try-catch-throw

This is a discussion on try-catch-throw within the C++ Programming forums, part of the General Programming Boards category; I don't really understand the advantages with the try-catch-throw statements. I took a look at msdn explanation of the statements ...

  1. #1
    Algorithm engineer
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    try-catch-throw

    I don't really understand the advantages with the try-catch-throw statements. I took a look at msdn explanation of the statements and here's an example they've given:

    Code:
    int main() {
       char *buf;
       try {
          buf = new char[512];
          if( buf == 0 )
             throw "Memory allocation failure!";
       }
       catch( char * str ) {
          cout << "Exception raised: " << str << '\n';
       }
    }
    If I would have done the same thing I would have written:

    Code:
    int main() {
       char *buf;
       buf = new char[512];
       if (buf == 0) {
          cout << "Memory allocation failure!\n";
          cout << "Exception raised: " << str << '\n';
       }
    }
    I don't really se the difference and my is much more compact
    Last edited by TriKri; 07-03-2007 at 03:34 PM.
    Come on, you can do it! b( ~_')

  2. #2
    (?<!re)tired Mario F.'s Avatar
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    Yeah. They repeat that kind of stuff on tutorials all over the web.
    The hint is: forget about that throw.

    Here's something a little better. http://www.gamedev.net/reference/art...article953.asp

    Just keep reading past the first code you see.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


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    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  3. #3
    Registered User
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    The advantage is that exceptions automates the practice of bubbling up boolean returns. For example, this might be exception-less error checking code.
    Code:
    // Reads a person's age from the string database
    // Returns 0 on error and sets errno
    long getAge(char const * name) {
       char const * age_str;
       long age_num;
    
       age_str = database_lookup (name);
       if (errno) // Database lookup error
          return 0;
    
       if (age_str == NULL)
       { // Can't read a null value.
          errno = 1;
          return 0;
       }
    
       char * endptr;
       age_num = strtol (age_str, &endptr, 10);
    
       if (errno) // Range error
          return 0;
    
       if (*endptr != 0) // Didn't read the whole string, it's not an int
       {
          errno = 1;
          return 0;
       }
          
       return age_num;
    }
    The hope with exceptions is that we could change that to something like this...
    Code:
    // Reads a person's age from the string database
    // Throws on err.
    long getAge(char const * name) {
       char const * age_str = database_lookup (name);
    
       if (age_str == NULL)
       { // Can't read a null value.
          std::ostringstream err;
          err << "DB field USERS.AGE_STR contains NULL field for USERS.NAME = " << name;
          throw database_error (err.str());
       }
    
       return boost::lexical_cast<long>(age_str);
    }
    The second snippet handles all the same errors in all the same places, it's just all done under the hood.


    The following links goe pretty into "Why exceptions are great" and how to properly use them, albeit light on examples.

    http://www.parashift.com/c++-faq-lite/exceptions.html
    http://www.scottcollins.net/articles/exceptions.html
    Callou collei we'll code the way
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  4. #4
    Registered User
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    Quote Originally Posted by TriKri View Post
    Code:
    int main() {
       char *buf;
       buf = new char[512];
       if (buf == 0) {
          cout << "Memory allocation failure!\n";
          cout << "Exception raised: " << str << '\n';
       }
    }
    I don't really se the difference and my is much more compact
    compact and wrong. to make this do what you expect you'll have to use
    Code:
    buf = new(std::nothrow) char[512];

  5. #5
    Algorithm Dissector iMalc's Avatar
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    It doesn't help that the msdn example is out of date to the point of being wrong!
    As hinted at, 'new' never returns NULL any more; it just throws.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

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