Explain the concept of double exceptions and how they differ from regular exceptions.

Double exceptions in C++ occur when, during the handling of one exception, another exception is thrown, for example, in a catch block or a destructor called during exception handling. This leads to a call to the std::terminate function because the system cannot properly handle two exceptions simultaneously.

The difference from regular exceptions is that a regular exception is a single error that the program attempts to handle, whereas a double exception is a situation where a second error occurs while handling the first, leading to the program terminating abruptly.

Example:

try {
    throw std::runtime_error("First exception");
} catch (...) {
    // During handling of the first exception, throw a second
    throw std::logic_error("Second exception");
}
// This will lead to a call to std::terminate

To avoid double exceptions, it is recommended not to throw exceptions from destructors and catch blocks.