Automatic C wrappers for C++ functions - c++

Question

Let's say I have some unspecified type called variant, as well as two functions allowing to convert to/from this type, with the following signature:
struct converter
{
template<typename T>
static variant to(const T&);
template<typename T>
static T from(const variant&);
};
Now, what I'd like to do is create wrappers for arbitrary C++ functions as in the following example:
SomeObject f_unwrapped(const std::string& s, int* x)
{
//... do something with the inputs...
return SomeObject();
}
extern "C" variant f(variant s, variant x)
{
return converter::to<SomeObject>(f_unwrapped(converter::from<std::string>(s), converter::from<int*>(x)));
}
Ideally I'd want the wrapper to be a one-line declaration or macro that would take only the f_unwrapped function and the name f as inputs.
I've tried to wrap the function into a function object, then do the bureaucratic work using variadic templates. While this does work, I don't know how to make the resulting function extern "C".
What is the most idiomatic way of achieving this goal?

Answer 1

If we use the EVAL, helper, Conditional, and map macros from the first two code blocks here.
The map will need to be made more general for our needs.
#define MM1() MM_CALL1
#define MM_NEXT1(Macro,a,...) \
IS_DONE(a)( \
EAT \
, \
OBSTRUCT(COMMA)() OBSTRUCT(MM1)() \
) \
(Macro,a,__VA_ARGS__)
#define MM_CALL1(Macro,a,...) \
Macro(a) \
MM_NEXT1(Macro,__VA_ARGS__)
#define MacroMap1(Macro,...) MM_CALL1(Macro,__VA_ARGS__,DONE)
#define MM2() MM_CALL2
#define MM_NEXT2(Macro,a,...) \
IS_DONE(a)( \
EAT \
, \
OBSTRUCT(COMMA)() OBSTRUCT(MM2)() \
) \
(Macro,a,__VA_ARGS__)
#define MM_CALL2(Macro,a,b,...) \
Macro(a,b) \
MM_NEXT2(Macro,__VA_ARGS__)
#define MacroMap2(Macro,...) MM_CALL2(Macro,__VA_ARGS__,DONE)
We will also want the WithTypes and WithoutTypes from here.
We can define AMACRO to do the job you wanted.
#define AsVariant(param) variant param
#define ConvertFrom(type,param) converter::from<type>(param)
#define HEADDER(type,func,params) type func ##_unwrapped (WithTypes params)
#define WRAPPER(type,func,params) \
extern "C" variant func (OBSTRUCT(MacroMap1)(AsVariant,WithoutTypes params)) \
{ \
return converter::to< type >(func ## _unwrapped( \
MacroMap2(ConvertFrom,IDENT params) \
)); \
}
#define AMACRO(type,func,params) \
EVAL( \
HEADDER(type,func,params); \
WRAPPER(type,func,params) \
HEADDER(type,func,params) \
)
Which will turn this:
AMACRO(SomeObject,f,(const std::string&, s, int*, x))
{
// ... do something with the inputs ...
return SomeObject();
}
Into this (after formatting):
SomeObject f_unwrapped (const std::string& s , int* x );
extern "C" variant f (variant s , variant x )
{
return converter::to<SomeObject>(f_unwrapped(converter::from<const std::string&>(s),converter::from<int*>(x)));
}
SomeObject f_unwrapped (const std::string& s , int* x )
{
return SomeObject();
}
NOTE:
If the const needs removing from the parameter, a conditional, similar to the ISDONE, can be made and added to the ConvertFrom macro.