Bits & Bytes

UML Class Diagrams – part 3

Continued . . .

So far, we have modeled our classes with considerable detail, but there is still more that we can show. C++ provides several levels of visibility for variables and operations. The three primary visibility designations are public, protected, and private. Additionally, variables can have package level visibility, which means that they can only be accessed within the file in which they are defined. Package visibility is seldom used in C++, but is common in C.

The first three visibility settings are commonly used in most object-oriented languages. The public setting means that the member function or variable can be accessed by anything that can access the main object. The protected setting means that the member can only be accessed by functions from the class that the member is declared in and all functions of classes that inherit the class. Finally, private visibility means that the member can only be accessed by functions within the class where the member is declared.

The C++ class below shows the header for a car class, called CCar. This class has three member functions and three member variables, each with a different visibilty setting. In C++, members of a class have private visibility by default. Otherwise, they have the visibility of the last specification that is above them in the class. So, for example, mdPosition has protected visibility. For clarity, we have listed the visibility specifiers for the class in the order of decreasing visibility, from public to private.

class CCar
    void Accelerate();
    double mdTopSpeed;
    double GetSpeed();
    double mdPosition;
    bool HasAntilock();
    double mdSpeed;

This car class can be modeled by the UML class diagram shown below. The visibility of a member is specified by a symbol that precedes the variable or function name. We use +, #, and for public, protected, and private, respectively. So, we see that in the UML diagram, we have one variable and function with each type of visibilty.

Finally, we remark that we can have variables with package visibility. This is designated by a ~ symbol. Package visibility is more common in other programming languages like Java, but has been passed down to C++ from C. To declare a package level variable, we declare the variable in global scope and precede the declaration by the word static, like this

static int i;

Of course, these visibility specifiers are not strictly meaningful in C++. In C++, we can break the encapsulation of a visibility specifier via the friend specification, but that is uncommon and can be modeled by other means.

To be continued . . .

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By: Michael Hall

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