Question

Is there any tips one can give me about passing pointers to structs, doubles, functions, … from a C program to a C++ library and back?

Answer 1

Assuming you’re coding these in two different libraries static or dynamic (DLLs on windows shared libraries on Linux and other *nix variants) The biggest concerns I have are as follows:

1. They are compiled with the same compiler. While this isn’t necessary if all C++ exports are exported with a C-style naming convention it is necessary for C++ to C++ calls to class instances between the two C++ modules. This is necessary due to how different compilers mangle C++ exports differently.

2. Do not cast a C++ class as a C struct. They aren’t the same under the covers, even if the layout of fields are the same. C++ classes have a “v-table” if they have any virtual members; this v-table allows the proper calling of inherited or base class methods.

3. This is true of C to C or C++ to C++ as well as C to C++. Ensure both use the same byte alignment for the output library. You can only determine this by reading your compiler or development environments documentation.

4. Don’t mix malloc/free with new/delete. More specifically don’t allocate memory with new and free memory with “free” and vice versa. Many compilers and operating systems handle memory management differently between the two.

5. Passing function pointers: So long as they are exposed to/from C++ as ”extern “C”” this should be fine. (You’ll either need to reference your compilers documentation on how to determine when a header is being compiled as C or C++ to maintain this in one file, or you will need two separate copies of the same function declaration in each project — I recommend the first)

6. Passing doubles: This is a built-in type in both C and C++ and should be handled the same.

7. If you must share an instance of a C++ object with a C function, and act on it from within C code, expose a set of C-exported helper functions which call the appropriate methods on the C++ object. Pure C code cannot properly call methods on C++ objects.

    Pseudocode-ish Example:
    // C++ class
    class foo {
       public:
           void DoIt();
    };

    // export helper declarations
    extern "C" void call_doit(foo* pFoo);
    extern "C" foo* allocate_foo();
    extern "C" deallocate_foo(foo* pFoo);


    // implementation
    void call_doit(foo* pFoo)
    {
        pFoo->DoIt();
    }

    foo* allocate_foo()
    {
        return new foo();
    }

    deallocate_foo(foo* pFoo)
    {
       delete pFoo;
    }

    // c consumer
    void main()
    {
        foo* pFoo= allocate_foo();
        call_doit(pFoo);
        dealocate_foo(pFoo);
    }