CODEWITHSUNDEEP
c++classc++11forward-declaration
visitor
visitor

Reputation: 702

Behaviour of member functions when forward declaring classes

When the following code is compiled with C++11, it behaves as expected.

class Student;

class University
{
    vector <Student*> students;
public:
    University();
    void print();
};

class Student
{
    string name;
public:
    Student(string nom) : name(nom) {}
    friend ostream& operator << (ostream& out, Student* S)
    {
        return out << S -> name;
    }
};

University::University()
{
    for (string name: {"Alice", "Bob"})
        students.push_back(new Student(name));
}

void University::print() { for (auto s: students) cout << s << '\n'; }

int main()
{
    University uni;
    uni.print();
}

The printout is

Alice
Bob

However, when the print() function is implemented inside its class declaration like this:

class University
{
    vector <Student*> students;
public:
    University();
    void print() { for (auto s: students) cout << s << '\n'; }
};

The printout becomes

0x20055ff0
0x20056028

Question: Why does the print() function behave like this even though Student has been declared in the very first line?

Solution: Declare all classes before implementing any of their member functions. So long as print() and operator << are both implemented after their classes' declaration, print() will work correctly even if it's implemented before operator <<

Upvotes: 1

Views: 97

Answers (1)

Christian.M
Christian.M

Reputation: 173

The default behavior of printing a pointer with operator<< is to print the hex representation of the memory address being pointing at.

When the implementation of print() is inside the class declaration, the compiler has not yet seen the operator<< override for a Student* pointer, so it doesn't know that it should behave any differently than the default.

When you define the print() implementation after the operator<< override, the compiler knows to use that override instead of the default.

Upvotes: 6

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