throw-expression: throw assignment-expressionopt
Evaluating a throw-expression with an operand throws an exception; the type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand and adjusting the type from “array of T” or function type T to “pointer to T”.
A throw-expression with no operand rethrows the currently handled exception. The exception is reactivated with the existing exception object; no new exception object is created. The exception is no longer considered to be caught. [ Example: Code that must be executed because of an exception, but cannot completely handle the exception itself, can be written like this:
try {
// ...
} catch (...) { // catch all exceptions
// respond (partially) to exception
throw; // pass the exception to some other handler
}— end example ]
If no exception is presently being handled, evaluating a throw-expression with no operand calls std::terminate().