Re: Custom constructor and the Meyers Singleton
irotas schrieb:
After thinking about this for a while, one wild idea came to mind.
What if we create a second singleton, called SingletonConfigurator,
that the application can use to set the parameters needed for
Singleton's constructor. SingletonConfigurator could even set default
value where appropriate, so that the user doesn't even need to know
that the SingletonConfigurator exists unless they really need it.
Trying to get a way of one time initialization of a singleton you can
do as following (see code), but note: I'm not sure about the behavior
when
using this singleton in another static context (may be another file).
It might
happen that the startparameter is not set yet):
However, on windows with microsofts compiler I can change the order
of the two lines with the mark "xxx" without problems. No assertion
occurs (for this example). At least it's not a good idea two use two
singletons to initialize one!
CODE:
struct Singleton
{
static Singleton& getIt()
{
static Singleton instance;
return instance;
}
Singleton()
: _value(_startupParameter) {}
int _value;
static int _startupParameter;
};
struct X
{
X()
: _value(Singleton::getIt()._value){}
int _value;
};
// mark: xxx
int Singleton::_startupParameter = 10;
static X instanceOfX;
int main()
{
assert(Singleton::getIt()._value == 10);
assert(instanceOfX._value == 10);
}
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