Build a Simple Program Using Boost in(std::cin) : What does it mean?

From:http://stackoverflow.com/questions/8262994/instdcin-what-does-it-mean

In the first example of Boost,

in(std::cin)

is
used. I think

in()

get
an

istream

and
create some kind of iterator. However, I could not find any C++ documentation that explain it in detail. Could you please help me to find one?

here is the copy and paste of the example from the Boost webpage:

#include <boost/lambda/lambda.hpp>
#include <iostream>
#include <iterator>
#include <algorithm>

int main()
{
using namespace boost::lambda;
typedef std::istream_iterator<int> in;
std::for_each(
in(std::cin), in(), std::cout << (_1 * 3) << " " );
}

otice the typedef in the code:

typedef std::istream_iterator<int> in;

Thus,

in(...)

is
the same as

std::istream_iterator<int>(...)

:
it's calling the constructor for that type. There is a 1-argument constructor which accepts a

std::istream

,
creating an iterator that represents the current point in that stream; and a 0-argument constructor creating an iterator that represents the end of any given stream. So

std::for_each

will
iterate over every value provided by

std::cin

from
now until it runs out.

std::istream_iterator<int>

takes
a stream and provides an iterator over the

int

s
in the stream, using

operator>>

to
read them out of the stream.

However, I could not find any C++ documentation that explain it in detail.

I don't know how you could possibly fail. I put

std::istream_iterator<int>

into
Google and the first result was http://www.sgi.com/tech/stl/istream_iterator.html which
is pretty thorough, assuming you're already familiar with iterators. The next result ishttp://www.cplusplus.com/reference/std/iterator/istream_iterator/ which
makes another attempt to explain things and is also fully detailed. Next comes http://stdcxx.apache.org/doc/stdlibref/istream-iterator.html ,
similarly, which finally explicitly mentions

operator>>

instead
of just talking about formatted I/O operations (which is what

operator>>

does).
Next comes a page with some C++ example snippets, then a couple of StackOverflow questions where people were trying to do something similar, etc....

in

is
just a

typedef

for

std::istream_iterator<int>

so
the example is just calling

std::for_each

on
a "range" defined by the two temporary iterators:

std::istream_iterator<int>(std::cin)

and

std::istream_iterator<int>()

.

A value-inititalized

istream_iterator

is
just a universal "end" iterator for streams.

How

std::cout
<< (_1 * 3) << " "

works is more subtle. Because

_1

comes
from the

boost::lambda

namespace,
it ensures that the operators

*

and
then

<<

from
the

boost::lambda

namespace
are used, rather than an

operator<<

that
acts directly on a

std::ostream

.
This way the whole expression becomes a lambda rather than (any part of it) being executed as a conventional expression at the call site of

for_each

.

typedef std::istream_iterator<int> in;

Thus

in(std::cin)

is
an iterator over integers, used by

std::for_each

,
that reads from stdin (cin) and multiplies them by 3 and prints them out.