Must "try" Have "catch" To Pair?

sunnysky

Mar 11 2007, 01:40 AM

Hi Programmers,

To my knowledge, a try block can stand alone. But my compiler keep
complaining untill I put a catch block after a try block. Below is
the code, what wrong did I do? Thanks.

Brian

CODE

#include <iostream>

using namespace std;

int f(int i)
{
  try {
    cout << ++i << endl;
  }

  return 1;

}

int main()
{
  int n = 1;

  f(n);

  return 1;

}

QUOTE

Compiler Message:
$ g++ test_try_catch.cpp
test_try_catch.cpp: In function `int f(int)':
test_try_catch.cpp:11: error: expected `catch' before numeric constant
test_try_catch.cpp:11: error: expected `(' before numeric constant
test_try_catch.cpp:11: error: expected identifier before numeric
constant
test_try_catch.cpp:11: error: invalid catch parameter
test_try_catch.cpp:11: error: expected `)' before numeric constant
test_try_catch.cpp:11: error: expected `{' before numeric constant

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WeaponX

Mar 11 2007, 04:39 AM

Please include quotes for your code and lines of errors shown. All members here must do that for all programming code they post. I have edited your post to enclose them in quotes.

Now to your question, I'm not a C++ guru or anything, but it looks like the Try, Catch and Throw statement requires this:

http://msdn2.microsoft.com/en-us/library/6dekhbbc.aspx

Does your program work out as expect when you put catch after the try block?

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mastercomputers

Mar 11 2007, 05:25 AM

A Try block can not stand alone, because you're using it to test if an exception is thrown and what to do to handle it. If an exception gets thrown it needs to know what to do with it when it's been caught. If you're not testing for an exception then there's no need to use a try/catch statement.

e.g.

CODE

int main() {
  try {
    throw 13;
  }
  catch(int exception) {
    cout << "An exception occured. Code: " << exception << endl;
  }
  return 0;
}

What this code does is throw an exception with an integer value of 13. The catch statement, caught this exception and does what I tell it to do when an exception occurs, which is to output a string to standard out, and display the exception value thrown.

Cheers,

MC

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pyost

Mar 11 2007, 09:55 AM

What's more, you could just add a catch block that would be empty.

CODE

try {
    cout << ++i << endl;
  }
  catch {
  }

Not quite clean, but works.

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bluefish

Mar 18 2007, 05:33 AM

A try block really does nothing on its own. If you want it to ignore exceptions, just put an empty catch block as shown above. The compiler doesn't do this automatically because there is to reason for it to "think" that this is the intended effect.

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vizskywalker

Mar 18 2007, 08:21 PM

Also, C++ was designed to not be a thinking language. It was designed for powerful speedy code. The assumption is that if a programmer wants to do something, they can explicitly have the program do it, so there is no reason for the compiler to do it, but if the programmer doesn't want it done, it is wasteful to do it. So the compiler does not put it an empty catch block moreso because it doesn't assume anything than because there is no reason for it to assume an empty catch block.

In regards to the empty catch block, however, it is bad practice to use one. The only time to use a try block is if the program is or could throw an exception. Since this exception will almost definitely impact later code's performance, it should be handled with at least a message to the user explaining the exception. Otherwise, the try block should be left out so the program will fail. Otherwise, unintended and possibly bad results may occur. For example, take a move file function. If it throws an exception when trying to create the new file because it fails, but handles it with a blank catch block, it may then delete the old file anyway. There goes some important data. So if you are using a try statement, always do something, at least output, in the catch.

~Viz

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