Exception specifications on functions useless?
l.lunak at suse.cz
Tue Mar 20 14:50:02 PDT 2012
I've just found out that exception specification on functions in LO have been
just pretty comments, for about 10 years.
Just to make sure what I'm talking about, it's the throw() in e.g. this
void foo( int a, void* b ) throw( css::uno::RuntimeException );
which says what exceptions a function can throw.
I found out by compiling LO with Clang, running it and having it crash where
gcc-compiler version had no problem. Stephan Bergmann and repo history
revealed that in 2001 -fno-enforce-eh-specs was added to gcc's flags, turning
off code generated that ensures these specifications are actually followed
(and MSVC has reportedly never cared). Clang does not have the option. I've
already actually fixed the problem (in
vbahelper/inc/vbahelper/vbaccesshelper.hxx) and it appears the specifications
somehow magically haven't rotten, as now LO runs fine for me (as far as I can
say, I can't test everything).
That however brings up the question of what the specification are for in the
codebase. The 'Deprecated Exception Specifications, Added noexcept' part of
is quite interesting read, the summary is:
- the specifications enforce that disallowed exceptions are not thrown at
runtime, adding code to each such function to check it, thus actually making
the performance worse
- boost, and pretty much everybody else it seems, do not consider it worth the
hassle of specifying what a function may or may not throw, except for
flagging a non-inline method as not throwing any exception at all as a means
of optimization for places where such functions are called
- C++11 deprecates it and instead introduces a noexcept keyword which forbids
the function to throw anything
I'm not strongly biased either way, but what we have right now are really
just pretty comments on functions. I think we should either say that we use
the specifications, in which case -fno-enforce-sh-specs should not be used at
least in debug builds, or we can say we follow the C++11/Boost/etc. trend and
not use them, in which case we can have one macro for portable way of saying
noexcept and have an EasyHack for removing the other specifications. BTW, it
should be also noted that SAL_THROW() expands to nothing with gcc.
Opinions on this?
l.lunak at suse.cz
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