Re: NVI idiom and patterns such as Decorator and Proxy

From:
 James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 11 Jun 2007 15:38:17 -0000
Message-ID:
<1181576297.879655.221470@q75g2000hsh.googlegroups.com>
On Jun 11, 3:17 pm, Christian Hackl <h...@sbox.tugraz.at> wrote:

I've got a design question related to the combination of the NVI idiom
(non-virtual interfaces, [1]) and popular object-oriented patterns such
as Proxy or Decorator, i.e. those which have the basic idea of deriving
from a base class and delegating to an object of it at the same time.

My problem is that I cannot seem to combine those two techniques in a
flawless way. For a very simple, non real life example (for which I
shall omit smart pointers and std::strings :)), let's say I've got an
abstract base class Printer, from which ConcretePrinter is derived. I'd
express this in C++ as follows:

class Printer
{
//...
public:
   virtual ~Printer() {}
   void print(char *str);
private: // could also be protected; doesn't matter for my problem
   virtual void doPrint(char *str) = 0;
};

class ConcretePrinter : public Printer
{
//...
public:
   virtual ~ConcretePrinter() {}
private:
   virtual void doPrint(char *str);
};

The point is that Printer::print can do some parameter
checking before delegating to doPrint:

void Printer::print(char *str)
{
   if (str)
     doPrint(str);
}

The derived class then actually performs the operation:

void ConcretePrinter::doPrint(char *str) { cout << str << endl; }

So far, that's fine. But now let's say I want to use a Decorator to add
some extra output:

class PrinterDecorator : public Printer
{
public:
   virtual ~PrinterDecorator() {}
   PrinterDecorator(Printer *decorated_printer) :
     decorated_printer_(decorated_printer) {}
   //...
private:
   virtual void doPrint(char *str)
   {
     cout << "some decoration..." << endl;
     decorated_printer_->print(str); // <-- line that bugs me
     cout << "some decoration..." << endl;
   }
   Printer *decorated_printer_;
};

The print() call in this piece of code is what bugs me. I cannot call
decorated_printer_'s doPrint() because it is non-public in this context,
but calling print() means that all parameter checking performed in
Printer::print() is uselessly duplicated, and it would be duplicated
again for all further decorators or proxies I might add.


Why do you say "uselessly duplicated"? Isn't PrinterDecorator a
client of Printer, as well as being an implementation? Is the
author of PrinterDecorator somehow protected from making the
same stupid errors as other clients might make, and so not need
the same protection.

After all, by
the time PrinterDecorator::doPrint() is called, all necessary checking
already took place in Printer::print(). It's like the derived class
telling the base class, "I know you already checked the data, but please
check it again anyway."


No, it's like a client telling the service that it has already
checked the data, but please check it again anyway.

Granted, in this stupid example, it's just a pointer check, but think of
more expensive operations such as "do files exist", "can server be
accessed", or "lock for other threads". A program that duplicates such
operations "by design" doesn't strike me as very well designed.


Presumably, however, this holds for all checks. If the contract
says you should not call the function with a null pointer,
presumably, all clients will take steps to ensure that the
pointer they pass is not null. You check it anyway, because you
don't trust the client. Why is the client PrinterDecorator any
different? The fact that it also derives from Printer doesn't
mean that its authors are infallible.

Of course, if the profiler shows that this is a problem, you
could define an AbstractPrinterDecorator from which all
PrinterDecorator derive, declare it a friend of Printer, and
provide a function in it to forward to the unchecked version in
the target Printer. But I while an AbstractPrinterDecorator
isn't necessarily a bad idea in itself (since all
PrinterDecorator's have the common behavior of holding a pointer
to the decorated Printer object), I wouldn't start skipping any
of the intermediate checks until the profiler said I had to.

How do I cope with this situation? Is it just an unfortunate fact of
life that NVI and Decorator/Proxy-like patterns don't mix? Or am I
missing something?

In fact, I thought of a possible solution. If I gave Printer an
additional protected "print" method that took a Printer object and that
did *nothing* but delegate to doPrint(), I could call that protected
method from the decorator:


But this supposes that *all* Printer have the behavior of a
Decorator.

class Printer
{
//...
public:
   void print(char *str); // clients of Printer keep calling this one
protected:
   void print(Printer *printer, char *str) // <-- new method, to be

                                           // called by derived classes
                                           // if they need to
   {
     printer->doPrint(str);
   }
private:
   virtual void doPrint(char *str) = 0;
};

void PrinterDecorator::doPrint(char *str)
{
   cout << "some decoration..." << endl;
   print(decorated_printer_, str); // <-- now calling the new
                                   // protected method
   cout << "some decoration..." << endl;

}

Is this a good idea? It looks a bit confusing even to me although I came
up with it myself :)


Putting it in Printer is probably not a good idea, but I can see
it (and have done something similar in the past) in an abstract
base class of the decorators, which is a friend of Printer.

--
James Kanze (GABI Software, from CAI) email:james.kanze@gmail.com
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