Implicit use of symbol from a static library - c++

Question

I come to submit a problem I have while trying to import symbols from static library implicitly.
Let me set up the problem with a peace of simplified code:
I have 3 projects:
- a static plugin handler project that defines a registry class, it is linked in the other two projects
- a static plugin project
- a final project that has a plugin handler and "load statically" the plugin.
first, the static plugin handler project:
struct PluginType
{
virtual void doMyStuff() = 0 ;
};
typedef PluginType* (*PluginCreator)();
struct Registry
{
std::map<std::string, PluginCreator> _item_list;
static Registry& instance()
{
static Registry singleton;
return singleton;
}
static PluginType* create(std::string const& name)
{
// call the creator function that returns an PluginType*
return instance()[name]();
}
};
struct RecordInRegistry
{
RecordInRegistry(std::string const& key, PluginCreator new_item_creator)
{
Registry::instance()._item_list[key] = new_item_creator;
}
};
Now, a static plugin project
struct ADerivedItem : public PluginType
{
virtual void doMyStuff() override
{
//some really interesting stuffs
}
static PluginType* create() { return new ADerivedItem() ; }
}
export "C" const RecordInRegistry i_record_derived_item_class("derived_item", &ADerivedItem::create);
Then, the final project where i use the plugin. This project links the two other porjects !
int main()
{
PluginType* my_instance = Registry::create("derived_item");
return 0;
}
What I hope is that I will be able to register my static plugin just by linking the static library plugin
to the project.
It almost worked ! The only problem I got is that the symbol "i_record_derived_item_class" from the static
plugin is not explicitly used in the final project, thus this symbol is not imported from the static plugin lib !
I work on visual studio, i found a simple fix that consists in forcing the symbol import, something like
/INCLUDE:i_record_derived_item_class, and everything works fine.
I want to know if you see a work around to this, what I would like is to be able to add this kind of static
plung just by linking it, without having to add anything more than /LINK:myStaticPlugin.lib
Any idea ?
Moreover it would be really nice to avoid MSVC-specific solution.
Thanks in advance !

Answer 1

The problem is that symbols in library are used only when required. Draft n4659 says at 5.2 Phases of translation [lex.phases]:
All external entity references are resolved. Library components are linked to satisfy external references
to entities not defined in the current translation.
As your current source has no external references to i_record_derived_item_class, nor to the class ADerivedItem, nothing will be loaded by default from the plugin project. Worse, even the plugin handler module has no reference either, so you need to declare an undefined reference to the build system itself, be it MSVC or anything else.
Alternatively, you could link a minimal translation unit (means a .cpp or .o file) containing an external reference to i_record_derived_item_class, to force the resolution from a library.