Re: HOWTO use a Factory

"Alf P. Steinbach" <>
Thu, 23 Aug 2007 00:01:50 +0200
* Goran:

Hi all,

i have some questions/problems about "Factories".

My origin is a constructor which needs a string in special format.

An example:

class Example_t {

                Example_t(std::string &aString);

My first idea was to check "aString" IN the constructor with
"boost::regex" AND to throw an exception if "aString" was bad.

Now I know it's bad to throw an exception in a constructor.

No, it isn't: it's the one and only rational way to deal with
construction failure.

But what else?

How can I AVOID elegant a construction of an instance of "Example_t"
if "aString" is in bad format?

Throw an exception from the constructor.

I've read a lot at the net but I'm still unhappy. At some other forum
I got the information to use something like that:

class Validator
  virtual bool validate(std::string) = 0;

class A

class Factory
  static A createA(std::string, *Validator);
  static B createB(std::string, *Validator);

But I don't understand how to AVOID a construction with classes above
if the string is in bad format?

It's just meaningless, very bad advice.

However, you might and should probably consider introducing a class
ValidXXX that represents valid strings of whatever the format is.

Then the failure to validate can be detected as near to the source of
the string as possibly (by having /that/ class' constructor throw), and
valid strings can then be passed around as ValidXXX instances with no
need to check them everywhere for validity: you know they're valid.

This general principle of not passing possibly invalid data around tends
to simplify code enormously, thus also reducing the number of bugs.

And it's very very closely tied in with the concept of throwing from
constructors, so that you know that if you have an object at hand, then
that is perforce a /valid/ object with some class invariant established.

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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