How can I expose C++ function pointers in C?

Question

I have two types of function pointers defined in my C++ that look like this:

typedef void(*CallbackFn)(bool, std::str         


        

Answer 1

In order to expose any C++ functions to C, you should wrap the C++ calls in C functions in a plain C++ library. And export only the C functions from it. Use a common header for C functions declarations inside and outside the library. These functions will be callable from any C environment. All the C++ types wrap in a class and pass a pointer to that class across function wrappers, as a handle to C++ environment. The pointer to class should be void* or just long. And only in C++ side you will reinterpret it to own environment class.
Update 1:

1. You should keep C and C++ separated. It means don't do conversions between C and C++. Keep separated C versions and C++ versions of XX_log_callback functions. For instance your C++ functions uses std::string, py::array_t&. There is no way you can use it is C. No conversion available, and no way to take advantages of it in C. You can take advantage of C++ only in C++, so make a separate version for C++ only and one available for C developers.

2. This is a by the way. There is a technique of passing around C++ interfaces to C and back to C++. But be attentive, it uses only C compatible return and argument types. It means creating a structure with a pointer to a table of function pointers. In C++ it is an interface but in C it is a struct. This technique is used in COM/OLE2 in Windows. https://www.codeproject.com/Articles/13601/COM-in-plain-C To use such a technique you should understand very well how to make a C++ class compatible with a C struct.

Now I will just copy/paste some pieces of code from the codeproject with little explanations. The rule of thumb when passing interfaces between C and C++, use only types compatible with C as function arguments and as return type. The first four bytes in the interface is a pointer to an array of functions, called Virtual Table:

typedef struct
{
   IExampleVtbl * lpVtbl;//<-- here is the pointer to virtual table
   DWORD          count;//<-- here the current class data starts
   char           buffer[80];
} IExample;

Here you add the pointers to functions in the virtual table. The IExampleVtbl is a structure filled with pointers, and binary it is equivalent to a contiguous array of pointers

static const IExampleVtbl IExample_Vtbl = {SetString, GetString};

IExample * example;

// Allocate the class
example = (IExample *)malloc(sizeof(IExample));

example->lpVtbl = &IExample_Vtbl;//<-- here you pass the pointer to virtual functions
example->count = 1; //<-- initialize class members
example->buffer[0] = 0;

Now this is how you call the methods:

char buffer[80];
example->lpVtbl->SetString(example, "Some text");
example->lpVtbl->GetString(example, buffer, sizeof(buffer));

Keep in mind, all of above is C. In the above example you refer explicitly the virtual table member, and also you pass it explicitly as first parameter in the functions. The C++ equivalent of call to GetString/SetString is:

example->SetString("Some text");
example->GetString(buffer, sizeof(buffer));

Here is the SetString/GetStrinf functions and the virtual table structure:

HRESULT STDMETHODCALLTYPE SetString(IExample *this, char * str)
{
   memcpy(this->buffer, str, length);//be attentive, it is almost pseudocode
   return(0);
}
HRESULT STDMETHODCALLTYPE GetString(IExample *this, char *buffer, int buffer_len)
{
   memcpy(str, this->buffer, length);//be attentive, it is almost pseudocode
   return(0);
}
typedef struct {
   SetStringPtr       *SetString;
   GetStringPtr       *GetString;
} IExampleVtbl;

The STDMETHODCALLTYPE is to make it compatible with C++ calling of member function classes, so you will be able to pass the IExample between C and C++. I believe this will be really a nightmare for the C programmers, but not an easy task for C++ counterparts.
To access that when interface is passed from C, you declare interface like this:

class IExample
{
public:
   virtual HRESULT  SetString(char * str) = 0;//<-- see first parameter gone away in both functions
   virtual HRESULT GetString(char *buffer, int buffer_len) = 0;
};

If you implement in C++ to pass in C equivalent of above code will be:

class IExample
{
    int count = 1; //<-- initialize class members
    char buffer[80] = "";
public:
   virtual HRESULT  SetString(char * str)
   {
      memcpy(this->buffer, str, length);//be attentive, it is almost pseudocode
      return(0);
   }
   virtual HRESULT GetString(char *buffer, int buffer_len)
   {
      memcpy(str, this->buffer, length);//be attentive, it is almost pseudocode
      return(0);
   }
};

One more thing. You don't use the C declaration in C++ and vice-versa. This is by the COM approach to address the issue. It might be not portable to different compilers but keep in mind, similar approach is done in the old CORBA. Only you should keep in mind. You create one interface for C and one for C++. On C++ part hide the C interface and on C hide the C++ interface. Pass around only the pointers.