Re: sorting 5million minute data: choose std::list or std::vector

From:

Ian Collins <ian-news@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 15 Feb 2011 11:38:08 +1300

Message-ID:

<8rtp6gFg8vU3@mid.individual.net>

On 02/15/11 11:29 AM, Paavo Helde wrote:

Pete Becker<pete@versatilecoding.com> wrote in news:2011021415103940206-
pete@versatilecodingcom:

On 2011-02-14 14:03:02 -0500, Paavo Helde said:

I would certainly prefer std::string to use a signed type for

size_type.

For example, the npos value -1 would be something much more concrete

and

would be representable in any signed type, regardless of whether the
string::size_type is 32 or 64 bit.

I don't get the point here. -1 is representable in every unsigned type.
It doesn't matter whether the underlying size_type is 32 or 64 bits.

#include<string>
#include<iostream>

int main() {
    std::string s = "abc";
      // unsigned used because "indexes are not negative"
    unsigned int k = s.find('x');
    if (k==s.npos) {
        std::cout<< "ok\n";
    } else {
        std::cout<< "FAIL\n";
    }
}

This works fine if size_type is the same size than unsigned int (e.g.
32bit), but fails in a common 64-bit compilation.

Even worse, if you were to use unsigned long, it will work on Unix and
Unix like 64 bit systems, but fail on windows.

If npos was signed and -1, then this would work fine with "int k",
regardless of the size of std::string::size_type. Actually, it seems to
work fine with "int k" even now in a 64-bit MSVC compilation, but this
involves unsigned -> signed conversion of an out-of-range value and is
thus implementation-defined behavior.

Currently the correct solution is to use std::string::size_type instead
of int or unsigned int. However, this is an extra piece of detail to
follow and not so easy to get correct at first attempt. If I know that my
app is only dealing with ordinary strings, easily addressable by a plain
int, why should I need to take care about such details?

I've often wondered why std::string find doesn't return an iterator, to
be consistent with std::find and container find methods. Then there
would be no need for std::string::npos.

--
Ian Collins