Exception Handling

[Captain Cosine]

Hello. I've been working recently on a recursive expression parser and have gotten to the phase of handling exceptions, such as division by zero, using non-numerical non-operator characters, and other syntax errors. I've read a few tutorials and references on exception handling, but I'm still a bit lost. I have code that works fine to handle errors, but just because it works doesn't mean it's right. I'm pretty certain that I have (1) unnecessary statements and (2) missing statements that would allow my program to run more efficiently.

Essentially, my question is in regards to the proper syntax and conventions when it comes to handling exceptions in C++.

Here is a snippet of the .cpp which contains a class, constructor (I think), and function to handle a few syntax errors:

class syntaxErr{
	public:
		syntaxErr();
};

syntaxErr::syntaxErr(){}

void syntax(const string & s){
	//Array of operators which cannot be next to one another
	const char ops[] = "*/";
	//Parentheses counter
	int parenth = 0;
	char ind, rInd;
	if(s.at(s.length() - 1) == ')' || s.at(s.length() - 1) == '(')
		parenth++;
	for(unsigned int i = 0; i < s.length() - 1; i++){
		ind = s.at(i);
		rInd = s.at(i + 1);
		if((ind == ops[0] || ind == ops[1]) && (rInd == ops[0] || rInd == ops[1]))
			//If the two aformentioned operators are next to one another
			//without a number between them, throw a syntax error.
			throw syntaxErr();
		else if(ind == '(' || ind == ')')
			parenth++;
	}
	if(parenth % 2 != 0)
		//If open and closed parentheses are not equal in number
		//throw a syntax error
		throw syntaxErr();
}

Again, it works fine, but it doesn't seem correct.

And here is a snippet of code from a separate .cpp which is used to catch exceptions:

try{
		syntax(s);
	}
	catch(syntaxErr){
		cerr << "Error:  syntax error" << endl;
		cin.get();
		exit(1);
	}

Any help/tips would be greatly appreciated.

[firedraco]

1) Derive your base exception from std::exception (or std::runtime_exception if you can)
2) I'd probably make syntaxErr a base class or at least add a .what() method that let's the user know what went wrong so you can use it like this:

throw syntaxErr("Divide by zero");
later
[code]catch(syntaxErr& err) {
std::cerr<<"Error: "<<err.what()<<"\n";
}

As for your parathenses handler, it seems wrong to me.

[Captain Cosine]

As I work on your suggestions in regards to the exception class itself, may I ask what you believe is wrong with the parentheses handler?

Edit:

Is this what you meant?

class syntaxErr : public exception{
	public:
		string message;
		syntaxErr(string msg);
		virtual const char* what() const throw(){
			return message.c_str(); 
		}
};

syntaxErr::syntaxErr(string msg) : message(msg){}

void syntax(const string & s){
	//Chunk of code from first post
		if((ind == ops[0] || ind == ops[1]) && (rInd == ops[0] || rInd == ops[1]))
			throw syntaxErr("syntax error.");
	}
	if(parenth % 2 != 0)
		throw syntaxErr("mismatched parentheses.");
}

And called with:

try{
		syntax(s);
	}
	catch(syntaxErr & err){
		cerr << "Error:  " << err.what() << endl;
		cin.get();
		exit(1);
	}