why constructor is called in for_each funobj everytime?

[suresh]

Hi,
Kindly look at the code below of a function object and its application
in for_each.
class DistBwObjFunObj
{
public:
DistBwObjFunObj(Object o);
void operator()(Point p); //finds the distance and accumulates them
double totalDist(); //returns the accumulated dist
private:
Object obj;
double dist;
};
DistBwObjFunObj:istBwObjFunObj(Object o)bj(o),dist(0){}

void DistBwObjFunObj:perator()(Point p){
dist += distPoint(p,obj.closestPoint(p));
}

double DistBwObjFunObj::totalDist(){
return dist;
}

//Application code segment:
DistBwObjFunObj mydist(o);
for_each(f.begin(),f.end(),mydist);
double d = mydist.totalDist();

My problem: the value of d is always equal to the value of dist at the
time of construction, which is zero now. Why is it so?
I feel that, the constructor is called everytime inside the for_each.
Why is it so?


Thanks for your comments
suresh

 

[Ian Collins]

Hi,
Kindly look at the code below of a function object and its application
in for_each.
class DistBwObjFunObj

What on Earth prompted you to use such a bizarre name?

{
public:
DistBwObjFunObj(Object o);
void operator()(Point p); //finds the distance and accumulates them
double totalDist(); //returns the accumulated dist
private:
Object obj;
double dist;
};
DistBwObjFunObj:istBwObjFunObj(Object o)bj(o),dist(0){}

void DistBwObjFunObj:perator()(Point p){
dist += distPoint(p,obj.closestPoint(p));
}

double DistBwObjFunObj::totalDist(){
return dist;
}

//Application code segment:
DistBwObjFunObj mydist(o);
for_each(f.begin(),f.end(),mydist);
double d = mydist.totalDist();

My problem: the value of d is always equal to the value of dist at the
time of construction, which is zero now. Why is it so?

Have you looked? Does your distPoint function return non-zero values?

 

[Öö Tiib]

Hi,
Kindly look at the code below of a function object and its application
in for_each.
class DistBwObjFunObj
{
public:
        DistBwObjFunObj(Object o);
        void operator()(Point p); //finds the distance and accumulates them
        double totalDist(); //returns the accumulated dist
private:
        Object obj;
        double dist;};

DistBwObjFunObj:istBwObjFunObj(Object o)bj(o),dist(0){}

void DistBwObjFunObj:perator()(Point p){
        dist += distPoint(p,obj.closestPoint(p));

}

double DistBwObjFunObj::totalDist(){
        return dist;

}

//Application code segment:
DistBwObjFunObj mydist(o);
for_each(f.begin(),f.end(),mydist);
double d = mydist.totalDist();

My problem: the value of d is always equal to the value of dist at the
time of construction, which is zero now. Why is it so?
I feel that, the constructor is called everytime inside the for_each.
Why is it so?

Thanks for your comments

Maybe this will help:

mydist = std::for_each(f.begin(),f.end(),mydist);

 

[Kai-Uwe Bux]

Hi,
Kindly look at the code below of a function object and its application
in for_each.
class DistBwObjFunObj
{
public:
DistBwObjFunObj(Object o);
void operator()(Point p); //finds the distance and accumulates them
double totalDist(); //returns the accumulated dist
private:
Object obj;
double dist;
};
DistBwObjFunObj:istBwObjFunObj(Object o)bj(o),dist(0){}

void DistBwObjFunObj:perator()(Point p){
dist += distPoint(p,obj.closestPoint(p));
}

double DistBwObjFunObj::totalDist(){
return dist;
}

//Application code segment:
DistBwObjFunObj mydist(o);
for_each(f.begin(),f.end(),mydist);
double d = mydist.totalDist();

My problem: the value of d is always equal to the value of dist at the
time of construction, which is zero now. Why is it so?

Because, you don't change it.

I feel that, the constructor is called everytime inside the for_each.
Why is it so?

Not everytime. However, for_each() takes the functor argument by value.
Thus, a copy is created. Luckily, it returns the copy that is used
internally. Thus:

mydist = for_each( f.begin(), f.end(), mydist );

should do the trick (provided, the type has a correctly working assignment
operator).


Best

Kai-Uwe Bux

 

[suresh]

What on Earth prompted you to use such a bizarre name?










Have you looked?  Does your distPoint function return non-zero values?

Hi Ian,

It does return non zero values. In fact I can see that dist variable
gets accumulated. But when I call mydist.totalDist(), I get a zero!
Sorry for bizzare names..
suresh

 

[suresh]

Maybe this will help:

  mydist = std::for_each(f.begin(),f.end(),mydist);

I tried std::for_each, no change!
suresh

 

[suresh]

Because, you don't change it.


Not everytime. However, for_each() takes the functor argument by value.
Thus, a copy is created. Luckily, it returns the copy that is used
internally. Thus:

  mydist = for_each( f.begin(), f.end(), mydist );

should do the trick (provided, the type has a correctly working assignment
operator).

Best

Kai-Uwe Bux

Hi,
Thank you!
It worked.....so for each takes a copy of the object....and I was
querying the original object!!!
Thanks again
suresh

 

[Öö Tiib]

I tried std::for_each, no change!

But "mydist = " part you missed.

 

[suresh]

But "mydist = " part you missed.

oops, I missed the mydist=part. Only saw std::for_each...stupid guy
here
thanks a lot for helping
suresh