Every executable image (EXE or DLL) can have its own statically linked CRT, or can dynamically link to a CRT. The version of the CRT statically included in or dynamically loaded by a particular image depends on the version of the tools and libraries it was built with. A single process may load multiple EXE and DLL images, each with its own CRT. Each of those CRTs may use a different allocator, may have different internal structure layouts, and may use different storage arrangements. This means allocated memory, CRT resources, or classes passed across a DLL boundary can cause problems in memory management, internal static usage, or layout interpretation. For example, if a class is allocated in one DLL but passed to and deleted by another, which CRT deallocator is used? The errors caused can range from the subtle to the immediately fatal, and therefore direct transfer of such resources is strongly discouraged.
You can avoid many of these issues by using Application Binary Interface (ABI) technologies instead, as they are designed to be stable and versionable. Design your DLL export interfaces to pass information by value, or to work on memory that is passed in by the caller rather than allocated locally and returned to the caller. Use marshaling techniques to copy structured data between executable images. Encapsulate resources locally and only allow manipulation through handles or functions you expose to clients.
It's also possible to avoid some of these issues if all of the images in your process use the same dynamically loaded version of the CRT. To ensure that all components use the same DLL version of the CRT, build them by using the /MD option, and use the same compiler toolset and property settings.
Be careful if your program passes certain CRT resources across DLL boundaries. Resources such as file handles, locales, and environment variables can cause problems, even when using the same version of the CRT. For more information on the issues involved and how to resolve them, see Potential Errors Passing CRT Objects Across DLL Boundaries.