How Curly Brackets work in Boost::extension, how to make such macros on my own?

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"?

the macro doesn't understand anything(?) it is simply replaced - so in effect (if you ignore the calling convention stuff) becomes `void boost_extension_exported_type_map_function(boost::extensions::type_map& types)` (I've left out the `extern "C"` bit too), i.e. a function... — Nim, Aug 08 '11 at 15:30

score 4 · Accepted Answer · answered Aug 08 '11 at 15:39

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...

How Curly Brackets work in Boost::extension, how to make such macros on my own?

1 Answers1