C++ Variadic Macros in Boost.Fusion?

Question

So, according to this answer, C++ doesn't support variadic macros, and the C++ standard doesn't mention variadic macros anywhere. I know that C99 introduced variadic macros with __VA_ARGS__, and certain C++ compilers (like GCC) even provide extensions to allow this in C++, but the fact remains that variadic macros simply aren't part of standard C++.

Now, there's a feature in Boost.Fusion where you can bind a Fusion sequence to an arbitrary class or struct using the BOOST_FUSION_ADAPT_STRUCT macro. This allows you to use your class or struct as if it was a Fusion sequence.

Here is an example of how this is used (taken from the Boost docs):

namespace demo
{
    struct employee
    {
        std::string name;
        int age;
    };
}

// demo::employee is now a Fusion sequence
BOOST_FUSION_ADAPT_STRUCT(
    demo::employee,
    (std::string, name)
    (int, age))

Now, how is this code possible without variadic macros? The BOOST_FUSION_ADAPT_STRUCT macro seems to take an arbitrary number of arguments, since presumably it can work with any arbitrary user-defined class or struct.

I know that Boost is famous for bending C++ in interesting ways, but this seems outright impossible without compiler support. So what sort of wizardry is Boost.Fusion doing to pull this off?

PS: Yes, I know Boost is open source. The first thing I did was to look at the source code. It seems to be using the Boost Preprocessor library to somehow concatenate macros. But I don't understand how this can work for any arbitrary number of arguments, and the source code is a very dense collection of preprocessor code which is very difficult to understand.

It probably doesn't work for an arbitrary number of arguments (unless the compiler actually supports variadic macros), but just a sufficiently large number. — Bo Persson
– Bo Persson, Commented May 18, 2011 at 15:55

Dark Falcon · Accepted Answer · 2011-05-18 14:22:49Z

2

BOOST_FUSION_ADAPT_STRUCT(
    demo::employee,
    (std::string, name)
    (int, age))

This is a single macro which takes two arguments: Argument 1: demo::employee Argument 2: (std::string, name)(int, age)

Argument 2 is concatenated with a string to form another macro invocation which also takes 2 parameters:

BOOST_FUSION_SOME_INTERNAL_MACRO(std::string, name)
BOOST_FUSION_SOME_INTERNAL_MACRO(int, age)

answered May 18, 2011 at 14:22

Dark Falcon

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