228 Part III: Introduction to Classes

Avoiding the “object declaration trap”

Look again at the way the Student objects were declared in the ConstructorWDefaults example:

Student noName;

Student freshMan(“Smell E. Fish”); Student xfer(“Upp R. Classman”, 80, 2.5);

All Student objects except noName are declared with parentheses surrounding the arguments to the constructor. Why is noName declared without parentheses?

To be neat and consistent, you may think you could have declared noName as follows:

Student noName();

Unfortunately, C++ allows a declaration with only an open and close parentheses. However, it doesn’t mean what you think it does at all. Instead of declaring an object noName of class Student to be constructed with the default

constructor, it declares a function that returns an object of class Student by value. Surprise!

The following two declarations demonstrate how similar the new C++ format for declaring an object is to that of declaring a function. (I think this was a mistake, but what do I know?) The only difference is that the function declaration contains types in the parentheses, whereas the object declaration contains objects:

Student thisIsAFunc(int);

Student thisIsAnObject(10);

If the parentheses are empty, nothing can dif­ ferentiate between an object and a function. To retain compatibility with C, C++ chose to make a declaration with empty parentheses a function. (A safer alternative would have been to force the keyword void in the function case, but that would not have been compatible with existing C programs.)

Constructing Class Members

In the preceding examples, all data members are of simple types, such as int and float. With simple types, it’s sufficient to assign a value to the variable within the constructor. Problems arise when initializing certain types of data members, however.

Constructing a complex data member

Members of a class have the same problems as any other variable. It makes no sense for a Student object to have some default ID of zero. This is true even if the object is a member of a class. Consider the following example:

//

//ConstructingMembers - a class may pass along arguments

//	to the members’ constructors
//
#include <cstdio>
#include <cstdlib>

Chapter 17: Making Constructive Arguments 229

#include <iostream> #include <strings.h> using namespace std;

const int MAXNAMESIZE = 40;

int nextStudentId = 0; class StudentId

{

public:

StudentId()

{

value = ++nextStudentId;

cout << “Assigning student id “ << value << endl;

}

protected: int value;

};

class Student

{

public:

Student(char* pName)

{

strncpy(name, pName, MAXNAMESIZE); name[MAXNAMESIZE - 1] = ‘\0’; semesterHours = 0;

gpa = 0.0;

}

// ...other public members...

protected:

char name[MAXNAMESIZE]; int semesterHours; float gpa;

StudentId id;

};

int main(int argcs, char* pArgs[])

{

Student s(“Chester”);

//wait until user is ready before terminating program

//to allow the user to see the program results system(“PAUSE”);

return 0;

}

A student ID is assigned to each student as the student object is constructed. In this example, IDs are handed out sequentially using the global variable nextStudentId to keep track.

This Student class contains a member id of class StudentId. The constructor for Student can’t assign a value to this id member because Student does not

230 Part III: Introduction to Classes

have access to the protected members of StudentId. You could make Student a friend of StudentId, but that violates the “you take care of your business, I’ll take care of mine” philosophy. Somehow, you need to invoke the construc­ tor for StudentId when Student is constructed.

C++ does this for you automatically in this case, invoking the default con­ structor StudentId::StudentId() on id. This occurs after the Student constructor is called, but before control passes to the first statement in the constructor. (Single-step the preceding program in the debugger to see what I mean. As always, be sure that inline functions are forced outline.) Following is the output that results from executing this program:

assigning student id 1 constructing Student Chester Press any key to continue . . .

Notice that the message from the StudentId constructor appears before the output from the Student constructor. (By the way, with all these construc­ tors performing output, you may think that constructors must output some­ thing. Most constructors don’t output a bloody thing.)

If the programmer does not provide a constructor, the default constructor provided by C++ automatically invokes the default constructors for data members. The same is true come harvesting time. The destructor for the class automatically invokes the destructor for data members that have destructors. The C++-provided destructor does the same.

Okay, this is all great for the default constructor. But what if you want to invoke a constructor other than the default? Where do you put the object? For example, assume that a student ID is provided to the Student construc­ tor, which passes the ID to the constructor for class StudentId.

Let me first show you what doesn’t work. Consider the following program segment (only the relevant parts are included here — the entire program, ConstructSeparateID, is on the CD-ROM that accompanies this book):

class Student

{

public:

Student(char *pName = “no name”, int ssId = 0)

{

cout << “constructing student “ << pName << endl; strncpy(name, pName, MAXNAMESIZE);

name[MAXNAMESIZE - 1]	= ‘\0’;
// don’t try this at home kids. It doesn’t work
StudentId id(ssId);	// construct a student id

}

protected:

char name[MAXNAMESIZE]; StudentId id;

};

Chapter 17: Making Constructive Arguments 231

The constructor for StudentId has been changed to accept a value exter­ nally (the default value is necessary to get the example to compile, for rea­ sons that will become clear shortly). Within the constructor for Student, the programmer (that’s me) has (cleverly) attempted to construct a StudentId object named id.

If you look at the output from this program, you notice a problem:

assigning student id 0 constructing student Chester assigning student id 1234 This message from main

Press any key to continue . . .

The first problem is that the constructor for StudentId appears to be invoked twice, once with zero and a second time with the expected 1234. Then you can see that the 1234 object is destructed before the output string in main(). Apparently the StudentId object is destructed within the Student constructor.

The explanation for this rather bizarre behavior is clear. The data member id already exists by the time the body of the constructor is entered. Instead of constructing the existing data member id, the declaration provided in the constructor creates a local object of the same name. This local object is destructed upon returning from the constructor.

Somehow, you need a different mechanism to indicate “construct the existing member; don’t create a new one.” This mechanism needs to appear before the open brace, before the data members are declared. C++ provides a con­ struct for this, as shown in the following ConstructDataMembers program:

//

//ConstructDataMember - construct a data member

//	to a value other than the default
//
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <strings.h>

using namespace std;

const int MAXNAMESIZE = 40; class StudentId

{

public:

StudentId(int id = 0)

{

value = id;

cout << “assigning student id “ << value << endl;

}