How to Force Inclusion of "Unused" Object Definitions in a Library

How to force inclusion of unused object definitions in a library

Well, the other answers where good attempts but ultimately fruitless. I am going to use the refit trick but the rest appears to have been a red herring; it kind of makes sense since the template in question isn't actually used anywhere else so the fact that it's not explicitly instantiated shouldn't make a difference...the declaration of the global still happens in A translation unit, it has side effects...I don't think the standard allows it to be optimized away.

The unfortunate bit about the standard not saying whether or not it is required to include a translation unit at all is the ultimate issue. I think C++0x is doing something about this but maybe not... At any rate, MS feels free to not include the unit at all, and since it doesn't the global isn't ultimately included in the executable and thus none of the other crap happens.

What I've decided to do, and there are of course many other ways, is to create a file 'tag' variable. That tag is then assigned to in a function that is globally accessible (it HAS to assign or assign from or the reference is optimized away). Then that function has to be called from the executable.

I decided to do it this way because then the rest still works the same as it always has. I don't end up ultimately changing behavior like I could if I simply wrote a registration function that hand-registered the types. Plus I can do other things this way...I just have to make sure that anything that might fall into this classification of fucktardery has a tag and that tag is accessed.

I'll be writing a bunch of helper macros to make this mostly painless.

How to force inclusion of an object file in a static library when linking into executable?

It turns out my original attempt was mostly there. The following works:

extern "C" void Af(void);
void (*Af_fp)(void) = &Af;

For those that want a self-contained preprocessor macro to encapsulate this:

#if defined(_WIN32)
# if defined(_WIN64)
#  define FORCE_UNDEFINED_SYMBOL(x) __pragma(comment (linker, "/export:" #x))
# else
#  define FORCE_UNDEFINED_SYMBOL(x) __pragma(comment (linker, "/export:_" #x))
# endif
#else
# define FORCE_UNDEFINED_SYMBOL(x) extern "C" void x(void); void (*__ ## x ## _fp)(void)=&x;
#endif

Which is used thusly:

FORCE_UNDEFINED_SYMBOL(Af)

How to force gcc to link an unused static library

Use --whole-archive linker option.

Libraries that come after it in the command line will not have unreferenced symbols discarded. You can resume normal linking behaviour by adding --no-whole-archive after these libraries.

In your example, the command will be:

g++ -o program main.o -Wl,--whole-archive /path/to/libmylib.a

In general, it will be:

g++ -o program main.o \
    -Wl,--whole-archive -lmylib \
    -Wl,--no-whole-archive -llib1 -llib2

How to force gcc to link unreferenced, static C++ objects from a library

You can use -Wl,--whole-archive -lyourlib , see the manpage for ld for more info.

Any static libraries mentioned after -Wl,--whole-archive on the command line gets fully included, you can turn this off again too if you need to , as in e.g. -Wl,--whole-archive -lyourlib -Wl,--no-whole-archive -lotherlib

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