[boost] [Exception] Why is there no non-const version of get_error_info?

Subject: [boost] [Exception] Why is there no non-const version of get_error_info?
From: Adam Badura (abadura_at_[hidden])
Date: 2009-08-19 08:14:39

    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)?

    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).

    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.)

    With current code (lack of non-const get_error_info) it is not possible. 
I have to get the vector from exception object. Then add data to the vector 
(this may fail). Then again add vector to the exception object (overwriting 
old vector; this may fail). And in the end throw (if needed).
    Alternatively I could preconstruct vector, reserve space add data to it. 
Then in the end if I will throw the data vector is there for me. However I 
still have to add it to the exception object and this still may fail.

    Adam Badura