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several input-output questions

 
 
Jess
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      06-17-2007
Hello,

When I copy contents from an input string to a vector, I typically use
something like this

vector<string> v;
ifstream in("file");
copy(istream_iterator<string> (in), istream_iterator<string>(),
back_inserter(v));

However, I also tried to define istream_iterator separately. I did

istream_iterator<string> it(in);

this works, but there's no way I can define something equivalent to
istream_iterator<string>() above. I tried

istream_iterator<string> it;

and this failed. What does "istream_iterator<string>()" mean? Does
it define an iterator that's not bound to any istream? If so, why my
code above "istream_iterator<string> it;" doesn't work?

There are some error-detection functions, such as "bad(), "good()",
"fail()". I'm not sure what constitutes "errors". If I read from a
file and hit the end of the file, does it leave the stream in an error
state? If the reading reaches EOF, then do I need to use "clear()" to
reset it? If I use "clear()" to reset a stream (for whatever error
reasons), are the remaining data on the stream lost?

Thanks a lot!
Jess

 
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John Harrison
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      06-17-2007
Jess wrote:
> Hello,
>
> When I copy contents from an input string to a vector, I typically use
> something like this
>
> vector<string> v;
> ifstream in("file");
> copy(istream_iterator<string> (in), istream_iterator<string>(),
> back_inserter(v));
>
> However, I also tried to define istream_iterator separately. I did
>
> istream_iterator<string> it(in);
>
> this works, but there's no way I can define something equivalent to
> istream_iterator<string>() above. I tried
>
> istream_iterator<string> it;
>
> and this failed.


It should work (apart from the fact that you seem to have used the
variable name 'it' twice). What error did you get?

What does "istream_iterator<string>()" mean? Does
> it define an iterator that's not bound to any istream? If so, why my
> code above "istream_iterator<string> it;" doesn't work?
>
> There are some error-detection functions, such as "bad(), "good()",
> "fail()". I'm not sure what constitutes "errors". If I read from a
> file and hit the end of the file, does it leave the stream in an error
> state?


Yes

If the reading reaches EOF, then do I need to use "clear()" to
> reset it?


Yes, but be careful here, reaching the end of file is not the same as
trying to read past the end of file. Only the latter is an error.

If I use "clear()" to reset a stream (for whatever error
> reasons), are the remaining data on the stream lost?


No

>
> Thanks a lot!
> Jess
>

 
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Jess
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Posts: n/a
 
      06-17-2007
On Jun 17, 11:16 pm, John Harrison <(E-Mail Removed)>
wrote:
> Jess wrote:
> > Hello,

>
> > When I copy contents from an input string to a vector, I typically use
> > something like this

>
> > vector<string> v;
> > ifstream in("file");
> > copy(istream_iterator<string> (in), istream_iterator<string>(),
> > back_inserter(v));

>
> > However, I also tried to define istream_iterator separately. I did

>
> > istream_iterator<string> it(in);

>
> > this works, but there's no way I can define something equivalent to
> > istream_iterator<string>() above. I tried

>
> > istream_iterator<string> it;

>
> > and this failed.

>
> It should work (apart from the fact that you seem to have used the
> variable name 'it' twice). What error did you get?


Thanks, I tried it again, and this error disappears. By the way,
it seems both "istream_iterator it();" and "istream_iterator it;"
work, is there any difference?
Jess

 
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Robert Bauck Hamar
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Posts: n/a
 
      06-17-2007
Jess wrote:

> On Jun 17, 11:16 pm, John Harrison <(E-Mail Removed)>
> wrote:
>> Jess wrote:
>> > Hello,

>>
>> > When I copy contents from an input string to a vector, I typically use
>> > something like this

>>
>> > vector<string> v;
>> > ifstream in("file");
>> > copy(istream_iterator<string> (in), istream_iterator<string>(),
>> > back_inserter(v));

>>
>> > However, I also tried to define istream_iterator separately. I did

>>
>> > istream_iterator<string> it(in);

>>
>> > this works, but there's no way I can define something equivalent to
>> > istream_iterator<string>() above. I tried

>>
>> > istream_iterator<string> it;

>>
>> > and this failed.

>>
>> It should work (apart from the fact that you seem to have used the
>> variable name 'it' twice). What error did you get?

>
> Thanks, I tried it again, and this error disappears. By the way,
> it seems both "istream_iterator it();" and "istream_iterator it;"
> work, is there any difference?


istream_iterator it();

declares it as a function returning an istream_iterator, whereas

istream_iterator it;

defines it as an istream_iterator object and calls its default constructor.

--
rbh
 
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John Harrison
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      06-17-2007
Jess wrote:
> On Jun 17, 11:16 pm, John Harrison <(E-Mail Removed)>
> wrote:
>> Jess wrote:
>>> Hello,
>>> When I copy contents from an input string to a vector, I typically use
>>> something like this
>>> vector<string> v;
>>> ifstream in("file");
>>> copy(istream_iterator<string> (in), istream_iterator<string>(),
>>> back_inserter(v));
>>> However, I also tried to define istream_iterator separately. I did
>>> istream_iterator<string> it(in);
>>> this works, but there's no way I can define something equivalent to
>>> istream_iterator<string>() above. I tried
>>> istream_iterator<string> it;
>>> and this failed.

>> It should work (apart from the fact that you seem to have used the
>> variable name 'it' twice). What error did you get?

>
> Thanks, I tried it again, and this error disappears. By the way,
> it seems both "istream_iterator it();" and "istream_iterator it;"
> work, is there any difference?
> Jess
>


Well this time I will say that it should NOT work. This

istream_iterator<string> it;

declares a variable called 'it' of type istream_iterator<string>, no
problem. But this

istream_iterator<string> it();

declares a FUNCTION called 'it' which takes zero arguments and returns a
istream_iterator<string>. This is a common newbie mistake, by adding the
brackets you've written a function prototype.

I guess this was the source of your original confusion.

john
 
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Obnoxious User
Guest
Posts: n/a
 
      06-17-2007
On Sun, 17 Jun 2007 06:52:21 -0700, Jess wrote:

> I see my problems now, thanks!
>
> On the other hand, in program
>
> copy(istream_iterator<string> (in), istream_iterator<string>(),
> back_inserter(v));
>
> Does it convert "in" to an istream_iterator<string>? If so, what is
> "istream_iterator<string>()"? Neither of them looks like a function
> call or constructing an istream_iterator<string> object...
>
> Jess


'istream_iterator<string>(in)' creates a temporary object based on 'in'
'istream_iterator<string>()' creates a temporary object representing the
end iterator.
'back_inserter(v)' creates a temporary object

Consider:

class T {};

T t = T();

--
Obnoxious User
 
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Jess
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Posts: n/a
 
      06-17-2007
I see my problems now, thanks!

On the other hand, in program

copy(istream_iterator<string> (in), istream_iterator<string>(),
back_inserter(v));

Does it convert "in" to an istream_iterator<string>? If so, what is
"istream_iterator<string>()"? Neither of them looks like a function
call or constructing an istream_iterator<string> object...

Jess

 
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Jess
Guest
Posts: n/a
 
      06-17-2007
On Jun 17, 11:44 pm, Obnoxious User <O...@127.0.0.1> wrote:
> On Sun, 17 Jun 2007 06:52:21 -0700, Jess wrote:
> > I see my problems now, thanks!

>
> > On the other hand, in program

>
> > copy(istream_iterator<string> (in), istream_iterator<string>(),
> > back_inserter(v));

>
> > Does it convert "in" to an istream_iterator<string>? If so, what is
> > "istream_iterator<string>()"? Neither of them looks like a function
> > call or constructing an istream_iterator<string> object...

>
> > Jess

>
> 'istream_iterator<string>(in)' creates a temporary object based on 'in'
> 'istream_iterator<string>()' creates a temporary object representing the
> end iterator.
> 'back_inserter(v)' creates a temporary object
> Consider:
>
> class T {};
>
> T t = T();


I'm sometimes confused by the object creation syntax. To create an
object t of type T, I think I can use the following statements.

T t; //default constructor
T t(arg); //constructor with argument arg
T t = T(); //call default constructor for T()? then call copy
constructor to create t using the temporary object created from T()
T t = T(arg); //does the right hand side call the constructor with
argument arg, then call the copy constructor to get t?
T t = existing_t_object;
T* tp = new T; //call default constructor
T* tp = new T(arg); //constructor with argument arg

Are they correct? Have I missed something?

Thanks,
Jess

 
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James Kanze
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Posts: n/a
 
      06-17-2007
On Jun 17, 3:52 pm, Jess <(E-Mail Removed)> wrote:
> I see my problems now, thanks!


> On the other hand, in program


> copy(istream_iterator<string> (in), istream_iterator<string>(),
> back_inserter(v));


> Does it convert "in" to an istream_iterator<string>? If so, what is
> "istream_iterator<string>()"? Neither of them looks like a function
> call or constructing an istream_iterator<string> object...


According to the standard, they are both function style casts.
The first converting in into an istream_iterator<string>, and
the second converting nothing into an istream_iterator<string>.
In practice, most people view them as explicit creation of a
temporary, the first initialized with "in", and the second
initialized using the default constructor. (Converting nothing
into something doesn't sound much like a cast in most people's
mind, regardless of what the standard calls it.)

--
James Kanze (Gabi Software) email: http://www.velocityreviews.com/forums/(E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

 
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James Kanze
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Posts: n/a
 
      06-17-2007
On Jun 17, 4:25 pm, Jess <(E-Mail Removed)> wrote:
> On Jun 17, 11:44 pm, Obnoxious User <O...@127.0.0.1> wrote:
> > On Sun, 17 Jun 2007 06:52:21 -0700, Jess wrote:
> > > I see my problems now, thanks!


> > > On the other hand, in program


> > > copy(istream_iterator<string> (in), istream_iterator<string>(),
> > > back_inserter(v));


> > > Does it convert "in" to an istream_iterator<string>? If so, what is
> > > "istream_iterator<string>()"? Neither of them looks like a function
> > > call or constructing an istream_iterator<string> object...


> > 'istream_iterator<string>(in)' creates a temporary object based on 'in'
> > 'istream_iterator<string>()' creates a temporary object representing the
> > end iterator.
> > 'back_inserter(v)' creates a temporary object
> > Consider:


> > class T {};


> > T t = T();


> I'm sometimes confused by the object creation syntax. To create an
> object t of type T, I think I can use the following statements.


> T t; //default constructor
> T t(arg); //constructor with argument arg
> T t = T(); //call default constructor for T()? then call copy
> constructor to create t using the temporary object created from T()
> T t = T(arg); //does the right hand side call the constructor with
> argument arg, then call the copy constructor to get t?
> T t = existing_t_object;
> T* tp = new T; //call default constructor
> T* tp = new T(arg); //constructor with argument arg


> Are they correct? Have I missed something?


They're all correct for creating named objects. There are,
however, two types unnamed objects: those created using operator
new (dynamically allocated objects), and temporary objects. A
temporary object is the result of an expression: in the case of
an object of class type, a function call or a "cast". Thus, for
example: "static_cast< T >( arg )" behaves exactly like "T
t(arg)", except that the resulting object is unnamed. Of
course, there are two other ways of writing
"static_cast<T>(arg)": "(T)arg" and "T(arg)". In the latter
case, the standard also allows 0 or more than one argument; it
still calls it a cast, even if it doesn't seem very logical.

--
James Kanze (Gabi Software) email: (E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

 
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