Jarl Lindrud wrote:
Robert Ramey <ramey <at> rrsd.com> writes:
I'm still intrigued as to how such a situation could come up where something is saved through a pointer but in another program it isn't. I looked at the above link and didn't find this. Basically, my view is that anyone tripped up by this error can't be writing anything that makes sense anyway. I'm looking for a counter example to this.
Robert Ramey
I don't know this arose originally - there may well have been a situation where the vector<char> was serialized through a pointer in one program, and deserialized as a value, in another program. That is indeed an error, although in this case it does not seem that any exception was thrown to signify the error (the vector just silently failed to deserialize properly).
a) don't include class information in the archive advantage: faster performance since class info doesn't have to be looked up or each instance disadvantage: no way verify that the type being loaded is the same as that being stored b) include information in the archive advantage: slower performance since class info doesn't have to be looked up or each instance disadvantage: library can verify that the type being loaded is the same as that being stored I tried to choose reasonable defaults- it's the best I can do.
On a more fundamental level, I don't understand how this code can *ever* fail:
(1) std::ostringstream ostr; boost::archive::text_oarchive(ostr) & v0;
std::istringstream istr(ostr.str()); boost::archive::text_iarchive(istr) & v1;
bool ok = (v0 == v1);
This code can only fail if the saving template for V is different from that for V. Such a program can never work. Without class information inside the archive, I can't detected this type of error.
Here the vector is serialized and deserialized as a value. How is it that "tracking" or "implementation" settings for a particular type can be affected by the mere presence of this code.
I minor correction - "implementation_level" is not effected. tracking is effected if it has been set to "track_selectively" Do you mean how is something like this implemented? The short answer is that T *t ... ar << t instanciates code which refers to a special static object which "remembers" that this code has been invoked. This static object is initialized at pre-main time. So if anywhere else in the program one invokes T t ... ar << t Tracking is turned on for this operation. Note that this has the counter ituitive effect of seeming to "look ahead" in the code.
(2) ar << pVec; // pVec is a vector<char> *
, or this code
(3) ar >> pVec; // pVec is a vector<char> *
, in a completely different part of the program?
Snippet (2), if it appears in a program without snippet (3) also occurring, makes the program incorrect,
No it doesn't. The program is still correct. A valid archive is created. This program or any other can read this archive without problem by including (3). What cannot be done is
ar >> Vec; // Vec is a vector<char> (note missing *)
Since saving and loading types must match.
And even if snippet (3) is also present somewhere in the program, thus making the program correct in your view, IIUC the archives produced in (1) will then have a different format than those produced, when snippets (2) and (3) were absent. That would imply that archive formats can change silently, through mere addition of source code, anywhere else in the program. And the format change wouldn't be noticed until some *other* program tries to read the archives and fails to do so. Have I understood that correctly?
sort of, The format change will be notices when some tries to load a type different than the type saved. In this case, loading a non-pointer type where a pointer type was saved. Just remember this: If each load must use the exact same type as each corresponding save. If you follow this rule, you will never have a problem of this nature. Robert Ramey