Question about assigning default constructor to *this in C++?

Question

I am reading some C++ text. In an example, the text written:

class Student {
     int no;
     char grade[M+1];
 public:
     Student();
     Student(int, const char*);
     const Student& set(int, const char*);
     void display() const;
 };

Student::Student() {
    no = 0;
    grade[0] = '\0';
 }

 Student::Student(int n, const char* g) { 
     *this = Student(); // initialize to empty
     set(n, g);         // validate, reset if ok
 }

I don't understand this line: *this = Student();

Why do we have to do that, while just calling Student(); also makes the default constructor invoked? Thanks

Answer 1

It's not possible to call the default constructor directly (C++ FAQ). ie.

Student::Student(int n, const char* g){
Student();
set(n, g); // validate, reset if ok
}

doesn't work. However I'm not sure about the solution you have, either.

*this = Student()

will call Student::operator=(const Student&). In this particular class it's OK (that function is the default member copy) but it might not be in general, because the Student object is only 'partly constructed' when that method is called.

Better to have a private init function

void Student::init() {
 no = 0;     
 grade[0] = '\0';
}

and call it from both constructors.

Answer 2

Although, imho, it would be better to use common initialization function, something like this:

class Student {
    (...)
    private:
        void initMe();
 };

 Student::Student() {
    initMe();
 }

 Student::Student(int n, const char* g) { 
     initMe(); // initialize to empty
     set(n, g);         // validate, reset if ok
 }


void Student::initMe() {
    no = 0;
    grade[0] = '\0';
 }

That would avoid unnecessary creation of objects.