GCC style weak linking in Visual Studio?

You can do it, here is an example in C:

/*
 * pWeakValue MUST be an extern const variable, which will be aliased to
 * pDefaultWeakValue if no real user definition is present, thanks to the
 * alternatename directive.
 */

extern const char * pWeakValue;
extern const char * pDefaultWeakValue = NULL;

#pragma comment(linker, "/alternatename:_pWeakValue=_pDefaultWeakValue")

MSVC++ has __declspec(selectany) which covers part of the functionality of weak symbols: it allows you to define multiple identical symbols with external linkage, directing the compiler to choose any one of several available. However, I don't think MSVC++ has anything that would cover the other part of weak symbol functionality: the possibility to provide "replaceable" definitions in a library.

This, BTW, makes one wonder how the support for standard replaceable ::operator new and ::operator delete functions works in MSVC++.

MSVC used to behave such that if a symbol is defined in a .obj file and a .lib it would use the one on the .obj file without warning. I recall that it would also handle the situation where the symbol is defined in multiple libs it would use the one in the library named first in the list.

I can't say I've tried this in a while, but I'd be surprised if they changed this behavior (especially that .obj defined symbols override symbols in .lib files).

The only way i know. Place each symbol in a separate library. User objects with overrides also must be combined to library. Then link all together to an application. User library must be specified as input file, your lib's must be transfered to linker using /DEFAULTLIB: option.