Visitor and double dispatch without overriding "accept" method in c++

Question

Ok: here´s my problem: I have a base composit class which accepts a visitor an then iterates over its nodes. Works like a charm. But then, I have to used a derived from this composit and recognized that I have to override the "accept()" method in the derived class to have correct double dispatching (which I did not understand).

This brings out two flaws: first, I have to break the hidden structure of the base, and second, I have to duplicate code. To clearify, here´s my pseudo-code:

struct Visitor
{
    void visit( BaseComposit*)    { throw( "not expected"); };
    void visit( DerivedComposit*) { throw( "ok"); };
};

class BaseComposit 
{
private:  

    std::vector< BaseComposit*> nodes;

public:

    virtual void accept( Visitor* v)
    {
        v->visit( this);

        for( int i = 0; i < nodes.size(); i++)
            nodes[ i]->accept( v);
    }
};

class DerivedComposit : public BaseComposit
{
public:
};

Any ellegant solution on that ? Thank you !

Edit: added "virtual" to "accept()" to make it more precise...

Nitpick, but a space after an opening brace that is unmatched before a closing brace is a bit of an eye-sore. — StoryTeller - Unslander Monica, Dec 07 '17 at 08:57
Why don't you declare accept as virtual in the base class and implement an override in the derived? the visitor pattern is just that. — Oliv, Dec 07 '17 at 08:58
Related read: http://www.stroustrup.com/multimethods.pdf Stroustrup already has something in mind to make multidispatch a language feature — king_nak, Dec 07 '17 at 09:21
My `( point)( is)( that)( this)` sort of style may bring up a lot of discussions in code review. A one space can stand out as much as many, believe me. — StoryTeller - Unslander Monica, Dec 07 '17 at 09:21
@StoryTeller: after 35 years in programming, this will not change my brain anymore ;-) Same as "Type\* type" vs. "Type /*type" vs. "Type/* pType" vs. and so on.... — affenärschle, Dec 07 '17 at 09:28
I can get `( this )` or `(this)`, but I'm afraid `( this)` is always gonna be an issue with me in code review. But YMMV :) — StoryTeller - Unslander Monica, Dec 07 '17 at 09:30
sry - first time user: how can I place an asterisk in text ? Ahh - found by myself that I have to backslash it :-( — affenärschle, Dec 07 '17 at 09:33
I got you, don't worry. If you want to learn about SO's markup capabilities, including in comments, there's a [help page for it](https://stackoverflow.com/editing-help). — StoryTeller - Unslander Monica, Dec 07 '17 at 09:34

StoryTeller - Unslander Monica · Answer 1 · 2017-12-07T09:11:44.390

1

Any ellegant solution on that ?

Not really. That's what makes the visitor pattern a bit of a pain. Though you could mitigate the duplication a bit with the help of a template:

class BaseComposit 
{
private:  

    std::vector<BaseComposit*> nodes;

protected:

    template<class T>
    void accept_impl( Visitor* v, T* this_ )
    {
        v->visit( this_ );

        for(int i = 0; i < nodes.size(); i++)
            nodes[i]->accept(v);
    }

public:

    virtual void accept( Visitor* v ) { accept_impl( v, this ); }
};

Now the duplication that accept must incur is smaller.

Also, as @Oliv pointed out, your example really ought to have accept be a virtual function. Otherwise the whole thing won't work.

If you feel really adventurous, you can introduce a macro to ease the "injection" of accept into each class. Like so:

#define INJECT_ACCEPT() void accept( Visitor* v ) override { accept_impl( v, this ); }

But you'd still need to use it in each derived class to make the definition appear.

class DerivedComposit : public BaseComposit
{
public:
    INJECT_ACCEPT();
};

The semi-colon is allowed by a curious feature of the C++ grammar. So I suppose one may argue the above looks "natural".

edited Dec 07 '17 at 09:11

answered Dec 07 '17 at 08:58

StoryTeller - Unslander Monica

165,132
21
377
458

But "this" is always a BaseComposit*? – Oliv Dec 07 '17 at 09:00
@Oliv - Making it virtual is another matter :) – StoryTeller - Unslander Monica Dec 07 '17 at 09:00
Thanks - I´ll go with the template and yes, of course it has to be virtual. This is why I wrote "pseudo-code" ;-) Thanks again for the quick resonance! Great community! – affenärschle Dec 07 '17 at 09:05
@user9066185 - Well, I added another tidbit. It avoids a bit more repetition, but it's a bit questionable. – StoryTeller - Unslander Monica Dec 07 '17 at 09:07
What's the use of the template method here? Is it just to make the code more C++y? – king_nak Dec 07 '17 at 09:23
@king_nak - No, it's to avoid writing the whole function body over and over just because we want the static type of `this` to be different each time. You may call it C++y, I call it D.R.Y. – StoryTeller - Unslander Monica Dec 07 '17 at 09:25
You can achieve this with other means, too. Just separate the algorithm (iterating through the nodes), and the action (what each node does). Using the, well, template method pattern. E.g. like this https://ideone.com/u9tEzy – king_nak Dec 07 '17 at 09:26
@king_nak - Yes, a NVI is also a worthy alternative. And it was just suggested as well, coincidentally. Which to use, and that may even involve a mixture, is up to the implementer, based on their needs. – StoryTeller - Unslander Monica Dec 07 '17 at 09:27
Ah, just saw the other answer. Didn't remember the term NVI. And with your solution: I often have a hard time interpreting template methods that handle with hierarchy, so I try to avoid them ;) – king_nak Dec 07 '17 at 09:31
@king_nak - I suppose it's your prerogative :) – StoryTeller - Unslander Monica Dec 07 '17 at 09:32

score 1 · Answer 2 · answered Dec 07 '17 at 09:25

1

If you want to force some code to be always executed, use the non-virtual interface (NVI) pattern:

class BaseComposit 
{
private:  
    std::vector<BaseComposit*> nodes;

    virtual void call_visit(Visitor* v) { v->visit(this); }

public:

    void accept(Visitor* v)
    {
        call_visit(v);

        for(int i = 0; i < nodes.size(); i++)
            nodes[i]->accept(v);
    }
};

class DerivedComposit : public BaseComposit
{
    void call_visit(Visitor* v) override { v->visit(this); }
public:
};

answered Dec 07 '17 at 09:25

Sebastian Redl

69,373
8
123
157

@affenärschle: so get rid of `override`. – Jarod42 Dec 07 '17 at 10:07
@Jarod42: jup - works. But I can´t see why the compiler can select the right vtable entry of Base/Derived and not in the visitor ... Anyway, Sebastian, good solution. Ok - visitor has no vtabel ... shame on me – affenärschle Dec 07 '17 at 10:20

Visitor and double dispatch without overriding "accept" method in c++

2 Answers2