C struct 指向 C++ 类的指针

Question

I am writting a C shared library that will be loaded into a C++ program. There is an object in this program that I need access to, so I am gonna be using a struct pointer. But I'm not really sure how can I make a C struct pointer to a C++ class, as it also constains functions (constructor, destructor, etc). I think it is possible though, as a struct is just holds data, meaning that I can arrange it in a way it matches the C++ class in the memory. Example:

//C++ class
class test
{
    public:
    int   number;
    char* string;

    public:
    test();
    ~test();
    void function(int new_number);
    void function(char* new_string);
} obj;

//C struct pointer

struct test_t
{
    //???
};

//...
struct test_t* p_obj = (struct test_t*)addr_of_obj;

Any ideas?

EDIT: I cannot edit the source code of this program that I'm loading the library into. I'm forcing it to load this library that will access and modify memory for me.

Answer 1

After some testing, I think the answer to this question is: depends on the compiler and on the class. The examples I will give below were compiled with GCC and G++ on Linux (seems to work with clang and clang++ too).

Example 1 - A class with some variables, a constructor, a destructor, and an overloaded function - nothing too special about it.

//C++ class
class test
{
    private:
    int number;
    char* string;

    public:
    test() { this->number = 0; this->string = (char*)""; this->boolean = false; }
    ~test() {  }
    void function(int num) { this->number = num; }
    void function(char* str) { this->string = str; }
    int    get_number() { return this->number; }
    char*  get_string() { return this->string; }

    bool boolean;
};

//C structure
//To create a struct pointer for this class, I just had to ignore the functions. 

struct test_t
{
    int number;
    char* string;
    bool boolean;
};

Example 2 - a class that contains virtual functions. In this case, we'd have to add an extra pointer parameter on the beginning of our structure. That would be aligned with the virtual function table of that class, which is no longer hidden because of the virtual function.

//C++ class
class test
{
    private:
    int number;
    char* string;

    public:
    test() { this->number = 0; this->string = (char*)""; this->boolean = false; }
    ~test() {  }
    virtual void v_function()  { printf("virtual function\n"); }
    virtual void v_function2() { printf("another virtual function\n"); }
    void function(int num) { this->number = num; }
    void function(char* str) { this->string = str; }
    int    get_number() { return this->number; }
    char*  get_string() { return this->string; }

    bool boolean;
};

//C structure
//It is the same as before, but with the VFT pointer
struct test_t
{
    void** vptr; //A pointer to function addresses (void**)
    int number;
    char* string;
    bool boolean;
};

In conclusion, the only way to be 100% sure your C structure pointer aligns with the C++ class is looking on the memory and aligning it manually. And you can aways skip some variables by putting pads in your structure:

//C++ class
class test
{
    private:
    int number;
    std::vector<std::string> vector;
    char* string;

    public:
    test() { this->number = 0; this->string = (char*)""; this->boolean = false; }
    ~test() {  }
    virtual void v_function()  { printf("virtual function\n"); }
    virtual void v_function2() { printf("another virtual function\n"); }
    void function(int num) { this->number = num; }
    void function(char* str) { this->string = str; }
    int    get_number() { return this->number; }
    char*  get_string() { return this->string; }

    bool boolean;
};

//C structure
struct test_t
{
    void** vptr;
    int number;
    char vector[24]; //std::vector<std::string> pad (24 bytes long)
    char* string;
    bool boolean;
};

Answer 2

The way to do that and keep code easy to support is to use C definitions from C++ code. Perhaps using private implementation to lessen compatibility issues.

//C struct - non-static data

struct test_t
{
    int   number;
    char* string;
};

C++ wrapper for test_t .

// C++ interface

struct  test_t;
struct test
{
    test_t *priv; //private implementation
public:
    // methods.
};

Answer 3

How class es and struct s are laid out in memory are not defined by the C and C++ standards the are however laid out in platform specific Application Binary Interface (ABI) documents. To ensure safety you must ensure the layouts match. You will certainly run into issues if any functions are virtual.

A safer option will be to either embed the extern "C" struct in your C++ class or hide the class from the C code and just allow access via extern "C" accessor functions.