Re: Query: want a function that is called at initialisation time
tropostropos@hotmail.com wrote:
On 13 Dec, 03:52, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:
Not only it serves no real purpose except to invoke the 'initialise'
member, but I am not sure it's going to actually be instantiated
(it's static member of a template), unless you actually use it.
<snip>
Fair enough. I didn't show all the code. Suppose additionally that
the class has another static function f1 and there is some code
somewhere that calls it:
Singleton<MyClass>::f1( );
So that guarantees that the class will be instantiated, does it not?
The class yes, but not all of its members. Only those members that are
actually used will be instantiated. This also holds for static members.
See the difference in output if you uncomment the line 'if
(initialised)' in the program below.
#include <iostream>
using namespace std;
template <class T>
class Singleton {
public:
void tell_me() const;
static Singleton& instance();
private:
static bool do_initialise();
static bool initialised;
Singleton() {};
};
template <class T>
bool Singleton<T>::do_initialise()
{
cout << "initialised" << endl;
return true;
}
template <class T>
bool Singleton<T>::initialised = do_initialise();
template <class T>
Singleton<T>& Singleton<T>::instance()
{
static Singleton<T> single;
// if (initialised)
cout << "Has been initialised?" << endl;
return single;
}
template <class T>
void Singleton<T>::tell_me() const
{
cout << "Hello" << endl;
}
int main()
{
cout << "Start of main()" << endl;
Singleton<double>::instance().tell_me();
cout << "End of main()" << endl;
}
and that dummy will be initialised. So the question remains: what's
the best way to run initialise?
I don't see any problem with such a dummy variable (although I usually
use 'bool initialised = /* whatever */;')
Bart v Ingen Schenau
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