• Marc Celani's avatar
    exception wrapper · ad55cd0e
    Marc Celani authored
    Summary:
    folly::exception_wrapper is a different take on std::exception_ptr to
    suit a specific use case.
    
    The good: std::exception_ptr is not templated, so it can easily be used in
    different classes without template creep. You can pass errors around between
    threads or simply between modules. Rethrowing the exception throws the *proper*
    derived class exception, not some base class.
    
    The bad: Getting access to the exception requires throwing, which is expensive.
    Many users of popular frameworks that take advantage of std::exception_ptr
    check if the exception is set, and if so do some error handling without actually
    knowing the type of the exception or logging its message, just to avoid the cost
    of rethrowing the exception.
    
    The ugly: Creating an exception_ptr requires throwing the exception as least
    once. This is bad in the performance sensitive case where users will not even
    inspect the exception.
    
    This class takes advantage of the good while avoiding the requirement to throw.
    By using a templated deleter and thrower function, we can create an
    exception_wrapper which is properly managed, can be thrown, and can be retrieved
    as a void* with a get() function. Users that previously caught exceptions are
    now able to dynamically cast to different exception types they formerly caught
    to avoid the unwind cost while still getting details about the error.
    
    Test Plan: unit test
    
    Reviewed By: davejwatson@fb.com
    
    FB internal diff: D1305470
    
    @override-unit-failures
    ad55cd0e
ExceptionWrapperBenchmark.cpp 7.12 KB