17: Making Constructive Arguments in Visual Studio .NET

Generator PDF-417 2d barcode in Visual Studio .NET 17: Making Constructive Arguments

As far as C++ is concerned, every class must have a constructor; otherwise, you can t create objects of that class If you don t provide a constructor for your class, C++ should probably just generate an error, but it doesn t To pro vide compatibility with existing C code, which knows nothing about con structors, C++ automatically provides a default constructor (sort of a default default constructor) that sets all the data members of the object to binary zero Sometimes I call this a Miranda constructor you know, if you cannot afford a constructor, a constructor will be provided for you If you define a constructor for your class, any constructor, C++ doesn t pro vide the automatic default constructor (Having tipped your hand that this isn t a C program, C++ doesn t feel obliged to do any extra work to ensure compatibility) The result is that if you define a constructor for your class but you also want a default constructor, you must define it yourself Some code snippets help demonstrate this point The following is legal:

class Student { // all the same stuff as before but no constructors }; int main(int argcs, char* pArgs[]) { Student noName; return 0; }

The following code snippet does not compile properly:

class Student { public: Student(char *pName); }; int main(int argcs, char* pArgs[]) { Student noName; return 0; }

The seemingly innocuous addition of the Student(char*) constructor pre cludes C++ from automatically providing a Student() constructor with which to build object noName

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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, 25);

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);

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

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)

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

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>

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