Hellowuvrld Hellowuvrld - 3 months ago 7
C++ Question

about ctors and inheritance

Im kinda new to C++.
I have an assignment where I should implement c class' default ctor and cctor so that theyll print "c::ctor" and "c::cctor" respectively.
I have no idea of how to deal with this, help would be appreciated.

#include <iostream>
#include <string>
using namespace std;

class a {
public:
a(const char* sname)
: _sname(sname) {
cout << "a::ctor" << endl;
}
a(const a& s) { cout << "a::copy ctor" << endl; }
virtual ~a() { cout << "a::dtor " << endl; }
void f1() { cout << "a::f1()" << endl; f2(); }
virtual void f2() = 0;
private:
string _sname;
};

class b1 : virtual public a {
public:
b1(const char* saname, const char* ssname)
: _sname1(saname), a(ssname) {
}
b1(const b1& b1) : a(b1) { cout << "b1 :: copy ctor << endl"; }
~b1() { cout << "b1::dtor" << endl; }
virtual void f1() { cout << "b1::f1()" << endl; }
virtual void f2() { cout << "b1::f2()" << endl; }
virtual void f3() { cout << "b1::f3()" << endl; }
private:
string _sname1;
};

class b2 : virtual public a {
public:
b2(const char* saname, const char* ssname)
: _sname2(saname), a(ssname) {
cout << "b2::ctor" << endl;
}
b2(const b2& b2) : a(b2) { cout << "b2::copy ctor" << endl; }
~b2() { cout << "b2::dtor" << endl; }
virtual void f3() { f1(); cout << "b2::f3()" << endl; }
private:
string _sname2;
};

class c : public b1, public b2 {
public:
c();
c(const c& c);
~c() { cout << "c::dtor" << endl; }
virtual void f1() { a::f1(); cout << "c::f1()" << endl; }
void f3() { cout << "c::f3()" << endl; }
};

Answer

In case of virtual inheritance, things are more complicated for the constructors and destructors. The constructors and the destructors of the virtual base class a should also be called in the c class since the compiler cannot choose between b1 or b2 to build the (unique) a part in the c object. For the destructor, the compilers handles things to call the destructor of b1, the destructor of b2 and the destructor of a.

So you should implement

class c : public b1, public b2 {
public:
  c() : b1("", ""), b2("", ""), a("") {}
  c(const c& src) : b1(src), b2(src), a(src) {}
};

In general, you should try to avoid virtual inheritance when not necessary. But it is a good exercise to understand how it works.