异常处理

Question

I get the following core dump msg:

terminate called after throwing an instance of 'std::out_of_range' what(): basic_string::substr Abort - core dumped

I am reading a 14 digit hexadecimal number from a large file. periodically i notice that there are these blank lines (well i am not sure what it is. How do I handle this exception? maybe a try catch thingy? It looks something like below:

123456789ABCDE
123456789ABCDE
123456789ABCDE

123456789ABCDE

I am not sure what hidden symbol is occupying the space but its causing issues and i am not sure how to handle this..any ideas? maybe i could get a sample of how to handle it?

Answer 1

You need to post more code but from the exception it looks like you're reading the file line by line and storing the line in an std::string , then calling std::string::substr() to extract 14 characters you want.

Assuming your code looks like this:

std::string str;         /* the lines are stored in this string */
std::string substring;   /* extracted substring stored here */

/* Read file line by line */
// ...

substring = str.substr( index, 14 );  //extract 14 characters starting at 'index'

Change this to:

if( str.size() > index ) {
  substring = str.substr( index, 14 );
}

Answer 2

You probably need to include input validation before you try the conversion. Never accept external input as is and always validate.

Answer 3

The Robustness principle (also known as Postel's law):

Be conservative in what you send; be liberal in what you accept.

So, one possibility is just to ignore/skip the malformed lines.

Answer 4

Others have given the specific answer for this specifc issue.

But in general run this in a debugger and see where it throws. For gdb do 'catch throw' gdb will then break at the point where the error is. It will be pretty obvious what the cause is.

Answer 5

The following code likely shows the possible issue:

#include <iostream>

int main(){
    std::string s = "Hi";

    std::string sb = s.substr(14, 0);  // extract 0 characters from 
                                       // an out-of-range 
                                       // start location
}

basic_string<charT,traits,Allocator>
substr(size_type pos = 0, size_type n = npos) const;

If the start index(first argument) is invalid (greater than the size of the string object), the standard mandates the method to throw 'out_of_range' error.

It also looks like you are not having the string operations with try/catch. This is not a good practise. Have all code that can throw exceptions within a try catch block. This gives you a chance to handle the exception (if you so wish).