Nested classes

[kedaman]

Why doesn't this work unless i comment out "public:" before class aa?

class a{
//public:
	class aa{
	public:
		aa(){};
		aa(int x,int y){};
	};
	class ab{
		aa _aa;
	public:
		ab(){_aa=aa(1,2);};
	};
};

It complains that it can't access underlined line from with the bold statement.

Quote from http://www.icce.rug.nl/docs/cplusplus/cplusplus16.html

The constructor SecondWithin() and the member function getVar() of the class SecondWithin can also only be reached by the members of Surround (and by the members of its nested classes).

Which seems not to be the case in VC++

[Zaei]

This ONLY works if i dont comment out the "public:". If I do:

...
class ab{
	aa _aa;
public:
	ab(){aa abc(1, 2);_aa=abc;};
};
...

It works fine, commented public: or not.

Z.

[kedaman]

Okay that's one way around it, thanks, but will the compiler be smart enough to not make a copy of it?

[Zaei]

I doubt it, but it will be removed when the constructor exits. You could also declare _aa as an aa*, and use "new" in the constructor for ab (ie, _aa = new aa(1, 2)), then delete _aa in ab's destructor.

Z.

[kedaman]

Originally posted by Zaei
I doubt it, but it will be removed when the constructor exits. You could also declare _aa as an aa*, and use "new" in the constructor for ab (ie, _aa = new aa(1, 2)), then delete _aa in ab's destructor.

Z.

I wouldn't go for any freaky solutions A pointer would be plain painful overhead. I was rather wondering if the local variable had to be instantiated at all but instead assign aa immediately. I know the compiler tries to use the registrers as efficient as possible, but I have no guarantee.

Anyways, thanks for your solution, I still though await a "better" less awkward solution

[Zaei]

Ah, figured it out. Make "class aa" a friend of ab:

class ab;
class aa
{
     friend class ab;
     ...
};

class ab
{
       ...
public:
       ab() { _aa = aa::aa(1, 2);}
};

Z.

[kedaman]

Originally posted by Zaei
Ah, figured it out. Make "class aa" a friend of ab:

class ab;
class aa
{
     friend class ab;
     ...
};

class ab
{
       ...
public:
       ab() { _aa = aa::aa(1, 2);}
};

Z.

What a nasty little solution you came up with I had tried similar but forgot to put class ab; on top.

But I don't like it as said, It's against my moral, I clearly see that this undermines OOP, aa shouldn't need to specify which classes it can be instantiated from.

Well thanks anyway

[Zaei]

Only if aa is not public in a. If it was, you could instantiate it like so:

aa _aa;
...
_aa = a::aa::aa(1, 2);

If not, you can make the classes friends, and get by the private/public thing.

Z.

[kedaman]

Man I'm happy to find a reasonable solution!

class a{
	class aa{
	public:
		aa(){};
		aa(int x,int y){};
	};
	class ab{
		typedef aa aa2;
		aa _aa;
	public:
		ab(){_aa =aa2(1, 2);};
	};
};

Allthough it still is annoying, I would call it OOP at least

[kedaman]

Whoa!

typedef aa aa;

is even better

[Zaei]

Nice one!

Z.

[CornedBee]

Have you tried this ab constructor?:

ab() : _aa(1, 2)
{}

This is the clean way of doing such things.

[kedaman]

Yes, but that only works for the contructor. In the project i'm developing I rather got stuck with the calls to contructors of classes at same level, within the class but not in its own constructor.

A qwestions since you seem to know pretty much, what's the actual difference after compilation between

ab() : _aa(1, 2){};

and

ab() {_aa=aa(1, 2)};

?

Thanks.

[CornedBee]

ab() {_aa=aa(1, 2)};
creates _aa with the standardconstructor, creates a temporary aa object with the (x, y) constructor and assigns it to _aa.

ab() : _aa(1, 2){};
executes the (x, y) constructor for _aa

if you didn't have a standard constructor, but only the (x, y), you couldn't use the first method.

[kedaman]

CornedBee> Why do I get
"error C2064: term does not evaluate to a function"
when I do the following:

aa x;
x(2,3);

I also tried:

aa x;
x.aa(2,3);

but get "error C2274: 'function-style cast' : illegal as right side of '.' operator"

Does this mean I can only call constructors from other constructors?

Isn't the compiler smart enough to compile x=aa(2,3); so that a temporary variable won't be instantiated?

Thanks.

[CornedBee]

a) You can call the constructor only on construction time.
This line:
aa x;
already calls the constructor (the one without arguments).
Therefore, neither x(1, 2) nor x.aa(1, 2) can work because the object is already constructed.
You can, however, write
aa x(1, 2);
as this calls the (x, y) constructor instead of the () one.

b) I'm not sure if compilers optimize this. You can look at the asm code to find out (compare with and without optimization).

[kedaman]

Thanks again I've witnessed that whole classes never got instantiated even if they were used to evaluate an expression, (an article at flipcode.com) based on that i would guess that the compiler nowadays are smart enough, but I still have no guarantee. I need to generate some assembly listings