c++ - Why use an initialization method instead of a constructor?

Question

I just got into a new company and much of the code base uses initialization methods instead of constructors.

struct MyFancyClass : theUberClass
{
    MyFancyClass();
    ~MyFancyClass();
    resultType initMyFancyClass(fancyArgument arg1, classyArgument arg2, 
                                redundantArgument arg3=TODO);
    // several fancy methods...
};

They told me that this had something to do with timing. That some things have to be done after construction that would fail in the constructor. But most constructors are empty and I don't really see any reason for not using constructors.

So I turn to you, oh wizards of the C++: why would you use an init-method instead of a constructor?

Answer 1

Since they say "timing", I guess it's because they want their init functions to be able to call virtual functions on the object. This doesn't always work in a constructor, because in the constructor of the base class, the derived class part of the object "doesn't exist yet", and in particular you can't access virtual functions defined in the derived class. Instead, the base class version of the function is called, if defined. If it's not defined, (implying that the function is pure virtual), you get undefined behavior.

The other common reason for init functions is a desire to avoid exceptions, but that's a pretty old-school programming style (and whether it's a good idea is a whole argument of its own). It has nothing to do with things that can't work in a constructor, rather to do with the fact that constructors can't return an error value if something fails. So to the extent that your colleagues have given you the real reasons, I suspect this isn't it.