How do I safely pass objects, especially STL objects, to and from a DLL?

Question

How do I pass class objects, especially STL objects, to and from a C++ DLL?

My application has to interact with third-party plugins in the form of DLL files, and I c

Answer 1

You cannot safely pass STL objects across DLL boundaries, unless all the modules (.EXE and .DLLs) are built with the same C++ compiler version and the same settings and flavors of the CRT, which is highly constraining, and clearly not your case.

If you want to expose an object-oriented interface from your DLL, you should expose C++ pure interfaces (which is similar to what COM does). Consider reading this interesting article on CodeProject:

HowTo: Export C++ classes from a DLL

You may also want to consider exposing a pure C interface at the DLL boundary, and then building a C++ wrapper at the caller site.
This is similar to what happens in Win32: Win32 implementation code is almost C++, but lots of Win32 APIs expose a pure C interface (there are also APIs that expose COM interfaces). Then ATL/WTL and MFC wrap these pure C interfaces with C++ classes and objects.