Exception Handling

[lunchbearington]

Hi Guys,

I do not quite understand how exception handling is done exactly. I understand the "big picture" of what it is supposed to do, but I am having trouble making it work.

Here is a simple program. I kept it small for the sake of simplicity.

#include <iostream>
#include <math>

int main()
{
  int var1=0;
  cout<<"please enter in a number (positive only)."<< endl;
  try
  {
    cin>>var1;
  }
  catch (var1<0))
  {
      cout<<"negative numbers are not allowed"<<endl;
  }
  return 0;
}

The gist of this program is that I want someone to enter in a number that is positive only. If they type in a negative number, I want the catch block to throw up the message telling the user that negative numbers are not allowed. Please advise me on what I am doing wrong.

[kevinkjt2000]

Exception handling is for handling exceptions; it is not for things that are normal. Normal things, such as checking if an integer is less than zero, are not exceptions. In C++ you can throw pretty much anything, but people usually just throw some derivative of an exception class object.
http://www.cplusplus.com/doc/tutorial/exceptions/

[lunchbearington]

Hi Kevinkjt2000,

I actually have seen that link, but I'm confused by the first example. See line in bold in the code below:

// exceptions
#include <iostream>
using namespace std;

int main () {
  try
  {
    throw 20;
  }
  catch (int e) //I didn't realize that the bold wasn't going to show up very well. this is the line that confuses me.
  {
    cout << "An exception occurred. Exception Nr. " << e << '\n';
  }
  return 0;
}

The code says "Catch (int e)" but I do not understand entirely what this line means or the significance of it. This is where I am struggling.

Please note: I am not a C++ expert. I am simply some guy trying to teach himself C++. All comments/feedback/etc are welcomed.

[lunchbearington]

int main()
{
	int var1 = 0;
	cin >> var1;
	try
	{
		if (var1 < 0){throw 1;}
	}
	catch (int x)
	{
		
		cout << "Error Number: " << x << ". Your value: " << var1 << ". Is not valid." << endl;
	}
	system("Pause");
	return 0;
}

This works without giving me any issues. For reasons that I do not understand, "catch (int x)" takes a value of 1. Why is this referencing the number in my throw value?

[Disch]

That's how throw/catch works.

You have to throw something. Then, in your catch statement, it catches whatever you throw. Here, you are throwing 1, so that's what you're catching.

Note that usually you would throw something that is derived from the std::exception class, rather than just throwing an int.

[lunchbearington]

Note that usually you would throw something that is derived from the std::exception class, rather than just throwing an int.

Are there any advantages/disadvantages to throwing an exception class rather than an int?

My experience with C++ is limited to being on like chapter five of the book I'm reading haha, so sorry for the dumb question.

[E0p9LyTq]

Are there any advantages/disadvantages to throwing an exception class rather than an int?

http://www.cplusplus.com/reference/exception/

"This header <exception> defines the base class for all exceptions thrown by the elements of the standard library: exception, along with several types and utilities to assist handling exceptions"

[Disch]

lunchbearington wrote:

My experience with C++ is limited to being on like chapter five of the book I'm reading

Exceptions aren't really a beginner topic. Honestly I would say don't worry too much about them yet. They're a pretty complex topic to understand properly, and you're almost certainly going to misuse them as a beginner.

Are there any advantages/disadvantages to throwing an exception class rather than an int?

It's almost entirely advantageous to throw proper exception objects, but it'd be difficult to explain why without getting into a big spiel about how exceptions are typically used -- which I'm too tired to do right now =P