OzOz

Hi all. I'm working on improving my C++, and I've just come across something that is really puzzling me.

I have the following class hierarchy:

class A
{
public:
A(Stuff *pStuff);
};


class AA : public A
{
public:
AA(Stuff *pStuff);
};


class AAA : public AA
{
public:
AAA(Stuff *pStuff);
};


Constructors for AA and AAA call their base class constructors through their initialiser lists:

AA::AA(Stuff *pStuff)
: A(pStuff)
{
...
}


AAA::AAA(Stuff *pStuff)
: AA(pStuff)
{
...
}


When I throw this at the compiler (Microsoft Visual Studio 2005), it rejects it at AAA::AAA, as it says it can't find a default constructor for A. This is what puzzles me.

My definition of AAA::AAA tells the compiler which constructor I'm using for AA - but shouldn't THAT tell it which constructor I want to use for A?

I've found two ways to make the compiler happy, but I don't understand things well enough to be all that comfortable yet with either method (one, because I simply don't understand at all what's going on, and the other because I'm not sure which way to order things in the initialiser list):

Method 1 - Include a declaration for the default constructor in A:

class A
{
public:
A(Stuff *pStuff);
A(void){};
};

As far as I can tell by stepping through this solution, A(void) never actually gets called. Why does this keep the compiler happy?

Method 2 - Expand the initialiser list for AAA to explicitly call A(Stuff *pStuff).

AAA::AAA(Stuff *pStuff)
: A(pStuff), AA(pStuff)
{
...
}

If I adopt this solution, is this the right way to order the initialiser list? Does it matter which way the list is ordered?

__________________
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spectre

I'm not entirely sure about this, but I think the problem is that you're assuming that "class AA : public A" and "class AAA : public AA" means that "class AAA : public A". I believe that class A's member functions, including constructors, are either private or protected. You need to either make class A's constructor public, or declare class A to be a friend of class AAA. I can only guess at this since you didn't include the constructor for class A.

This is, unless you're talking about the whether or not to include the declaring the constructor in the header file, in which case, definitely include it in the header file and make sure to include the header in the AAA class (making sure to use ifdef/ifndef statements as well).

__________________
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No goddamn slave
I will be different" ~ Machine Head

OzOz

My understanding is that public inheritance (as in this case) makes the base class's public and protected members public and protected, respectively, for the derived class as well, so it will carry on down the inheritance chain.

__________________
Maybe you should put some shorts on or something, if you wanna keep fighting evil today.

Daemon1313

I'm not completely sure of what you are trying to do but I'll address what I believe is the root problem and see if that answers the question.

Objects are constructed in the opposite order of derivation. In your case if you removed everything from your initializer list; the constructor for A is called, then AA and finally AAA.

So when the compiler tries to build AAA; since you did not tell it how to contrust A, it looks for a default constructor that dosn't exist.

As far as initialization lists goes, the values normally are set in the order they appear in class and not the order in the list.

If that dosn't clear it up, let me know and I'll try to explain it in a different way.

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OzOz

I've figured out what the problem was.

The actual class hierarchy was more involved than what was portrayed here (although I could not see at the time how that could be a problem, which is why I didn't include that detail here). What caused the problem was the fact that A was a virtual base for AA (and other classes that also fed into AAAA).

Without that detail, then, what I typed in my first post, and how I initialised things (AAAA's initialiser list tells which of AAA's constructors to use, and the initialiser list in whichever of AAA's constructors is called in turn tells AA which constructor to use, etc.) was in fact correct, as written.

When you have a virtual base class, however, things are different. With the following class structure:

class A{...};

class AA : public virtual A {...};

class AB : public virtual A {...};

class AAA: public AA, public AB {...};

without the virtual keyword, AAA would actually include two distinct copies of A. When virtual is included as shown here, only one common copy is used, and thus C++ needs to ensure that the constructor for A is called once only. The way it does that is by declaring that the most derived class (in this case, AAA) determines which of A's constructors is called, regardless of what is specified in AA or AB.

In my case, AAAA did not specify a constructor for A (I assumed that this would be specified from what I told AA to do). Since there was therefore no A constructor that the compiler was being told to use, it went looking for the default constructor, which of course it couldn't find. Hence, some very confusing error messages (until I finally understood where I had stuffed up).

Experience: Something you invariably acquire immediately after you needed it.

__________________
Maybe you should put some shorts on or something, if you wanna keep fighting evil today.