Re: What is this function object?
On 2007-11-04 23:58:13 -0500, "webinfinite@gmail.com"
<webinfinite@gmail.com> said:
Could anyone explain this snippet for me?
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
class PrintInt {
public:
void operator() (int elem) const {
cout << elem << ' ';
}
};
int main() {
vector<int> coll;
for (int i = 1; i <=9; ++i)
coll.push_back(i);
for_each (coll.begin(), coll.end(), PrintInt()); // I don't
understand here
}
for_each needs a function object, so if we are doing: PrintInt p, then
we call for_each(coll.begin(), coll.end(), p()); I will understand,
But that call would be wrong. The algorithm needs the object, not the
result of calling its operator(). So the correct call is
for_each(coll.begin(), coll.end(), p);
but what is this "PrintInt()" here? Is it a default constructor? Or
just a call to overloaded operator ()?
Yes, it's the default contructor. It creates an unnamed temporary
object, and for_each gets a copy of that object. for_each calls the
copy's operator() as needed.
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)