On Wed, Aug 19, 2009 at 5:14 AM, Adam Badura<abadura@o2.pl> wrote:
get_error_info takes a const exception object and returns a pointer to const data object of respective error_info. But why is there no way of getting non-const pointer (which would allow to modify the data)?
The easy answer is: because I haven't needed to modify the data, and nobody has requested it before. Logically, the error info interface is intended for recording various known facts about an exception object: "What's the name of the file that failed to open?" "What error code did the OS report?" Etc. etc. These things aren't mutable.
This is especially strange because operator << can be used to add data to a const exception object so I don't see reason for not being able to get modifiable data back (at least from non-const exception object).
Adding error info is supported for const exceptions because it isn't stored in the exception object itself, it's stored in a separate object the exception points to. This arrangement can't be just an implementation detail because the copy constructor is required to be nothrow. The other reason is to support the throw foo() << my_Info() syntax.
Having this would allow me to safely gather exception data before throwing or during exception propagation. What I wanted was to insert an empty vector of data to the preconstructed exception object. Then obtain the vector with get_error_info (since operator << adds a copy I have to get the actual object). Then call reserve on the obtained vector and start the actual operation. During the operation I can gather (possible) error data and safely add it to the vector since memory is already reserved. In the end if any error data was gathered I will throw the preconstructed exception object. In reasonable implementation this should be safe as well. (Similar thing might be done while propagating the exception.)
I'm curious, could you post more information about your use case? What's this vector, what kind of objects/info does it store? I'm not against your request, but I want to understand your motivation better. (Also note that a failure to add error info to an exception is not the end of the world, the system is exception-safe.) Emil Dotchevski Reverge Studios, Inc. http://www.revergestudios.com/reblog/index.php?n=ReCode