On 9/20/18 11:06 AM, H. S. Teoh wrote:
On Thu, Sep 20, 2018 at 08:48:13AM -0400, Steven Schveighoffer via
Digitalmars-d wrote:
[...]
But this means you still have to build msg when throwing the
error/exception. It's not needed until you print it, and there's no
reason anyway to make it allocate, even with RAII. For some reason D
forces msg to be built, but it does't e.g. build the entire stack
trace string before hand, or build the string that shows the exception
class name or the file/line beforehand.
[...]
IIRC, originally the stacktrace was also built at exception construction
time. But it was causing a major performance hit, so eventually someone
changed the code to construct it lazily (i.e., only when the catcher
actually tries to look it up).
I think it makes sense to also make .msg lazy, if the exception object
is already carrying enough info to build the message when the catcher
asks for it. And if the catcher doesn't ask for it, we saved an extra GC
allocation (which is a plus even if we're not trying to go @nogc).
Except we DON'T construct the stack trace string, even lazily. If you
look at the code I posted, it's output directly to the output buffer
(via the sink delegate), without ever having allocated.
I think we can do that for the message too (why not, it's all
supported). But either one (using GC at print time, or lazily outputting
to buffer at print time) solves the biggest problem -- being able to
construct an exception without the GC.
Plus, this nudges developers of exceptions to store more useful data. If
you catch an exception that has details in it, possibly it is only going
to be in the string, which you now have to *parse* to get out what the
problem was. If instead it was standard practice just to store the
details, and then construct the string later, more useful information
would be available in the form of fields/accessors.
Think about this -- every ErrnoException that is thrown allocates its
message via the GC on construction. Even if you catch that and just look
at the errno code. Even with dip1008:
https://github.com/dlang/phobos/blob/6a15dfbe18f9151379f6337f53a3c41d12dee939/std/exception.d#L1625
-Steve
