Question

I look at how we use Boost::Extension BOOST_EXTENSION_TYPE_MAP_FUNCTION macro.

For example like this:

BOOST_EXTENSION_TYPE_MAP_FUNCTION
{
    std::map<std::string, boost::extensions::factory<service> > &factories(types.get());
    factories["file_service"].set<file_service>();
}

BOOST_EXTENSION_TYPE_MAP_FUNCTION macro is defined in extension.hpp.

I wonder how this macro understands what is in Curly Brackets and how for example expand this macro to something that would cout anything like “Hello extended macro”?

Answer 1

Let me put my comment into an answer…

A macro is an instruction to the compiler (I use the collective term here) to substitute at that location the symbols defined as that macro, for example

#define FOO 1

int val = FOO; // at this point, FOO is replaced with 1

(p.s. please don’t do this in C++)

Now, what is happening in your case is that there is a set of symbols (the signature of a function) defined as a macro, so all that happens is the compiler will substitute the macro with the symbols, and the end result would look (roughly) like this:

void boost_extension_exported_type_map_function(boost::extensions::type_map& types)
{
    std::map<std::string, boost::extensions::factory<service> > &factories(types.get());
    factories["file_service"].set<file_service>();
}

Which as you can see is a simple function. You can do this too (but don’t unless you have a very good reason)

#define BOB void foo(std::string const& bar)

BOB
{
  std::cout << "HEllo: " << bar << std::endl;
}

It simply allows a user to define their own implementation for that function… presumably somewhere else – it takes the address of that function and uses it via a pointer…