Part IV: Inheritance in .NET framework

Painting PDF-417 2d barcode in .NET framework Part IV: Inheritance

namespace Schools { char* GraduateStudent::display() { // get description of basic student char* pFirst = Student::display(); // we ll add this text char* pSecond = -G ; // get a new string and tack second onto first char* pName = new char[strlen(pFirst) + strlen(pSecond) + 1]; strcpy(pName, pFirst); strcat(pName, pSecond); // don t forget to return the string returned by // Student::display() to the heap before passing // our new string to the caller delete pFirst; return pName; } }

The GraduateStudent version of display() concatenates a -G onto the end of whatever Student returns It begins by allocating a new character array that s large enough to handle the extra information Never assume that there s enough room in the original buffer for any extra characters to be tacked onto the end The program copies the contents of the original string into the newly allo cated array It then appends the - G The display() function must return the buffer allocated by Student::display() to the heap before continuing Forgetting to return buffers to the heap is known as a memory leak A pro gram with memory leaks executes properly at first; however, the program slows more and more as the available memory is lost to the leaks The pro gram eventually grinds to a halt Memory leaks are very difficult to find

The two classes, Student and GraduateStudent, have been separated into independent source files and included in the Schools namespace I wrote the following very simple application to invoke the two classes:

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// SeparatedMain - demonstrated an application separated // into two parts - the main() part #include <cstdio> #include <cstdlib> #include <iostream> #include graduateStudenth #include studenth using namespace std; //using namespace Schools; using Schools::GraduateStudent; int main(int nArgc, char* pszArgs[]) { Schools::Student s( Sophie Moore , 1234); cout << Student = << sdisplay() << endl; GraduateStudent gs( Greg U Waite , 5678); cout << Student = << gsdisplay() << endl; // wait until user is ready before terminating program // to allow the user to see the program results system( PAUSE ); return 0; }

The application includes both the studenth and graduateStudenth include files This gives the application access to the definition of the two classes You might notice that including graduatestudenth automatically includes studenth However, you shouldn t take it for granted; include studenth if you access the Student class directly, whether or not you include graduateStudenth The #ifndef, which you installed in studenth, will make sure that the contents of studenth are not processed twice by the C++ compiler

SeparatedMain is not a member of the Schools namespace When main() refers to the Student class, C++ does not know whether the programmer intends to use the Student found in the Schools namespace or a similarly

main() can completely specify a class without any possibility of ambiguity because Schools::Student refers specifically to the namespace and class

Alternatively, the programmer can specify her intentions at the beginning of the module: The phrase using Schools::GraduateStudent; tells C++ that any mention to GraduateStudent refers to the Schools namespace

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The programmer can gain access to all members of the Schools namespace by adding the command using namespace Schools The following version of main() builds successfully:

using namespace Schools; int main(int nArgc, char* pszArgs[]) { Student s( Sophie Moore , 1234); cout << Student = << sdisplay() << endl; GraduateStudent gs( Greg U Waite , 5678); cout << Student = << gsdisplay() << endl; // wait until user is ready before terminating program // to allow the user to see the program results system( PAUSE ); return 0; }

You began using the using namespace std statement at the beginning of the book The modules that make up the Standard C++ Library are members of the std namespace