Use purpose-designed user-defined types as exceptions (not built-in types)

Reason

A user-defined type can better transmit information about an error to a handler. Information can be encoded into the type itself and the type is unlikely to clash with other people's exceptions.

Example

throw 7; // bad

throw "something bad";  // bad

throw std::exception{}; // bad - no info

Deriving from std::exception gives the flexibility to catch the specific exception or handle generally through std::exception:

class MyException : public std::runtime_error
{
public:
    MyException(const string& msg) : std::runtime_error{msg} {}
    // ...
};

// ...

throw MyException{"something bad"};  // good

Exceptions do not need to be derived from std::exception:

class MyCustomError final {};  // not derived from std::exception

// ...

throw MyCustomError{};  // good - handlers must catch this type (or ...)

Library types derived from std::exception can be used as generic exceptions if no useful information can be added at the point of detection:

throw std::runtime_error("something bad"); // good

// ...

throw std::invalid_argument("i is not even"); // good

enum classes are also allowed:

enum class alert {RED, YELLOW, GREEN};

throw alert::RED; // good

Enforcement

Catch throw of built-in types and std::exception.