[boost] [system] error report refinement

From: Beman Dawes (bdawes_at_[hidden])
Date: 2006-08-28 09:49:28

For a library function:

Some system errors are best handled by aways throwing an exception. For 
example, a constructor should always throw if there is no way to leave 
the object in a valid state when a system error is encountered.

Some system errors are best handled by ignoring the error. For example, 
an error in a destructor.

But for all the other cases, the recommended approach is to provide 
throwing and non-throwing versions of the function, and let the user 
choose which is best for a given use:

   whatever foo();                   // always throws on error
   whatever foo( error_code & ec );  // never throws on error

Although this works well, it has the unfortunate consequence of doubling 
the number of functions. That makes the interface harder to understand 
and is a pain to implement, document, and maintain.

A refinement is for the <error_code.hpp> header to provide a dummy 
error_code that is interpreted as a request to thrown on errors:

   extern error_code throw_on_error;

Then only a single signature is required:

   whatever foo( error_code & ec = throw_on_error );

On an error, the implementation of foo throws if ec != &throw_on_error, 
otherwise it sets ec to the appropriate error_code and returns without 
throwing.

This approach gives the user the same choice, but halves the number of 
functions. I can't think of any downside.

Comments?

--Beman