Define forward declared C-struct as C++-struct

Question

Is it legal to forward-declare a struct as a C-struct

// api.h
#ifdef __cplusplus
extern "C" {
#endif

    typedef struct handle_tag handle_t;

    handle_t *construct();
    void destruct(handle_t *h);

    void func(handle_t *h);

#ifdef __cplusplus
}
#endif

and subsequently define it as a C++-struct, i.e. as a non-POD type?

// api.cpp
struct handle_tag {
    void func();
    std::string member;
};

void func(handle_t *h) {
    h->func();
}

The general intention is to get via a C interface an externally accessible opaque type handle_t which is internally implemented as an C++ data type.

Answer 1

Yes, that will work fine as long as the C code never needs to see "inside" the handle_tag structure, and the appropriate C++ construction/destruction is performed by the C++ code (which I preseume the construct and destruct are for).

All that the C code needs is a pointer to some datastructure - it won't know what the contents is, so the content can be anything you like it to to be, including constructor/destructor reliant data.

Edit: I should point out that this, or methods similar to it (e.g. using a void * to record the address of an object for the C portion to hold), is a fairly common way to interface C-code to C++ functionality.

Edit2: It is critical that the C++ code called doesn't "leak" exceptions into the C code. That is undefined behaviour, and very much liable to cause crashes, or worse, "weird things" that don't crash... So unless the code is guaranteed to not cause exceptions (and for example std::string is liable to throw bad_alloc in case of low memory), it is required to use a try/catch block inside code like construct anf func in the C++ side.