static istringstream

[sergey.lukoshkin]

Hello everyone!

My task is in converting numbers from string to int variables. I used
istringstream to perform it.
So I wrote simple test function. But it doesn't work as I expected
because val is not being changed while the program is running. Could
you please tell me why ??

#include <iostream>
#include <sstream>
#include <iomanip>
#include <stdint.h>

int main()
{
static std::string str;
static std::istringstream istr;
static unsigned val;

while( true )
{
std::cout << "Enter num: " << std::endl;
std::cin >> str;

istr.str(str);

istr >> val;
std::cout << "Number: " << val << std::endl;
}

return 0;
}

 

[Abhishek Padmanabh]

My task is in converting numbers from string to int variables. I used
istringstream to perform it.
So  I wrote simple test function. But it doesn't work as I expected
because val is not being changed while the program is running. Could
you please tell me why ??

#include <iostream>
#include <sstream>
#include <iomanip>
#include <stdint.h>

int main()
{
     static std::string str;
     static std::istringstream istr;
     static unsigned val;

    while( true )
    {
        std::cout << "Enter num: " << std::endl;
        std::cin >> str;

        istr.str(str);

        istr >> val;
        std::cout << "Number: " << val << std::endl;
    }

    return 0;
}

You need to clear the buffer held by the stringstream object. Make a
call istr.str("");

Alternatively, why don't you use boost::lexical_cast<> which is just a
header only library from boost? Saves you from error handling routines
as well which otherwise you need to employ yourself for cases where it
fails.

 

[Abhishek Padmanabh]

You need to clear the buffer held by the stringstream object. Make a
call istr.str("");

Sorry, correction : it seems eof() bit is being set for the
stringstream. And hence it further doesn't work. You need to clear
that flag for the next iteration. For that, you need to call the
clear() member. Verified that by putting in:

std::cout << std::endl << istr.eof() << " " << istr.fail() << " "
<< istr.bad() << std::endl;

You need to check those flags, eof() may be ok but if the rest of the
2 bits are set then that would probably mean an error with the IO
operation.

 

[sergey.lukoshkin]

Sorry, correction : it seems eof() bit is being set for the
stringstream. And hence it further doesn't work. You need to clear
that flag for the next iteration. For that, you need to call the
clear() member. Verified that by putting in:

std::cout << std::endl << istr.eof() << " " << istr.fail() << " "
<< istr.bad() << std::endl;

You need to check those flags, eof() may be ok but if the rest of the
2 bits are set then that would probably mean an error with the IO
operation.

Thanks a lot ! Now I call istr.clear() and it works properly.
Here my fixed code.

P.S I've never used boost lib.


int main()
{
static std::string str;
static std::istringstream istr;
static unsigned val;

while( true )
{
std::cout << "Enter num: " << std::endl;
std::cin >> str;

istr.clear();
istr.str(str);

istr >> val;
std::cout << "Number: " << val << std::endl;
}

return 0;

- Hide quoted text -
- Show quoted text -
}

 

[sergey.lukoshkin]

You should, lots of it will be included in the next C++ standard.




main() is called only once so the keyword 'static' does not change
anything here. Why do you think 'static' is needed?

And why do you define them here, not inside the loop where they are used?
Would have avoided all the trouble in the first place...

Well, I agree with you. In this case there is no benefit of using
static keyword. But if I take the contents of main() and put it in my
function that I'm going to call many times at a runtime it will reduce
the cost of function calling.

 

[Kai-Uwe Bux]

Well, I agree with you. In this case there is no benefit of using
static keyword. But if I take the contents of main() and put it in my
function that I'm going to call many times at a runtime it will reduce
the cost of function calling.

a) You should measure that.

b) This move will make your function unsuitable for multi-threaded client
code. You should definitely document that it is not reentrant.


Best

Kai-Uwe Bux

 

[sergey.lukoshkin]

a) You should measure that.

b) This move will make your function unsuitable for multi-threaded client
code. You should definitely document that it is not reentrant.

Best

Kai-Uwe Bux

Words of wisdom.....

But all in all I have to call istr.clear(). Because the specific of my
function is to process string in cycle. Thanks for answers!!

 

[sergey.lukoshkin]

IOW, the sooner you wander away from the perils of premature optimization
the better! For deciding what would be the best optimization you need to
be an expert, and even then you have to measure your hypothesis. Here I
think a possible optimization could be to use a certain C function
instead of multiple C++ objects construction, but I'm not sure at all.

Ok. Imagine I got a string representing IPv4 address "1ABC2DF1" and I
should convert it to classical view "26.188.45.241". It seems to me
that the best way to to this is using stringstream...

 

[sergey.lukoshkin]

(e-mail address removed) kirjutas:





Yes, why not? There is nothing wrong with stringstream. I objected only to
using static objects, and reusing the same objects without any sound
reason.

Cheers
Paavo

Thank you!

 

[James Kanze]

Well, I agree with you. In this case there is no benefit of
using static keyword. But if I take the contents of main() and
put it in my function that I'm going to call many times at a
runtime it will reduce the cost of function calling.

On the other hand, it will work correctly and be maintainable.

And who says it will reduce the cost of calling the function.
Until you have actual measurements indicating that this is
causing a real performance problem, it's just plain stupid to
add complexity for nothing. If you want a new, clean object,
you construct one.