String At function run time error

Question

Here is a snippet of the code that creates a run time error "Microsoft Visual C++ Runtime Library" http://www.flickr.com/photos/66130188@N07/6023459646/

string text = something;
size_t index = text.find("hoopla");
try{
    if(text.at(index-1)<'0'&&text.at(index-1)>'9')
       return false;
}catch(out_of_range){return true;}

I am running it in Qt creator. It is not triggering the catch block. When the program reaching the text.at(index-1) and index-1 is out of bounds, it creates the run time error in Qt http://www.flickr.com/photos/66130188@N07/6023453724/

I did not have a problem when I used MVS2010. Any suggestions?

(Never mind. I see you're not really coding in Qt.)

Hot Licks
– Hot Licks

2011-08-08 21:03:25 +00:00
Commented Aug 8, 2011 at 21:03 — Hot Licks
– Hot Licks, Commented Aug 8, 2011 at 21:03
@Daniel R Hicks I am using C++ object in a Qt C++ project.

ßee
– ßee

2011-08-08 21:59:20 +00:00
Commented Aug 8, 2011 at 21:59 — ßee
– ßee, Commented Aug 8, 2011 at 21:59

Kerrek SB · Accepted Answer · 2011-08-08 21:49:43Z

1

You can avoid the exception check entirely by simply checking the return value of find first:

if ((index == std::string::npos || index == 0)        ||
    (text[index - 1] < '0' && text[index - 1] > '9')    )
{
  return false;
}

In the first case, npos, the search string wasn't found, and in the second case it is right at the beginning of the ambient string so you cannot look at the character before it.

(This is called "offensive programming": Don't check for errors at runtime, but construct your algorithm so that you know that your access is correct. If you will, you can add an assertion assert(index < text.length()); to express your certainty that you are holding a correct value, which will not weigh down your release version.)

Update: Replaces .at() by [] since we're sure of ourselves.

edited Aug 8, 2011 at 21:49

answered Aug 8, 2011 at 20:59

Kerrek SB

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11 Comments

vines Over a year ago

as it's stated in cplusplus.com/reference/string/string/at : If the position passed is past the end of str, an out_of_range exception is thrown.

Kerrek SB Over a year ago

@vines: What's that got to do with my answer?

Hot Licks Over a year ago

But the position passed isn't "past the end of str", it's negative.

Thomas Matthews Over a year ago

Is it negative or an unsigned large value?

vines Over a year ago

string::at(size_t), where size_t is (obviously) unsigned. @Kerrek SB: it has to do the following: why there's (supposedly) no exception where it should be?

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