How to add exception handling - c++

Question

Hi everyone I would like to add exception handling for a couple conditions. Parentheses mismatch when unstacking and and if the expression entered by the user doesn't start and end with #. Mostly just looking for pointers and tips for expression handling in a class and main not a solution.
Here's what I was thinking but its obviously not correct.
This is done in my infixtopost class and I have class invalidExp(){} declared in my header file.
The other exception I tried to handle in main checking the line of input from the user with
if(line[0] != '#' || line[line.length()] != '#') throw "Invalid Expression"
but it would skip over this line and when I put it in a try catch block the rest of the code couldn't exectue.
while (!op->isEmpty() && op->top() != '(') throw invalidExp();
{
post.push_back(op->pop());
if (!op->isEmpty())
{
op->pop();
} //to remove ( from stack
}
main:
try{
IntoPost *infix = new IntoPost(token.length());
post = infix->inToPost(token);
cout << post << "=";
Evalexp *result = new Evalexp(post.length());
result->evalExp(post);
result->print();
}
catch (IntoPost::invalidExp){
cout << "Exception: Invalid Expression" << endl;
}

Answer 1

As I understand your question, you want to declare some custom exception types to handle errors in a clean way. Thats good and the following code will be an example of how this can be done. It's all just an example, to show what is possible.
0: Understanding throw
You can read more about throw here. But in general you can throw anything. throw 1; will throw an int for example. Your example throw "Invalid Expression" will throw a const char*. Usually you want to throw an object of a class that inherits from std::exception.
1: Declare a class for each type of exception you need to handle
It's good practice to let them extend from std::exception. In the constructor I usually create the exception message and pass it to the constructor of std::exception. So the following are just examples:
class ExceptionA : public std::exception {
public:
ExceptionA() : std::exception("Case A occurred!") { }
};
class ExceptionB : public std::exception {
public:
static string CreateMessage(int x, int y) {
// You can create a custom exception message here
}
ExceptionB(int x, int y) : std::exception(ExceptionB::CreateMessage(x, y)) {}
};
As you see in ExceptionB it's totally fine to attach additional data to your exception. This will help you to create a more specific exception message.
2: Sorround your code with a try-multicatch
You can read more about this here
It means, you can use multiple catch-blocks for each try-block.
void foo() {
try {
// Calcultion starts here
// Now you get unexpected input or something like this and signal it by throwing an exception.
if(errorCondition) { throw ExceptionA(); }
// Something else might happen here, so you throw the other type of exception
// in this example ExceptionB also takes two custom parameters.
if(otherErrorCondition) { throw ExceptionB(x, y); }
}
catch(ExceptionA &e) {
// Handle an occurence of Exception A
}
catch(ExceptionB &e) {
// Handle an occurence of Exception B
}
catch(std::exception &e) {
// Handle an occurrence of any other exceptions STL-components migth throw for example std::range_error
}
catch(...) {
// Handle the occurrence of any other thrown ... things. This will also catch things like throw 1, but usually you don't need this.
}
}
Here the order of the catch-blocks is important!
3: Enjoy clean exception handling!
I hope this answers your question!