Custom Exception Class

std::exception Class

In catching a std::bad_alloc exception, you actually caught an object of class std::bad_alloc thrown by new. std::bad_alloc is a class that inherits from C++ standard class std::exception , declared in header <exception>.

Class std::exception that is the base class supports a useful and important virtual method what() that gives a more descriptive reason on the nature of the problem causing the exception.

You can make use of std::exception being a base class for many exceptions types and create one catch(const exception&) that can catch all exceptions that have std::exception as base:

void someFunc()
{
    try
    {
        // code made exception safe
    }
    catch (const std::exception& exp) // catch bad_alloc, bad_cast, etc
    {
        std::cerr << “Exception encountered: “ << exp.what() << std::endl;
    }
}

You can throw an exception of whatever type you want. But, there is a benefit in inheriting from std::exception: all existing exception handlers that catch(const std::exception&) and work for bad_alloc , bad_cast and the like will automatically scale up to catch your new exception class as well because it has the base class in common with them.

# include <iostream>     // std::cout, std::endl
# include <string>       // std::string
# include <exception>    // std::exception

class CustomException: public std::exception
{
    private:
        std::string reason;
    public:
        // constructor of an exception
        CustomException(const char* why) : reason(why) {}

        ~CustomException() throw() {}

        // redefining virtual function to return ‘reason’
        virtual const char* what() const throw()
        {
            return reason.c_str();
        }
};

double divide(double a, double b)
{
    if(b == 0)
        throw CustomException("CustomException: Cant divide by 0");
    return (a / b);
}

int main()
{
    try
    {
        std::cout << "Result of division is: " << divide(12, 0);
    }
    catch(std::exception& exp)// catch CustomException, bad_alloc, etc
    {
        std::cerr << "Exception: " << exp.what() << std::endl;
    }
    return 0;
}