The uncaught_exceptions function is functionally equivalent to
unhandled_exceptions_count in Boost.Log and implements functionality
similar to the same named C++17 standard function. Tests and docs are
also included.
One notable difference from std::uncaught_exceptions is that the return
type is unsigned rather than signed. This is deliberate as uncaught_exceptions
must never return a negative value and unsigned int better documents that.
Theoretically, as a counter, it may also overflow.
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