Michal Wyrebski wrote:[color=blue]
> I'm new in this group and I don't know if my questions are too silly,
> but I'm intermediate programmer and don't have enought experience.
> Please be charitable.
>
> I don't know how to write an operator() class to be properly executed
> inside for_each()?
>
> Maybe example will be more accurate:
>
> #v+
> #include <algorithm>
> #include <iostream>
> #include <list>
>
> class Class1
> {
> private:
> /* ... */
> double last_value;
> public:
> Class1() : last_value(0) {}
>
> double show_result() {
> return last_value;
> }
>
> void operator()(double n) {
> if (n > last_value)
> last_value = n;
> }
> }; // class
>
> using namespace std;
>
> int main() {
> Class1 functor;
> list<int> List1;
>
> for (int i=0; i<6; ++i)
> List1.push_back(i);
>
> for_each(List1.begin(), List1.end(), functor);
>
> functor.show_result(); // (1)
>
> return 0;
> }
> #v-
>
> And the problem is, that (1) returns 0, and I want it to return 5.
> Where is a mistake in my reasoning?[/color]
'for_each' makes a copy of the functor.
[color=blue]
> How should it be written?[/color]
You need some kind of indirection.
class Class1
{
private:
/* ... */
double& last_value; /// reference!!!
public:
Class1(double& d) : last_value(d) {}
double show_result() {
return last_value;
}
void operator()(double n) {
if (n > last_value)
last_value = n;
}
}; // class
...
double a = 0;
Class1 functor(a);
for_each(List1.begin(), List1.end(), functor);
Now, 'functor' keeps the reference to the place where it stores the
last value, and the reference survives the copying of 'functor'.
V
