Question on usage of functional object.

[hn.ft.pris]

I've got following code test C++'s functor. For the sake of
easy-reading, I omit some declearations.

#include <algorithm>
#include <functional>

using namespace std;

template <typename Tclass Sum{
public:
Sum(T i=0):sum(i){};
inline void operator () (T x){
sum += x;
}
inline T output() const{
return sum;
}

private:
T sum;
};

int main( void ){
vector<intvec(10, 1);

Sum<intsum;

sum = for_each(vec.begin(), vec.end(), Sum<int>()); .......(1)
sum = for_each(vec.begin(), vec.end(), sum); .........(2)
sum = for_each(vec.begin(), vec.end(), sum.operator()(int) ); ...(3)

cout << sum.output() << endl;

return 1;
}

It's easy to understand that (2) works, because sum.operator()(int) is
called implicitly. (1) also works, it confuses me. Does Sum<int>()
create an implicit class Sum object? On the other hand, Sum<int>
doesn't work.
(3) fails, does it means the third argument of "for_each" couldn't be a
function pointer? What will the code be if I want to pass a function
pointer here? Thanks for help!

[Roland Pibinger]

On 22 Jan 2007 22:50:56 -0800, hn.ft.pris wrote:

>I've got following code test C++'s functor. For the sake of
easy-reading, I omit some declearations.

#include <algorithm>
#include <functional>

using namespace std;

template <typename Tclass Sum{
public:
Sum(T i=0):sum(i){};
inline void operator () (T x){
sum += x;
}
inline T output() const{
return sum;
}

private:
T sum;
};

This is a (probably) problematic 'stateful predicate', see:
http://www.gotw.ca/gotw/052.htm.

>int main( void ){
vector<intvec(10, 1);

Sum<intsum;

sum = for_each(vec.begin(), vec.end(), Sum<int>()); .......(1)
sum = for_each(vec.begin(), vec.end(), sum); .........(2)
sum = for_each(vec.begin(), vec.end(), sum.operator()(int) ); ...(3)

cout << sum.output() << endl;

return 1;
}

It's easy to understand that (2) works, because sum.operator()(int) is
called implicitly. (1) also works, it confuses me. Does Sum<int>()
create an implicit class Sum object? On the other hand, Sum<int>
doesn't work.
(3) fails, does it means the third argument of "for_each" couldn't be a
function pointer? What will the code be if I want to pass a function
pointer here? Thanks for help!

The third argument in (3) is neither a function object nor a pointer
to a (free) function.

Best wishes,
Roland Pibinger

[Noah Roberts]

sum = for_each(vec.begin(), vec.end(), sum.operator()(int) ); ...(3)

(3) fails, does it means the third argument of "for_each" couldn't be a
function pointer? What will the code be if I want to pass a function
pointer here? Thanks for help!

sum.operator()(int) isn't valid. You can't take an address of a member
function from an instance, you have to take it from the class. As such
its signature changes from what it was, (int) in this case, to
accepting a pointer to an instance of that class...so (Sum*, int) in
this case. To use such in an algorithm you need a binder. I use TR1
bind, which can be found as boost::bind.

sum = for_each(vec.begin(), vec.end(), boost::bind(&Sum::operator(),
&sum, _1));