将Gravity（脚本语言）调用转换为本机C函数调用

Question

I am currently looking into implementing a cleaner way to call native C functions from the Gravity scripting language.

So far, the most simplistic example would be this one:

int add(int lhs, int rhs) {
  return lhs + rhs;
}

static void gravity_wrap_add(
  gravity_vm* vm,
  gravity_value_t* args, uint32_t nargs, 
  uint32_t retIndex, void* data
) {
  int lhs, rhs, rt;

  // Unwrap
  lhs = VALUE_AS_INT(args[1]);
  rhs = VALUE_AS_INT(args[2]);

  // Perform call, capture return
  rt = add(lhs, rhs);

  // Forward the return
  gravity_vm_setslot(vm, VALUE_FROM_INT(rt), retIndex);
}

By using C++ (98) templating or C preprocessor magic, would there be a way to generate wrapper functions?

A very, very crunched example of the above wrapper function, would be this one:

static void gravity_wrap_add(
  gravity_vm* vm,
  gravity_value_t* args, uint32_t nargs, 
  uint32_t retIndex, void* data
) {
  gravity_vm_setslot(vm, 
   VALUE_FROM_INT(
     add(VALUE_AS_INT(args[1]), VALUE_AS_INT(args[2]))
   ), 
  retIndex);
}

This version is technically what I want to achieve - but through methods like the preprocessor or C++ templating. For cross-platform compatibility reasons, I'd like to stick with C++98 (since MSVC isn't exactly good on modern features).

Answer 1

It's pretty tedious without variadic templates, but you should be able to do a small number of arguments:

namespace gravity {
  template<class T> struct value {};
  // for brevity and deduction:
  template<class T> T get(const gravity_value_t &v)
  {return value<T>::get(v);}
  template<class T> gravity_value_t put(const T &t)
  {return value<T>::put(t);}
  template<> struct value<int> {
    int get(const gravity_value_t &v)
    {return VALUE_AS_INT(v);}
    gravity_value_t put(int i)
    {return VALUE_FROM_INT(i);}
  };
  // more specializations…

  template<class R,R f()>
  void wrap(gravity_vm* vm,
            gravity_value_t* args, uint32_t nargs, 
            uint32_t retIndex, void* data) {
    assert(nargs==0);
    gravity_vm_setslot(vm,put(f()),retIndex);
  }
  template<class R,class A,R f(A)>
  void wrap(gravity_vm* vm,
            gravity_value_t* args, uint32_t nargs, 
            uint32_t retIndex, void* data) {
    assert(nargs==1);
    gravity_vm_setslot(vm,put(f(get<A>(args[0]))),retIndex);
  }
  // more overloads with more arguments…

  /*gravity_register*/(wrap<int,int,int,add>);
}

As usual, void return types will be a pain; you have to repeat each argument-count overload for R of void .

It would be possible to use deduction to avoid repeating the signature when using wrap , but then you get fewer functions and extra closure objects to distinguish them (which you can presumably use via the data argument).