[PING] Re: Add const char* constructors for exception classes in <stdexcept>

[Oleg Endo]

On Wed, 2014-01-29 at 21:38 +0000, Jonathan Wakely wrote:
> On 29 January 2014 21:17, Oleg Endo wrote:
> > My original intention was to eliminate code bloat when doing something
> > like  throw std::logic_error ("cold coffee");
> > If the const char* overloads are inlined it will emit code to construct
> > an std::string from const char* in user code where the exception is
> > being constructed over and over again.  The idea was to move that code
> > into the std library.
> 
> That's exactly what happens today with the constructors that only take
> a std::string, so it wouldn't be any worse than what we have now,
> would it?

Sorry, I'm not sure I understand your question.  Maybe you meant "any
better than what we have"?  Anyway, I've attached two outputs that show
what I mean.  The version with the const char* ctor overloads
implemented in the library is significantly shorter in the throwing path
(5x function call vs. 7x function call + other inlined std::string ctor
code).

> 
> > BTW the original patch was posted during Stage 3 (19.12.2013).  I don't
> > mind waiting until Stage 1 if adding exports now is a problem.
> 
> OK, let's wait and decide how we want to do it properly in stage 1.

Sure.  Actually I've missed some of the other exception types in
system_error, which should be added, too.  That would eliminate the
   TODO: Add const char* ctors to all exceptions.

I'd also propose moving the system_error ctor implementations into the
library as well, for the same reasons as above.

> 
> (If we're going to make various changes that impact the ABI during the
> next stage 1 we might even want to consider changing the
> std::exception base class to store something like a
> std::shared_ptr<std::string> so that copying an exception object will
> never throw an exception, which is something I've been thinking about
> recently.)

Wouldn't using std::shared_ptr<std::string> introduce an additional heap
allocation when creating the exception, though?
How about storing the exception message in a plain zero terminated
string instead?  This would require only one heap allocation if the
string data is prefixed with a refcount variable.  Basically,
std::make_shared re-invented because it doesn't work with arrays.
Or maybe implement N3640 + N3641 first (even if for library internal use
only as a start)...

Cheers,
Oleg
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