Book Excerpt: Essential Guide to Managed Extensions in C++

Book Excerpt: Essential Guide to Managed Extensions in C++

hapter 9: Properties
You have heard about data encapsulation. Roughly speaking, what it means is that all data members should be hidden behind public interfaces. Quite often you might find yourself writing code like this:

class Student{int age_;//...public:// GetAge(){return age_;}void SetAge(int age){age_=age;}};

And then you might use class Student in the following manner:

int CompareByAge(Student*student1,Student*student2 ){return student1->GetAge()-student2->GetAge();}

What’s wrong with this code? The data encapsulation principle is obeyed?but at the price of code readability.

And this is exactly when properties are useful. A convenient syntactic notation long enjoyed by Visual Basic programmers, properties have actually been around in Visual C++ for quite some time supported by __declspec(property). However, the __declspec(property)syntax wasn’t very popular with C++ programmers.

Fortunately, properties have made their way into the .NET family of languages, including MC++. This chapter shows how to use scalar and indexed properties, explains what you can and cannot do with properties, and demonstrates how properties work.

Managed Properties: First Steps
Properties supported in MC++ are called managed properties. A property is declared using the __property keyword followed by a property method declaration.

Note that the name of property methods must start with the prefix get_ or set_. The method with the get_prefix is called a getter and the method with the set_prefix is called a setter. The name of the getter must be the same as the name of the setter except for the prefix. Here is how you would rewrite the preceding example using managed properties:

#using__gc class Student{int age_;//...public://...__property int get_Age(){return age_;}__property void set_Age(int age){age_=age;}};

Using properties is now just as simple as this:

int CompareByAge(Student*student1,Student*student2){return student1->Age -student2->Age;}

As you can see, a property can be used like a data member. In this example, the compiler replaces Age with a call to the appropriate method, which in this con-text is get_Age.

Note the keyword __gc in front of the class declaration. Managed properties can only be declared in managed classes?that is, either gc classes or value types.

Scalar and Indexed Properties
The common language runtime supports two types of properties: scalar and indexed. The MC++ compiler also supports both of them.

A scalar property is defined by a getter that does not take any parameters and a setter that takes exactly one parameter. A property is indexed if the getter takes one or more parameters and the setter takes more than one parameter.

To illustrate managed properties, let’s consider the following scenario.

Imagine that you need to design a database of students. Given a student’s name or ID, this simple database should be able to return the student’s address. Here is how you can do it:

#using using namespace System;__gc class Student{String*name_;String*address_;/*...*/public:__property String*get_Address(){return address_;}__property void set_Address(String*address ){address_=address;}/*...*/};__gc class Database{Student*students_[];int MapNameToId(String*name );public:__property String*get_Address(int id ){return students_[id ]->Address;}__property String*get_Address(String*name ){int id =MapNameToId(name );return students_[id ]->Address;}protected:__property void set_Address(int id,String *address ){students_[id ]->Address =address;}__property void set_Address(String*name,String *address ){int id =MapNameToId(name );students_[id ]-> Address =address;}};Database*OpenDatabase();

Access to a student’s address(es) is now achieved as follows:

int main(){Database*pDatabase =OpenDatabase();String*address1 =pDatabase->Address ["John "];String*address2 =pDatabase->Address [89640 ];}

In the preceding sample, method Student::get_Address takes no arguments and returns a String*. This is an example of a scalar property. In contrast, Database::get_Address takes an argument, an index. That’s why this kind of property is called indexed.

Implementing Property Access Methods
As you may have noticed, there are actually two get_Address methods defined in the class Database. One of them takes a student ID () as a parameter, whereas the other one takes String*, which demonstrates that property methods can be overloaded.

Overall, getters and setters are just regular functions. Not only can they be overloaded, they can also be declared virtual, pure virtual, or static. Property methods don’t have to have the same access level. As in the example in the preceding section, Database::get_Address is public but Database::set_Address is protected. Furthermore, a property does not have to have both a getter and a setter; having either one is enough to define a property. If property methods are declared as static, no object is needed to access the property. This is similar to how you would access a static data member, as demonstrated here:

#usingusing namespace System;void main(){//Print current directoryConsole::WriteLine(Environment::CurrentDirectory );}

This example uses the BCL class Environment, with a property called CurrentDirectory. Note that because the property is static, no instance of the class Environment is required. The same result could be achieved by calling the getter get_CurrentDirectory, which is a static member function:

void PrintDir(){//Print current directoryConsole::WriteLine(Environment::get_CurrentDirectory());}

If you are concerned about performance overhead associated with a function call when using a property?don’t worry. Property methods can be inlined, the same way as regular C++ member functions.

Parameters of Property Access Methods
Let’s go back to our class Student example presented earlier and look closer at the getter and the setter. As you can see, get_Age takes no arguments and returns an int (one could argue that unsigned char, for instance, would be more appropriate, but let’s ignore that for now).

The setter, set_Age, is a method that takes int as an argument. Note that the type of the setter parameter is the same as the return type of the getter?int. The MC++ compiler requires these two types to be identical. That understood, we could now say that the property Age has type int.

When it comes to indexed properties, keep in mind that to assign a value to the property you must pass it in the last argument of the setter. Let’s look at the class Database in the example in the section “Scalar and Indexed Properties” earlier. What if you mistakenly defined the set_Address methods with the wrong order of parameters? For example:

#using using namespace System;__gc class Database{//...protected:__property void set_Address(String *address,int id ){students_[id ]->Address =address;}__property void set_Address(String *address,String*name ){int id =MapNameToId(name );students_[id ]->Address =address;}};

In case of the first set_Address, you will get an error because the type of the setter’s last argument () doesn’t match the getter’s return type ().

The second case is far more dangerous: the code will compile but produce wrong results. We will get back to this issue in the next section. For now, just remember that the last argument of the setter is used for passing a value to the property.

A well-designed property behaves exactly as if it were a public data member. Consider an example of using our class Student, defined earlier in this chapter:

void ResetAges(Student*student1,Student*student2){return student1->Age =student2->Age =0;}

This would certainly work if Age were a public data member of class Student. However, this code gives an error:

'Student::set_Age ' ::cannot convert parameter 1 from 'void ' to 'int '

The problem lies in the return type of Student::set_Age , which is void. When properties are expanded into function calls, the compiler comes up with this:

void ResetAges(Student*student1,Student*student2){return student1->set_Age(student2->set_Age(0));}

This code doesn’t work because student2->set_Age(0)returns void and there is no conversion from void to int. The solution? Define your setter as a method returning the property type or a reference to the property type:

__property int set_Age(int age){age_=age;return age_;}

Bear in mind, however, that this approach has certain performance implications?returning a variable doesn’t come free. In some cases, the compiler can “optimize away” such overhead, but this is not always possible.

How Properties Work
When you define a property by declaring a getter or a setter, the MC++ compiler “pretends” a data member is defined. This data member is called a pseudo member because it doesn’t actually exist. The compiler replaces the pseudo member in your code by a call to the appropriate method which, depending on the context, is either the getter or the setter:

#using __gc class MyArray{//...public://...__property int get_Length();__property void set_Length(int);};void IncrementLength(MyArray*pArray ){int nLen =pArray->Length;//calls pArray->get_Length();pArray->Length =nLen +1;//calls pArray->set_Length(nLen +1);}

Remember, we warned you in the last section that the value of the property must be passed in the last argument of the setter. Now it’s time to shed some light on this situation. Here is how the compiler generates the function call for the setter: given an indexed property Address, the compiler will convert the expression

pDB->Address [S "John Smith "] ==S "Seattle ";


pDB->set_Address(S "John Smith ",S "Seattle ");

passing the value from the right side of the assignment in a last argument of the setter. That’s why if this last argument expects anything else other than the new value of the property, you will not get the result you want.What if you want to increment a property using operator++? You can do that as follows:

void IncrementLength2(MyArray*pArray ){++pArray->Length;//compiler generates://pArray->set_Length(pArray->get_Length()+1);pArray->Length++;//compiler generates://int tmp;tmp =pArray->get_Length(),//pArray->set_Length(tmp +1),tmp;}

As in unmanaged C++, the post-increment operator is less efficient than the pre-increment operator?it requires a temporary variable to hold the value of the property before the increment. Hence this advice: where you can, consider using a pre-increment operator instead of a post-increment operator.

What You Cannot Do with Properties
As we said earlier, a good property behaves like a data member. Is there anything you can do with a data member but not with a property? Unfortunately, yes. Let’s recall the earlier example with class Student. Consider the following:

#using __gc class Student{int age_;//...public://...__property int get_Age(){return age_;}__property void set_Age(int age){age_=age;}void Birthday();};void IncrementAge(int*pAge ){(*pAge)++;}void Student::Birthday(){IncrementAge(&Age );//error!}

With what you now know about properties, it should come as no surprise that this code won’t work. Still wondering why? Look at the call to IncrementAge?the function expects a parameter of type int*, but the property, Age, is provided instead. What can the compiler do? The pseudo member Age can be replaced with either get_Age or set_Age , neither of which would yield the desired result. That’s why taking address of a property is illegal?and results in a compile-time error.

There is also one restriction to overloading property methods. Examine the following code:

#using using namespace System;__gc class Product{//...};__gc class Inventory{//...public://...__property int get_ItemsSold(int ProductID);__property void set_ItemsSold(int ProductID,int value);__property int get_ItemsSold()__gc [];*///error!};void SellProduct(Inventory*pInventory,int ProductID ){pInventory->ItemsSold [ProductID ]++;}

As you can see, the method get_ItemsSold is overloaded: the first function takes one argument (), and the other one takes no arguments but returns a managed array. Now we have an ambiguity problem in the function SellProduct: how do you convert the property ItemsSold into a getter/setter function call? It is impossible to determine from the context whether either of the methods int get_ItemsSold(int)or int get_ItemsSold()__gc []should be called. So, what we want you to take away from this is an array property declaration shall not overload an indexed property.

As this chapter has shown, managed properties are easy to use because the syntax for defining them is much simpler than that of regular C++ properties. Being syntactic sugar for function calls, properties behave like data members, making code that uses them cleaner and easier to understand.

In the next chapter, we will dive into another advanced topic of MC++?operators. Like properties, operators provide a more natural way of coding by hiding function calls “under the hood.” You will see how to define and use operators and user-defined conversions for value types as well as gc classes.

Reproduced from the Essential Guide to Managed Extensions for C++, by permission of Apress. ISBN 1-893115-28-3, copyright 2002. All rights reserved.


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