Confusing function declaration

[feminine.aura]

I have to read a file containing integers into a vector. I could do
something like this:

ifstream data("file.dat");
istream_iterator<int> begin(data);
istream_iterator<int> end;
vector<int> c(begin,end);

Now this does what i want it to.

But I read in Scott meyer's effective C++ that if I try to get rid of
the second and the thir statement and do something like this

ifstream data("file.dat");
vector<int> c(istream_iterator<int>(data), istream_iterator<int>());

then this wouldn't work as expected because here the second line is
treated as a function declaration where c is the function which returns
a vector<int> and takes two parameters: a)istream_iterator<int> b) a
function pointer which doesnt take any arguments and return a
istream_iterator<int>

And that is the reason why this won't work.

But a friend of mine pointed me to this paper.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1798.html

where it says that such a declaration won't work, not because it is
considered as a function declaration but because an istream_iterator in
this case is considered as a forward iterator and that the
implementation of the constructor for vector which takes a forward
iterator is such that it is not possible to use an istream iterator
with them. I would request you to read the paper and correct me if I
misunderstood what the author said and let me know which one is right.

 

[Victor Bazarov]

I have to read a file containing integers into a vector. I could do
something like this:

ifstream data("file.dat");
istream_iterator<int> begin(data);
istream_iterator<int> end;
vector<int> c(begin,end);

Now this does what i want it to.

But I read in Scott meyer's effective C++ that if I try to get rid of
the second and the thir statement and do something like this

ifstream data("file.dat");
vector<int> c(istream_iterator<int>(data), istream_iterator<int>());

then this wouldn't work as expected because here the second line is
treated as a function declaration where c is the function [...]

Wrap the argument list in a second set of parentheses:

vector<int> c((istream_iterator<int>(data), istream_iterator<int>()));

V

 

[feminine.aura]

Victor,

Thanks for the reply. I know that if I wrap the arguments it would
work. But I was more curious about that paper which had a different
explanation and that is the reason why I posted it here. Two
contradicting statements and I was curious to know which one is right.

Cheers
Priya

 

[=?ISO-8859-15?Q?Juli=E1n?= Albo]

But a friend of mine pointed me to this paper.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1798.html

where it says that such a declaration won't work, not because it is
considered as a function declaration but because an istream_iterator in

Have you read the last sentence of the paper?

"We avoid these problems in current C++ by emulating explicit model
definitions using traits and category tags."

 

[against.inex]

Julian,

Aargh my bad. Thanks a lot []

Cheers
Priya

 

[David Harmon]

On Wed, 6 Sep 2006 11:03:45 -0400 in comp.lang.c++, "Victor Bazarov"

Wrap the argument list in a second set of parentheses:

vector<int> c((istream_iterator<int>(data), istream_iterator<int>()));

No, wrap each argument

vector<int> c((istream_iterator<int>(data)),
(istream_iterator<int>()));

 

[Thomas J. Gritzan]

Wrap the argument list in a second set of parentheses:

vector<int> c((istream_iterator<int>(data), istream_iterator<int>()));

This shouldn't work.

The comma here is the comma operator, which discards the left hand operand.