Re: for_each algorithm and ostream

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 15 Nov 2009 03:50:07 -0800 (PST)
Message-ID:
<a2184a84-c66a-4cba-a177-7d04d95e72e2@l13g2000yqb.googlegroups.com>
On Nov 15, 8:24 am, "subramanian10...@yahoo.com, India"
<subramanian10...@yahoo.com> wrote:

I want to write data stored in a container into standard
output or an output file using for_each algorithm. To acheive
this, the following program creates a function object of type
'Print'. The argument to the 'Print' class ctor is an ostream
object. This function object is passed as the third argument
to 'for_each' algorithm. The overloaded function call operator
in the 'Print' class, writes the data passed to it into the
ostream member data of the function object.

#include <cstdlib>
#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

class Print
{
public:
explicit Print(ostream& arg);
template <typename T> bool operator()(const T& arg);

private:
ostream& os;
};

Print::Print(ostream& arg) : os(arg)
{
}

template <typename T> inline bool Print::operator()(const T& arg)
{
        os << arg << endl;
        return true;
}

int main()
{
        typedef vector<int> Container;
        Container c;

        for (int i = 0; i != 10; ++i)
                c.push_back(i);

        for_each(c.begin(), c.end(), Print(cout));

        ofstream ofs("Descending_order");

        if (!ofs)
        {
                cout << "Could not create output file" << endl;
                return EXIT_FAILURE;
        }

        for_each(c.rbegin(), c.rend(), Print(ofs));
        return EXIT_SUCCESS;
}

This program works as expected. Is this approach of passing a
function object argument to for_each algorithm for printing
the container data, correct ?


Yes, but...

Kindly reply me if there is a better/cleaner solution and
please provide the same.


You're actually copying, so you should be using std::copy, with
a specialized output iterator. In this case, the specialized
output iterator already exists: std::ostream_iterator.
Otherwise, I'd say that it's more "idiomatic" in some sense to
define a new output iterator, and use it, but that has to be
weighed against the fact that writing an iterator is
considerably more complicated than writing a simple functional
object. (It shouldn't be, of course, but the STL idiom tends to
make things more difficult than they should be.)

--
James Kanze

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