[NEWBIE] istringstream-syntax

[Markus Kern]

Hi!

I want to do what has been tried before
by many: Converting a string into an integer
*the C++ way* (= using streams rather than atoi).
Knowing I'm not the first to wonder how, I
googled getting this solution only:

-----------------
int number;
string test = "1234";
istringstream buf(test);
buf >> number;
-----------------

However, what I want to do is to use buf *after*
defining test --or at least, after assing any
value to test, so something like:


-----------------
int number;
string test;
istringstream buf;

test="1234";
// and now turn "test" into an
// integer, maybe something like:
// buf(test) >> number;
// or
// buf=test; buf >> number;
------------------

Oh, BTW, I *have* read http://www.parashift.com/cpp-faq-lite/


Thanks for any suggestions in advance

M.K.

 

[Dietmar Kuehl]

int number;
string test;
istringstream buf;

test="1234";
// and now turn "test" into an
// integer, maybe something like:
// buf(test) >> number;
// or
// buf=test; buf >> number;

Maybe you should use something which will work:

/**/ buf.str(test); buf >> number;

Of course, you might want to use the more convenient
lexical cast from boost:

/**/ number = boost::lexical_cast<int>(test);

 

[Markus Kern]

Ah! Thank you, that was exactly what
I was looking for.

Never heard of boost:: before, however,
I googled and found boost.org where
I'm going to spend an hour or two

Thanks again!

M.K.

 

[Markus Kern]

Hi!

Still, the whole thing does not work the way I want it to.
Here's my code:

-----------------------------
#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main()
{
int number;
string test;
istringstream buf;

test="1234";
buf.str(test); buf >> number;
cout << test << " : " << number << endl;

test="555";
buf.str(test); buf >> number;
cout << test << " : " << number << endl;

return 0;
}
-------------------

Here's the output:

1234 : 1234
555 : 1234


Why is this? What can I do to get a "555 : 555" as second out?

Thanks

MK

 

[Karl Heinz Buchegger]

Hi!

Still, the whole thing does not work the way I want it to.
Here's my code:

-----------------------------
#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main()
{
int number;
string test;
istringstream buf;

test="1234";
buf.str(test); buf >> number;
cout << test << " : " << number << endl;

test="555";
buf.str(test); buf >> number;
cout << test << " : " << number << endl;

return 0;
}
-------------------

Here's the output:

1234 : 1234
555 : 1234

Why is this? What can I do to get a "555 : 555" as second out?

The problem is, that when you do the first extraction, the stream has already gone
into a fail state, since internally it reads past the 'end of file'. If you think
of it, it needs to do that in order to determine, that '4' was the last digit.
So you need to clear that stream from that fail state otherwise the stream won't
react to any other operation request. Member function clear() does this.

But: You should get into the habit of using an istringstream in such contexts
only one, and you will have no problems at all. Best encapsulate the functionality
into a function and forget about the details:

int ToValue( std::string Text )
{
istringstream buf( Text );
int number;

buf >> number;
return number;
}

int main()
{
std::cout << ToValue( "1234" ) << std::endl;
std::cout << ToValue( "555" ) << std::endl;
}

(You might want to add some error handling in the above, but in principle that's
it).