Correct way to inherit from std::exception

Question

I\'ve just created exception hierarchy and wanted to pass char* to constructor of one of my derived classes with a message telling what\'s wrong, but apparently

Answer 1

If you want to make use of the string constructor, you should inherit from std::runtime_error or std::logic_error which implements a string constructor and implements the std::exception::what method.

Then it's just a case of calling the runtime_error/logic_error constructor from your new inherited class, or if you're using c++11 you can use constructor inheritance.

Answer 2

If your goal is to create an exception so that you do not throw a generic exception (cpp:S112) you may just want to expose the exception you inherit from (C++11) with a using declaration.

Here is a minimal example for that:

#include <exception>
#include <iostream>

struct myException : std::exception
{
    using std::exception::exception;
};

int main(int, char*[])
{
    try
    {
        throw myException{ "Something Happened" };
    }
    catch (myException &e)
    {
        std::cout << e.what() << std::endl;
    }
    return{ 0 };
}

As Kilian points out in the comment section the example depends on a specific implementation of std::exception that offers more constructors than are mentioned here.

In order to avoid that you can use any of the convenience classes predefined in the header <stdexcept>. See these "Exception categories" for inspiration.

Answer 3

I use the following class for my exceptions and it works fine:

class Exception: public std::exception
{
public:
    /** Constructor (C strings).
     *  @param message C-style string error message.
     *                 The string contents are copied upon construction.
     *                 Hence, responsibility for deleting the char* lies
     *                 with the caller. 
     */
    explicit Exception(const char* message):
      msg_(message)
      {
      }

    /** Constructor (C++ STL strings).
     *  @param message The error message.
     */
    explicit Exception(const std::string& message):
      msg_(message)
      {}

    /** Destructor.
     * Virtual to allow for subclassing.
     */
    virtual ~Exception() noexcept {}

    /** Returns a pointer to the (constant) error description.
     *  @return A pointer to a const char*. The underlying memory
     *          is in posession of the Exception object. Callers must
     *          not attempt to free the memory.
     */
    virtual const char* what() const noexcept {
       return msg_.c_str();
    }

protected:
    /** Error message.
     */
    std::string msg_;
};

Answer 4

How about this:

class My_Exception : public std::exception
{
public:
virtual char const * what() const { return "Something bad happend."; }
};

Or, create a constructor accepting the description if you like...

Answer 5

The what method is virtual, and the meaning is that you should override it to return whatever message you want to return.