probleme with memory management in threads and dll modules

hi,

i've got a problem on a school project im working on. it's
a web server wich processes requests throught modules that
can hook at any stage of the request treatment.

these modules are classes that has the same base and do their
job in a member function. they are dlls loaded dynamically
throught configuration file or user command. modules store their
results in a class containing the request.

each client/request is managed in a thread and the data object
that store the request is created at the begining of the thread
proc function. it is passed throught each module and used to store
their result in public string members vars (most). those strings are
modified in the modules member functions with constant strings like :

data->result_string = "HTTP/1.1 200 OK";

so i come to my problem :) after that my server send the response,
when the thread proc is finished, the destructor of my data class
is called then destructors of the members strings. and at the
time the string destructor delete the _Ptr member of the string
class (in function deallocate), i've got a user breakpoint (the
programme stop) in debugging mode folowed (if i resume execution)
by a "debug assertion failed" error in dbgheap.c at expression
_CrtIsValidHeapPointer. the adress of _Ptr is the same that when
i initialized it in the modules functions.

I've tried some things like replacing the string member by a char*
initialized by strdup("mystr") but i can't free it in the destructor
and if i don't, it's the precedent member string in the data class
declaration that causes the same trouble.
i even thought that it was because the stack of the thread was freed
before the destructor of my class was called, so i tried allocating
the class with the new operator and deleting it at the end of the
thread. but it was strictly the same.

i don't really have anymore idea. i use dll like this for the first
time. it seems that the locals and allocated adresses are
not anymore valid after a module function was executed.
i really don't know how to produce data in a module if it's the case :)
does i have to allocate a big buffer before calling the modules
in order to put all the strings they produce in? it would suck a lot :)

thanks for your help,

--
Lucas
Montes