Virtual inheritance - gcc vs. vc++

Question

Hi all!

I have a problem with Visual Studio 2008 concerning virtual inheritance.

Consider the following example:

#include<iostream>

class Print {
    public:

    Print (const char * name) {
        std::cout << name << std::endl;
    }
};

class Base : public virtual Print {
    public:

    Base () : Print("Base") {}
};

class A : public Base {
    public:

    A () : Print("A") {}
};

class B : public A {
    public:

    B () : Print("B") {}
};

int main (int argc, char** argv) {
    A a; // should print "A"
    B b; // should print "B"
    return 0;
}

This code compiles just fine if I use gcc on my linux machine. But if I try to build the same on windows with Visual Studio, the compilation fails with error message "error C2614: 'B': illegal member initialization: 'Print' is not a base or member."

Why does this not work?

Answer 1

From the standard [class.base.init]: "Unless the mem-initializer-id names a nonstatic data member of the constructor’s class or a direct or virtual base of that class, the mem-initializer is ill-formed."

Evidently gcc interprets your case as legal as Print is a non-direct, but virtual base of B, however MSVC 2008 doesn't see Print as a virtual base of B - only a virtual base of a non-virtual base. (Incidentally, your example compiles on VS2005, so it's a surprising change in behaviour.)

I'd lean towards the gcc interpretation as being correct (otherwise the phrasing 'a direct base' would have been sufficient).

To work around this problem, you could derive B virtually from Print. As Print is already a virtual base of A this doesn't have any overall effect on the class layout or the number of base class Print sub-objects.

Alternatively, you could add pass through constructors to Base and A that take a const char* and pass the constructor parameter through to the Print constructor.

Answer 2

What version of Visual Studio are you using? The code you posted works fine for me with VC 9 (specifically 15.00.21022.08), and VC 6 as well as a few other compilers.

Are you sure that by mistake class B doesn't look like this in Visual Studio:

class B {  //  note: no base class
    public:

    B () : Print("B") {}
};