Using Constructors in VS .NET

Making PDF417 in VS .NET Using Constructors

The constructor is a member function that is called automatically when an object is created Its primary job is to initialize the object to a legal initial value for the class (It s the job of the remaining member functions to ensure that the state of the object stays legal)

You could initialize an object as part of the declaration that s the way the C programmer would do it for example:

struct Student { int semesterHours; float gpa; };

TEAM LinG - Live, Informative, Non-cost and Genuine!

void fn() { Student s1 = {0, 00}; // or Student s2; s2semesterHours = 0; s2gpa = 00; // function continues }

You could outfit the class with an initialization function that the application calls as soon as the object is created Because this initialization function is a member of the class, it would have access to the protected members This solution appears as follows:

class Student { public: void init() { semesterHours = 0; gpa = 00; } // other public members protected: int semesterHours; float gpa; }; void fn() { Student s; // create the object sinit(); // then initialize it // function continues }

The only problem with this solution is that it abrogates the responsibility of the class to look after its own data members In other words, the class must rely on the application to call the init() function If it does not, the object is full of garbage, and who knows what might happen What is needed is a way to take the responsibility for calling the init() func tion away from the application code and give it to the compiler Every time an object is created, the compiler can insert a call to the special init() function to initialize it That s a constructor!

TEAM LinG - Live, Informative, Non-cost and Genuine!

The constructor is a special member function that s called automatically when an object is created It carries the same name as the class to differentiate it from the other members of the class The designers of C++ could have made up a different rule, such as: The constructor must be called init() It wouldn t have made any difference, as long as the compiler could recognize the constructor In addition, the constructor has no return type, not even void, because it is only called automatically if the constructor did return something, there would be no place to put it A constructor cannot be invoked manually

With a constructor, the class Student appears as follows:

// // Constructor - example that invokes a constructor // #include <cstdio> #include <cstdlib> #include <iostream> using namespace std; class Student { public: Student() { cout << constructing student << endl; semesterHours = 0; gpa = 00; } protected: int semesterHours; float gpa; }; int main(int nNumberofArgs, char* pszArgs[]) { cout << Creating a new Student object << endl; Student s; cout << Creating a new object off the heap << endl; Student* pS = new Student; // wait until user is ready before terminating program // to allow the user to see the program results system( PAUSE ); return 0; }

TEAM LinG - Live, Informative, Non-cost and Genuine!

At the point of the declaration of s, the compiler inserts a call to the con structor Student::Student() Allocating a new Student object from the heap has the same effect as demonstrated by the output from the program:

Creating a new Student object constructing student Creating a new object off the heap constructing student Press any key to continue

This simple constructor was written as an inline member function Constructors can be written also as outline functions, as shown here:

class Student { public: Student(); // other public members protected: int semesterHours; float gpa; }; Student::Student() { cout << constructing student << endl; semesterHours = 0; gpa = 00; }

The output from this program can prove to you that constructors work as advertised, but to get the real effect, you really should single-step this simple program in your debugger (See 10 for instructions on using the debugger) Single-step through this example until control comes to rest at the Student s declaration Select Step Into and control magically jumps to Student:: Student() Continue single-stepping through the constructor When the function has finished, control returns to the statement after the declaration In some cases, Step Into will actually execute the entire constructor without stopping You may have to set a breakpoint in the constructor to get the effect Setting a breakpoint always works