On Tue, Nov 01, 2005 at 07:05:20PM -0600, David Wojtowicz wrote:
> I guess my main point was that it is too easy to create code that looks
> perfectly valid (like my example), but had unintended side effects, due to
> the hidden nature of destructors being called as objects go out of scope.
>
> Here is an even simpler piece of code that runs into the same problem.
>
> int main(int argc,char **argv) {
>
> ResUse use;
> Connection con(....);
> Query q=con.query();
> use=q.use("....");
>
> }
I do agree, it is too easy to write code like this, and I'm in favour
of making it hard to use code wrong. Good example too.
> I don't know if the C++ standard prescribes the order in which destructors
> must be called when multiple objects go out of scope or not, but gcc anyway
> reasonably destructs them in the reverse order they were constructed in.
Yes, the C++ destruction order guarantees a bug with the above example.
> In the above code 'con' gets destructed before 'use', which is a problem.
> Granted, if you understand the internals of mysql++, then you can see why
> this is a problem. But it's not entirely obvious to anyone else. After
> all, you can do without repercussions:
>
> {string A; string B; B="text"; A=B;}
The mysql++ classes are not completely independent like string, so even
if we wanted to, I don't think that goal is appropriate.
> 2) Make the default ctors for ResUse and Result private (which is not
> unlike there being no default ctors for Query and Connection) This would
> require making Query a friend of both classes. It would prevent you from
> compiling the problem code example above... and force you to create a ResUse
> from an existing object, thus in the proper order. Though it won't help in
> my original example where I explicitly called delete on the Connection.
I like this idea. It helps to force the correct order of construction.
The only problem I can think of is that it may make it harder to build
a class that contains a ResUse object. It forces the query to happen
in the constructor of such a class, or it forces dynamic allocation.
- Chris
