";
        phil[2] = L"Tom Huffman";
        phil[3] = L"Judith Jarvis Thompson";
        phil[4] = L"Thomas Hobbes";
        phil[5] = L"Cornell West";
        phil[6] = L"Jane English";
        phil[7] = L"James Rachels";
    }
};

int main()
{
    CPhilosopher ^ thinker = gcnew CPhilosopher;

    for(int i = 0; i < 8; i++)
        Console::WriteLine(L"Philosopher: {0}", thinker[i]);

    Console::WriteLine();
    return 0;
}

This would produce:

Philosopher: Aristotle
Philosopher: Emmanuel Kant
Philosopher: Tom Huffman
Philosopher: Judith Jarvis Thompson
Philosopher: Thomas Hobbes
Philosopher: Cornell West
Philosopher: Jane English
Philosopher: James Rachels

Press any key to continue . . .

In the same way, you can created a Boolean-based indexed property by simply making it return a bool type. Here is an example:

using namespace System;

public ref class CDrivingWhileIntoxicated
{
    array<bool>^ dwi;

public:
    property bool default[int]
    {
        bool get(int i) { return dwi[i]; }
    }

    CDrivingWhileIntoxicated()
    {
	dwi = gcnew array<bool>(7);

        dwi[0] = false;
        dwi[1] = true;
        dwi[2] = true;
        dwi[3] = false;
        dwi[5] = false;
        dwi[6] = false;
    }
};

int main()
{
    CDrivingWhileIntoxicated ^ driving = gcnew CDrivingWhileIntoxicated;

    Console::WriteLine(L"Police Report");
    Console::WriteLine(L"-------------------------------");
    for(int i = 0; i < 7; i++)
            Console::WriteLine(L"Driver Was Intoxicated: {0}", driving[i]);

    Console::WriteLine();
    return 0;
}

This would produce:

Police Report
-------------------------------
Driver Was Intoxicated: False
Driver Was Intoxicated: True
Driver Was Intoxicated: True
Driver Was Intoxicated: False
Driver Was Intoxicated: False
Driver Was Intoxicated: False
Driver Was Intoxicated: False

Press any key to continue . . .

Using a Non-Integer-Based Index

In Lesson 25, we saw how to create different arrays that are numeric or string based. Here is an example of a float array:

using namespace System;

int main()
{
    array<float>^  ages = gcnew array<float>(5);

    ages[0] = 14.50f;
    ages[1] = 12.00f;
    ages[2] = 16.50f;
    ages[3] = 14.00f;
    ages[4] = 15.50f;

    Console::WriteLine(L"Student Age: {0}", ages[2]);
    Console::WriteLine();
    return 0;
}

When we think of arrays, we usually consider passing an integer-based parameter to the square brackets of the variable, as done for the above ages array:

array<float>^  ages = gcnew array<float>(5);

When using an indexed property, you can use almost any type of index, such as a real value or a string. To do this, in the square brackets of the default property, pass the desired type as the index. Here is an example:

public ref class CStudentAge
{
    property float default[string]
    {
    }
};

When defining the indexed property, there are two rules you must follow and you are aware of them already because an indexed property is like a method that takes a parameter and doesn't return void. Therefore, when implementing an indexed property, make sure you return the right type of value in the get accessor and make sure you pass the appropriate index to the return value of the default property. Here is an example:

public ref class CStudentAge
{
public:
    property float default[String ^]
    {
        float get(String ^ name)
        {
            if(  name == L"Ernestine Jonas" )
                return 14.50f;
            else if( name == L"Paul Bertrand Yamaguchi" )
                return 12.50f;
            else if( name == L"Helene Jonas" )
                return 16.00f;
            else if( name == L"Chrissie Hanson" )
                return 14.00f;
            else if( name == L"Bernard Hallo" )
                return 15.50f;
            else
                return 12.00f;
        }
    }
};

Once you have defined the property, you can use it. To access any of its elements, you must pass the appropriate type of index. In this case, the index must be passed as a string and not an integer. You can then do whatever you want with the value produced by the property. For example, you can display it to the user. Here is an example:

using namespace System;

public ref class CStudentAge
{
public:
    property float default[String ^]
    {
        float get(String ^ name)
        {
            if(  name == L"Ernestine Jonas" )
                return 14.50f;
            else if( name == L"Paul Bertrand Yamaguchi" )
                return 12.50f;
            else if( name == L"Helene Jonas" )
                return 16.00f;
            else if( name == L"Chrissie Hanson" )
                return 14.00f;
            else if( name == L"Bernard Hallo" )
                return 15.50f;
            else
                return 12.00f;
        }
    }
};

int main()
{
    CStudentAge ^ sa = gcnew CStudentAge;
    float age = sa[L"Paul Bertrand Yamaguchi"];

    Console::WriteLine(L"Student Age: {0}", age);
    Console::WriteLine();
    return 0;
}

This would produce:

Student Age: 12.5

Press any key to continue . . .

You can also pass an enumeration as an index. To do this, after defining the enumerator, type its name in the square brackets of the default member, then define the property as you see fit. To access the property outside, apply an enumeration member to the square brackets on an instance of the class. Here is an example:

using namespace System;

public enum CategoryFee
{
    Children,
    Adult,
    Senior,
    Unknown
};

public ref class CGolfClubMembership
{
    array<double> ^ fee;

public:
	CGolfClubMembership()
    {
		fee = gcnew array<double>(4);
        fee[0] = 150.95;
        fee[1] = 250.75;
        fee[2] = 85.65;
        fee[3] = 350.00;
    }

    property double default[CategoryFee]
    {
        double get(CategoryFee cat)
        {
            if (cat == CategoryFee::Children)
                return fee[0];
            else if (cat == CategoryFee::Adult)
                return fee[1];
            else if (cat == CategoryFee::Senior)
                return fee[2];
            else
                return fee[3];
        }
    }
};

int main()
{
    CGolfClubMembership ^ mbr = gcnew CGolfClubMembership;

    Console::WriteLine(L"Membership Fee: {0}",
		mbr[CategoryFee::Senior]);

    Console::WriteLine();
    return 0;
}

This would produce:

Membership Fee: 85.65

Press any key to continue . . .

Practical Learning: Creating an Indexer

  1. To create an indexed propertyr, access the PropertyListing.h header file and change it as follows:
     
    #pragma once
    
    #include "RentalProperty.h"
    
    public ref class CPropertyListing
    {
    public:
        array<CRentalProperty ^> ^ props;
    
        property String ^ default[long]
        {
            String ^ get(long code)
    	{
                for(int i = 0; i < props.Length; i++)
                    if( code == props[i]->PropertyCode )
                        return L"Property #:   " + props[i]->PropertyCode + 
                                   L"\nCondition:    " + props[i]->PropertyCondition +
                                   L"\nBedrooms:     " + props[i]->Bedrooms + 
                                   L"\nBathrooms:    " + props[i]->Bathrooms +
                                   L"\nMonthly Rent: " + props[i]->MonthlyRent.ToString("C");
                return L"Unidentifiable Property";
    	}
        }
    
    public:
    	CPropertyListing(void);
    };
  2. Access the Exercise.cpp file and change it as follows:
     
    using namespace System;
    
    #include "PropertyListing.h"
    
    int main()
    {
        CPropertyListing ^ properties = gcnew CPropertyListing;
        long lngCode;
    
        Console::WriteLine(L"Here is a list of our properties by code");
        for(int i = 0; i < 6; i++)
            Console::WriteLine(L"Property Code: {0}",
    		properties->props[i]->PropertyCode);
    
        try
        {
    	Console::Write(L"Enter Property Code: ");
    	lngCode = long::Parse(Console::ReadLine());
    
            Console::WriteLine(L"======================================");
            Console::WriteLine(L"Property Information");
            Console::WriteLine(L"--------------------------------------");
            Console::WriteLine(properties[lngCode]);
            Console::WriteLine(L"======================================");
        }
        catch(FormatException ^)
        {
            Console::WriteLine(L"=- Invalid Property Code -=");
        }
    
        return 0; 
    }
  3. Press Ctrl + F5 to execute the application. Here is an example:
     
    Here is a list of our properties by code
    Property Code: 355443
    Property Code: 653004
    Property Code: 800118
    Property Code: 839375
    Property Code: 148561
    Property Code: 697001
    Enter Property Code: 697001
    ======================================
    Property Information
    --------------------------------------
    Property #:   697001
    Condition:    Good
    Bedrooms:     2
    Bathrooms:    1
    Monthly Rent: $1,050.00
    ======================================
    Press any key to continue . . .
  4. Close the DOS window

Topics on Indexed Properties

 

Multi-Parameterized Indexed Properties

The indexed properties we have used so far were taking only one parameter. You can create an indexed property whose array uses more than one dimension. To start an indexed property that would use various parameters, first declare the array. After declaring the array, create a default property that takes the parameters. Here is an example for an indexed property that relates to a two-dimensional array:

public ref class CNumbers
{
    array<double, 2> ^ nbr;

public:
    property double default[int, int]
    {      
    }
};

In the body of an accessor (get or set), use the parameter as appropriately as you see fit. At a minimum, for a get accessor, you can return the value of the array using the parameters based on the rules of a two-dimensional array. This can be done as follows:

public ref class CNumbers
{
    array<double, 2> ^ nbr;

public:
    property double default[int, int]
    {
        double get(int i, int j) { return nbr[i,j]; }
    }
};

After creating the property, you can access each element of the array by applying the square brackets to an instance of the class. Here is an example:

using namespace System;

public ref class CNumbers
{
    array<double, 2> ^ nbr;

public:
	property double default[int, int]
    {
        double get(int i, int j) { return nbr[i,j]; }
    }

    CNumbers()
	{
		nbr = gcnew array<double, 2>(2, 4);

        nbr[0, 0] = 927.93;
        nbr[0, 1] = 45.155;
        nbr[0, 2] = 2.37094;
        nbr[0, 3] = 73475.25;
        nbr[1, 0] = 186.72;
        nbr[1, 1] = 82.350;
        nbr[1, 2] = 712734.95;
        nbr[1, 3] = 3249.0057;
	}
};

int main()
{
    CNumbers ^ nbrs = gcnew CNumbers;

    for (int i = 0; i < 2; i++)
    {
        for (int j = 0; j < 4; j++)
        {
            double value = nbrs[i, j];
            Console::WriteLine(L"Number [{0}][{1}]: {2}", i, j, value);
        }

        Console::WriteLine();
    }

    return 0;
}

Remember that one of the most valuable features of an indexed property is that, when creating it, you can make it return any primitive type and you can make it take any parameter of your choice. Also, the parameters of a multi-parameter indexed property don't have to be the same type. One can be a character while the other is a bool type; one can be a double while the other is a short, one can be an integer while the other is a string. When defining the property, you must apply the rules of both the methods and the arrays. Here is an example of a property that takes an integer and a string:

using namespace System;

public ref class CCatalog
{
    array<long> ^ nbrs;
    array<String ^> ^  names;

public:
	property double default[long, String ^]
    {
        double get(long nbr, String ^ name)
        {
            if( (nbr == nbrs[0]) && (name == names[0]) )
                return 275.25;
            else if( (nbr == nbrs[1]) && (name == names[1]) )
                return 18.75;
            else if( (nbr == nbrs[2]) && (name == names[2]) )
                return 50.00;
            else if( (nbr == nbrs[3]) && (name == names[3]) )
                return 65.35;
            else if( (nbr == nbrs[4]) && (name == names[4]) )
                return 25.55;
            else
                return 0.00;
        }
    }

    CCatalog()
    {
        nbrs = gcnew array<long>(5);

        nbrs[0] = 273974;
        nbrs[1] = 539759;
        nbrs[2] = 710234;
        nbrs[3] = 220685;
        nbrs[4] = 192837;

        names = gcnew array<String ^>(5);

        names[0] = L"Women Double-faced wool coat";
        names[1] = L"Men Cotton Polo Shirt";
		names[2] = L"Children Cable-knit Sweater";
		names[3] = L"Women Floral Silk Tank Blouse";
        names[4] = L"Girls Jeans with Heart Belt";
    }
};

int main()
{
    CCatalog ^ cat = gcnew CCatalog;

    long itemNumber = 539759;
    String ^ itemDescription = L"Men Cotton Polo Shirt";
    double price = cat[itemNumber, itemDescription];

    Console::WriteLine(L"Item #:      {0}", itemNumber);
    Console::WriteLine(L"Description: {0}", itemDescription);
    Console::WriteLine(L"Unit Price:  {0}", price);

    Console::WriteLine();
    return 0;
}

This would produce:

Item #:      539759
Description: Men Cotton Polo Shirt
Unit Price:  18.75

Press any key to continue . . .

In the above example, we first declared the variables to be passed as parameters to the indexed property. You can also pass the parameter(s) directly to the instance of the class. Here is an example:

int main()
{
    CCatalog ^ cat = gcnew CCatalog;

    double price = cat[220685, L"Women Floral Silk Tank Blouse"];

    Console::WriteLine(L"Item #:      220685");
    Console::WriteLine(L"Description: {Women Floral Silk Tank Blouse");
    Console::WriteLine(L"Unit Price:  {0}", price);

    Console::WriteLine();
    return 0;
}

Just as you can create a two-dimensional indexed property, you can also create a property that takes more than two parameters. Once again, it is up to you to decide what type of parameter would be positioned where in the square brackets. Here is an example of an indexed property that takes three parameters:

using namespace System;

public ref class CCatalog
{
public:
    property String ^ default[long, String ^, double]
    {
        String ^ get(long nbr, String ^ name, double price)
        {
            return L"Item #:      " + nbr.ToString() + L"\n" +
                   L"Description: " + name + "\n" +
                   L"Unit Price:  " + price.ToString(L"C");
        }
    }
};

int main()
{
    CCatalog ^ cat = gcnew CCatalog;

    Console::WriteLine(L"Item Description");
    Console::WriteLine(cat[220685, L"Women Floral Silk Tank Blouse", 50.00]);

    Console::WriteLine();
    return 0;
}

Overloading an Indexed Property

An indexed property borrows various characteristics of a method. One of them is the ability to create various indexers in the same class but all of them must have the same name: default. Still, the various indexed properties of a class can return the same type of value. Because of this, when creating the indexed properties, you must find a way to distinguish them. One way you can do this, as seen with function or method overloading, consists of passing a different type of parameter to each indexed property. This is referred to as overloading.

To overload the default property, if two indexed properties take only one parameter, each must take a different (data) type of parameter than the other. Here is an example:

using namespace System;

public ref class CStudentIdentifications
{
    array<int> ^ studentIDs;
    array<String ^> ^  fullnames;

    // This property takes a student ID, as an integer,
    // and it produces his/her name
public:
    property String ^ default[int]
    {
        String ^ get(int i)
        {
            for(int i = 0; i < studentIDs->Length; i++)
                if( i == studentIDs[i] )
                    return fullnames[i];

            return L"Unknown Student";
        }
    }

    // This property takes a student name, as a string,
    // and it produces his/her student ID
    property int default[String ^]
    {
        int get(String ^ name)
        {
            for(int i = 0; i < fullnames->Length; i++)
                if( name == fullnames[i] )
                    return studentIDs[i];

            return 0;
        }
    }

    CStudentIdentifications()
    {
        studentIDs = gcnew array<int>(6);

        studentIDs[0] = 39472;
        studentIDs[1] = 13957;
        studentIDs[2] = 73957;
        studentIDs[3] = 97003;
        studentIDs[4] = 28947;
        studentIDs[5] = 97395;

        fullnames = gcnew array<String ^>(6);

        fullnames[0] = L"Paul Bertrand Yamaguchi";
        fullnames[1] = L"Ernestine Ngovayang";
        fullnames[2] = L"Patricia L Katts";
        fullnames[3] = L"Helene Mukoko";
        fullnames[4] = L"Joan Ursula Hancock";
        fullnames[5] = L"Arlette Mimosa";
    }
};

int main()
{
    CStudentIdentifications ^ std = gcnew CStudentIdentifications;

    Console::WriteLine(L"Student Identification");
    Console::WriteLine(L"Student ID: 39472");
    Console::WriteLine(L"Full Name:  {0}\n", std[39472]);

    Console::WriteLine(L"Student Identification");
    Console::WriteLine(L"Full Name:  Joan Ursula Hancock");
    Console::WriteLine(L"Student ID: {0}\n", std[L"Joan Ursula Hancock"]);

    return 0;
}

This would produce:

Student Identification
Student ID: 39472
Full Name:  Paul Bertrand Yamaguchi

Student Identification
Full Name:  Joan Ursula Hancock
Student ID: 28947

Press any key to continue . . .

An indexed property combines the features of an array and those of a method that takes one or more parameters. As an array, an indexed property can use one or more dimensions as we have seen so far. Borrowing the features of a method, an indexed property can take one or more parameters and it can return a value. Besides passing different types of parameters to various indexed properties, you can create some of them that take more than one parameter. Here is an example:

using namespace System;

public ref class CCatalog
{
    array<long> ^ nbrs;
    array<String ^> ^ names;
    array<double> ^ prices;

public:
    // This property produces the name of an item, as a string,
    // if it is given the item #, as a number
    property String ^ default[long]
    {
        String ^ get(long nbr)
        {
            for (int i = 0; i < nbrs.Length; i++)
                if (nbr == nbrs[i])
                    return names[i];

            return L"Unknown Item";
        }
    }

    // This property produces the price of the item, as a number,
    // if it is given the item name and its number
    property double default[String ^, long]
    {
        double get(String ^ name, long nbr)
        {
            for (int i = 0; i < 5; i++)
                if ((nbr == nbrs[i]) && (name == names[i]))
                    return prices[i];

            return 0.00;
        }
    }

    CCatalog()
    {
        nbrs = gcnew array<long>(5);
        nbrs[0] = 273974;
        nbrs[1] = 539759;
        nbrs[2] = 710234;
        nbrs[3] = 220685;
        nbrs[4] = 192837;

        names = gcnew array<String ^>(5);
        names[0] = L"Women Double-faced wool coat";
        names[1] = L"Men Cotton Polo Shirt";
        names[2] = L"Children Cable-knit Sweater";
        names[3] = L"Women Floral Silk Tank Blouse";
        names[4] = L"Girls Jeans with Heart Belt";

        prices = gcnew array<double>(5);
        prices[0] = 275.25;
        prices[1] = 18.75;
        prices[2] = 50.00;
        prices[3] = 65.35;
        prices[4] = 25.55;
    }
};

int main()
{
    CCatalog ^ cat = gcnew CCatalog;

    Console::WriteLine(L"Item Identification");
    Console::WriteLine(L"Item #:      539759");
    Console::WriteLine(L"Unit Price:  {0}\n", cat[539759]);

    Console::WriteLine(L"Item Identification");
    Console::WriteLine(L"Item #:      192837");
    Console::WriteLine(L"Description: Girls Jeans with Heart Belt");
    Console::WriteLine(L"Unit Price:  {0}\n",
            cat["Girls Jeans with Heart Belt", 192837]);

    return 0;
}

This would produce:

Item Identification
Item #:      539759
Unit Price:  Men Cotton Polo Shirt

Item Identification
Item #:      192837
Description: Girls Jeans with Heart Belt
Unit Price:  25.55

Press any key to continue . . .

Read/Write Indexed Properties

 

Introduction

So far, we have purposely used indexed properties that only produced a value. This type is used when you are (always) in charge of specifying the values of the elements of the array: the only action you would expect from the user is to retrieve the value of the property. This is referred to as a read-only property. In some cases, you will use a property created by someone else. You may want to specify the value of an element of the array. In the same way, you may create an indexed property and you want the users of that property to be able to specify the value of the array.

If you want an indexed property to be read/write, besides the get accessor as we have been using it so far, you should also include a set accessor.

A Read/Write Property of a Primitive Type

To create a read/write indexed property, you must include a set accessor for the property. When creating the set accessor, pass two parameters. The first should be an integer type that would represent the index of the item. The second parameter is used to represent the value that will be assigned the the property. Here is an example of how this can be done:

public ref class CNumbers
{
    array<double> ^ Nbrs;

public:
    property double default[int]
    {
        double get(int i) { return Nbrs[i]; }
        void set(int i, double value) { Nbrs[i] = value; }
    }

    CNumbers()
    {
	Nbrs = gcnew array<double>(5);
    }
};

After creating the read/write property, you can assign its values outside of the class. In other words, clients of the class can change the values to its elements. Remember that the advantage of an indexed property is that each element of the arrayed member variable can be accessed from the instance of the class by directly applying the square brackets and the (appropriate) index to it. Here is an example:

using namespace System;

public ref class CNumbers
{
    array<double> ^ Nbrs;

public:
    property double default[int]
    {
        double get(int i) { return Nbrs[i]; }
        void set(int i, double value) { Nbrs[i] = value; }
    }

    CNumbers()
    {
	Nbrs = gcnew array<double>(5);
    }
};

int main()
{
    CNumbers ^ values = gcnew CNumbers;
        
    values[2] = 249.37094;

    for (int i = 0; i < 5; i++)
            Console::WriteLine(L"Number {0}: {1}", i + 1, values[i]);

    Console::WriteLine();
    return 0;
}

This would produce:

Number 1: 0
Number 2: 0
Number 3: 249.37094
Number 4: 0
Number 5: 0

Press any key to continue . . .

We saw that the index of a property could be a value other than an integer-based. For example, we created an index that was a string type. For such a property, if you make it read/write, you can assign its values outside of the class. Here is an example of a read/write string-based indexed property:

using namespace System;

public ref class CPhilosopher 
{
    array<String ^> ^ phil;

public:
    property String ^ default[int]
    {
        String ^ get(int i) { return phil[i]; }
        void set(int i, String ^ value) { phil[i] = value; }
    }

    CPhilosopher()
    {
	phil = gcnew array<String ^>(8);
    }
};

int main()
{
    CPhilosopher ^ thinker = gcnew CPhilosopher;

    thinker[5] = L"Stuart Rachels";

    for(int i = 0; i < 8; i++)
        Console::WriteLine(L"Philosopher: {0}", thinker[i]);

    Console::WriteLine();
    return 0;
}

This would produce:

Philosopher:
Philosopher:
Philosopher:
Philosopher:
Philosopher:
Philosopher: Stuart Rachels
Philosopher:
Philosopher:

Press any key to continue . . .

The same rules would apply to a read/write indexed property that can receive Boolean or double values.


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