Subclassing from std::exception

[laserlight]

I prefer to subclass either std::logic_error or std::runtime_error from <stdexcept>, upon which you can simply write something like:

class MyException : public std::runtime_error
{
public:
    MyException() : std::runtime_error("My error message.") {}
};

EDIT:

However, I've found that I can also get the functionality I want by using the std::exception ctor that takes a string to set the error message & then I don't need to override what(). This seems to me to be preferable, so I'm wondering why this isn't the standard approach to subclassing std::exception. Is there a pitfall to this approach that I'm missing?

There is no such constructor for std::exception. If there is for you, then you're using an extension to the standard library, so it is non-standard.