several input-output questions

[Jess]

Hello,

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

vector<stringv;
ifstream in("file");
copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

However, I also tried to define istream_iterato r separately. I did

istream_iterato r<stringit(in) ;

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

istream_iterato r<stringit;

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

[John Harrison]

Jess wrote:

Hello,

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

vector<stringv;
ifstream in("file");
copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

However, I also tried to define istream_iterato r separately. I did

istream_iterato r<stringit(in) ;

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

istream_iterato r<stringit;

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_iterat or<string>()" mean? Does

it define an iterator that's not bound to any istream? If so, why my
code above "istream_iterat or<stringit;" 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

[Jess]

On Jun 17, 11:16 pm, John Harrison <john_androni.. [e-mail]>
wrote:

Jess wrote:

Hello,

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

vector<stringv;
ifstream in("file");
copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

However, I also tried to define istream_iterato r separately. I did

istream_iterato r<stringit(in) ;

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

istream_iterato r<stringit;

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_iterat or it();" and "istream_iterat or it;"
work, is there any difference?
Jess

[Robert Bauck Hamar]

Jess wrote:

On Jun 17, 11:16 pm, John Harrison <john_androni.. [e-mail]>
wrote:

>Jess wrote:

Hello,

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

vector<stringv;
ifstream in("file");
copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

However, I also tried to define istream_iterato r separately. I did

istream_iterato r<stringit(in) ;

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

istream_iterato r<stringit;

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_iterat or it();" and "istream_iterat or it;"
work, is there any difference?

istream_iterato r it();

declares it as a function returning an istream_iterato r, whereas

istream_iterato r it;

defines it as an istream_iterato r object and calls its default constructor.

--
rbh

[John Harrison]

Jess wrote:

On Jun 17, 11:16 pm, John Harrison <john_androni.. [e-mail]>
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_iterato r<string>(),
back_inserter (v));
However, I also tried to define istream_iterato r separately. I did
istream_itera tor<stringit(in );
this works, but there's no way I can define something equivalent to
istream_itera tor<string>() above. I tried
istream_itera tor<stringit;
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_iterat or it();" and "istream_iterat or it;"
work, is there any difference?
Jess

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

istream_iterato r<stringit;

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

istream_iterato r<stringit();

declares a FUNCTION called 'it' which takes zero arguments and returns a
istream_iterato r<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

[Jess]

I see my problems now, thanks!

On the other hand, in program

copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

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

Jess

[Obnoxious User]

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_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

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

Jess

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

Consider:

class T {};

T t = T();

--
Obnoxious User

[Jess]

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_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

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

Jess

'istream_iterat or<string>(in)' creates a temporary object based on 'in'
'istream_iterat or<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_obje ct;
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

[James Kanze]

On Jun 17, 3:52 pm, Jess <w...@hotmail.c omwrote:

I see my problems now, thanks!

On the other hand, in program

copy(istream_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

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

According to the standard, they are both function style casts.
The first converting in into an istream_iterato r<string>, and
the second converting nothing into an istream_iterato r<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: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[James Kanze]

On Jun 17, 4:25 pm, Jess <w...@hotmail.c omwrote:

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_it erator<string(i n), istream_iterato r<string>(),
back_inserter(v ));

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

'istream_iterat or<string>(in)' creates a temporary object based on 'in'
'istream_iterat or<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_obje ct;
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_cas t< 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: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]