Re: HOWTO use a Factory

"Victor Bazarov" <>
Wed, 22 Aug 2007 17:52:54 -0400
Goran wrote:

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's not bad. It's the only way to let the caller know that
construction cannot be completed.

But what else?


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

Check before trying to construct it.

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;

Try to pass by reference to const wherever possible.


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?

The only "elegant" way that comes to mind is to return *a copy* of some
kind of "invalid" or "generic" object that can be inquired about its own
validity and discarded by the caller.

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