Thread: atoi() equivalent in C++

Yes. And the main problem of atoi() in my opinion is the absence of error checking both for overflow situations, but also regarding the contents of the string.

It stops at the first non numeric character and returns an int. Seems simple but it's actually a quiet strange function:

Code:

int foo = atoi("643bar") // returns 643. No error checking.
int foo = atoi("bar") // return 0(!!). No error checking.
int foo = atoi("0")  // obviously returns 0 too.

There's no error with the third atoi() call. But it poses a serious problem... there's no user-made error checking possible when the atoi parameter can be 0.

Actually I like that it does this:

Code:

int foo = atoi("643bar") // returns 643. No error checking.

Usually there's no need to do anything when 0 is returned. But yes, this can be a problem.

Using atoi() and itoa() is a bad idea...

This is the overflow I was talking about

Code:

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
int main() {
  char *big = "12345678901234567890";
  int i = atoi(big);
  perror( "Anything?" );
  printf( "%s = %d\n", big, i );
  return 0;
}

$ gcc -W -Wall -ansi -pedantic -O2 new.c
$ ./a.exe
Anything?: Numerical result out of range
12345678901234567890 = 2147483647

http://www.hmug.org/man/3/atoi.php
Some key phases
"The atoi() function has been deprecated by strtol()"
"The function atoi() need not affect the value of errno on an error."

My implementation would seem to use the strtol() equivalence, so your mileage may vary.

>You can only make atoi() or itoa() "fail", if you make a serious bug in your program...
atoi is a serious bug in your program. Short of a complete integer validation on the string before calling atoi (which I've only seen in my own code), you're risking undefined behavior every time you use it.

So, which do you think is better to use in C++, stringstreams or strtol()?

Stringstreams, but as others said once lexical_cast makes its way into the standard, it will be the best option by far.

I would agree lexical_cast or stringstreams. However, truth be told, for simple conversions where the string is well contained within accepted values, strtol can become a more practical option. (at least until lexical_cast doesn't make it to the standard)

Originally Posted by Daved

And lexical_cast basically uses stringstreams internally itself.

Does it? I've never had the pleasure of seeing the code myself. Does it actually use stringstreams or just the same background logic as stringstreams? I guess once it's standard that may just depend on your compiler's implementation.

Originally Posted by MSDN

In Visual C++ 2005, in the case of overflow with large negative integral values, LONG_MIN is returned. atoi and _wtoi return INT_MAX and INT_MIN on these conditions. In all out-of-range cases, errno is set to ERANGE. If the parameter passed in is NULL, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, these functions set errno to EINVAL and return 0.

So it is MS specific. I didn't know that.