How to create wrapper for using C++ code in C?

Question

I'm trying to do a C wrapper for a C++ third-party library because I need to use it in my C project. I've seen examples of a C++ class wrapper but I don't undertood the process and I can't wrapper a C++ struct.

struct I want to wrapper:

struct confGlobal{
    long int device;
    string name;
    int number;
    string uid;
    string message;
    bool mode;

    confiGlobal():device{LONG_MAX}, number{INT_MAX}{}
};

struct product{
    string id;
    string name;
};
struct category{
    unsigned char id;
    string name;
    string description;

    category():id{UCHAR_MAX}{}
};
struct subCategory{
    unsigned char id;
    string name;
    string description;
    unsigned char idRoot;

    subCategory():id{UCHAR_MAX}, idRoot{UCHAR_MAX}{}
};
struct confPartner{
    vector<struct product> tableProduct;
    vector<struct category> tableCategory;
    vector<struct subCategory> tableSubCategory;
};

For call to this method:

class my_API {
public:
    static my_API* Instance(struct confGlobal cGlobal,
                        struct confPartner cPartner);
   ... another methods ...
private:
    virtual ~my_API();
    struct confGlobal cGlobal;
    struct confPertner cPartner;
};

I need to fill this structs and call my_API::Instance() from C but my attempts have been unsuccessful.

wrapper.h

#ifdef __cplusplus
extern "C" {
#endif
struct confGlobal_wpr; // An opaque type that we'll use as a handle
typedef struct confGlobal_wpr confGlobal_wpr;
confGlobal_wpr *confGlobal_create(unsigned int device,
                     char *name,
                     int number,
                     char *uid,
                     char *message,
                     unsigned char mode);

struct product_wpr{
    char id[4];
    char name[30];
};
typedef struct product_wpr product_wpr;

struct category_wpr{
    unsigned char id;
    char name[3];
    char description[30];
};
typedef struct category_wpr category_wpr;

struct subcategory_wpr{
    unsigned char id;
    char name[3];
    char description[30];
    unsigned char idRoot;
};
typedef struct subCategory_wpr subCategory_wpr;
struct confPartner_wpr; // An opaque type that we'll use as a handle
typedef struct confPartner_wpr confPartner_wpr;
confPartner_wpr *confPartner_create(Product_wpr tableProducts[],
                     unsigned char numProducts,
                     Category_wpr tableCategories[],
                     unsigned char numCategories,
                     SubCategory_wpr tableSubCategories[],
                     unsigned char numSubCategories);

struct my_api_wpr;
typedef struct my_api_wpr my_api_wpr;

my_api_wpr *my_api_create(struct confGlobal_wpr cGlobal,
                     struct confPartner_wpr cPartner);

#ifdef __cplusplus
}
#endif

wrapper.cpp

confGlobal_wpr *confGlobal_create(unsigned int device,
                     char *name,
                     int number,
                     char *uid,
                     char *message,
                     unsigned char mode)
{
    confGlobal_wpr *cg;
    struct confGlobal confiGlobal;
    confiGlobal.name = name;
    confiGlobal.device = device;
    confiGlobal.number = number;
    confiGlobal.uid = uid;
    confiGlobal.message = message;
    if (mode == 0)
        confiGlobal.mode = false;
    else
        confiGlobal.mode = true;
    return cg;
}
void confGlobal_destroy(confGlobal_wpr *cg)
{
    if (cg == NULL)
        return;
    delete static_cast<confGlobal_wpr *>(cg->instance); // ERROR: invalid static_cast from type ‘confGlobal’ to type ‘confGlobal_wpr*’
    free(cg);
}

confPartner_wpr *confPartner_create(product_wpr tableProducts_wpr[],
                     unsigned char numProducts,
                     category_wpr tableCategories_wpr[],
                     unsigned char numCategories,
                     subCategory_wpr tableSubCategories_wpr[], 
                     unsigned char numSubCategories)
{
    unsigned char i=0;

    confPartner_wpr *cc;
    struct confPartner cPartner;
    vector< struct product> tableProduct;
    vector< struct category> tableCategory;
    vector< struct subCategory> tableSubCategory;

    for (i=0; i<numProducts; i++)
    {
        struct product p;
        p.id = tableProducts_wpr[i].id;
        p.name = tableProducts_wpr[i].name;
        tableProduct.push_back(p);
    }
    cPartner.tableProduct = tableProducts;

    for (i=0; i<numCategories; i++)
    {       
        struct category c;
        c.id = tableCategories_wpr[i].id;
        c.nombre = tableCategories_wpr[i].name;
        c.descripcion = tableCategories_wpr[i].description;
        tableCategory.push_back(c);
    }
    cPartner.tableCategory = tableCategory;

    for (i=0; i<numSubCategories; i++)
    {
        struct subZona sc;
        sc.id = tableSubCategories_wpr[i].id;
        sc.name = tableSubCategories_wpr[i].name;
        sc.description = tableSubCategories_wpr[i].description;
        sc.idRoot = tableSubCategories_wpr[i].idRoot;
        tableSubCategory.push_back(sc);     
    }
    cPartner.tableSubCategory = tableSubCategory;
    return cc;

}

my_api_wpr *my_api_create(struct confGlobal_wpr confiGlobal_wpr,
                      struct confPartner_wpr confiPartner_wpr)
{
    my_api_wpr *my_api;
    my_API *obj;
    my_api = (typeof(my_api))malloc(sizeof(*my_api));
    obj    = my_API::Instance(confiGlobal_wpr, confiConsorcio_wpr);
    /* With this compile and linked OK
    confGlobal cg;
    confPartner cc;
    obj    = my_API::Instance(cg, cc);
    */
    my_api->obj = obj;

    return my_api;
}
void my_api_destroy(ct_api_wpr *my_api)
{
    if (my_api == NULL)
        return;
    delete static_cast<my_API *>(my_api->ptr_api); // ERROR: ‘virtual my_API::~my_API()’ is private within this context
    free(my_api);
}

The output error when compile and linked with:

g++ -shared -o libwrapper.so *.cpp wrapper.h -l:libthird-party.a -L. -ldl -lrt -Wl,-rpath /usr/local/lib -lc

In function ‘my_api_wpr* my_api_create(confGlobal_wpr, confPartner_wpr)’:
error: no matching function for call to ‘my_API::Instance(confGlobal_wpr&, confPartner_wpr&)’
  obj    = my_API::Instance(confiGlobal_wpr, confiConsorcio_wpr);
                                                               ^
my_API.h:30:20: note: candidate: static my_API* my_API::Instance(confGlobal, confPartner)
     static my_API* Instance(struct confGlobal cGlobal, struct confiPartner cPartner);
                    ^~~~~~~~
my_API.h:30:20: note:   no known conversion for argument 1 from ‘confGlobal_wpr’ to ‘confGlobal’

Instead of `char` with seemingly arbitrary lengths, where 30 characters is bound to be way too small in many cases, use `char*`. You may find that bridging C `char[]` or `char*` to C++ `std::string` isn't possible without C++. — tadman, Jun 06 '19 at 00:00
To use c++ code you need define wrapper functions with C calling convention,( extern C) and call them in your C project. Inside those exported functions you can use C++ code. — Vlad, Jun 06 '19 at 00:12
Possible duplicate of https://stackoverflow.com/questions/31903005/how-to-mix-c-and-c-correctly — Galik, Jun 06 '19 at 00:46

score 2 · Accepted Answer · answered Jun 06 '19 at 00:23

You're forgetting that CT_API::Instance doesn't understand the "handle" types that you have created in C to wrap the C++ structures. This is precisely what the error message is telling you, if you read it. You must translate those back to the appropriate C++ types.

Firstly, since you are using "create"-style routines to build the structures and return them as a pointer, you should consider making your my_api_create function accept pointers instead. Particularly because the resulting handle types are forward-declared structs with no definition visible in C and it will not be possible for your C client to dereference them.

That highlights another issue. You are also not using these handles correctly from C++.

So, one thing at a time...

Your creation routine in C should be declared as:

my_api_wpr *my_api_create(struct confGlobal_wpr* cGlobal, struct confPartner_wpr* cPartner);

On the C++ side, you need to actually define your handle types. Something like:

extern "C" struct confGlobal_wpr {
    struct confGlobal instance;
};

extern "C" struct confPartner_wpr {
    struct confPartner instance;
};

extern "C" struct my_api_wpr {
    my_API *ptr;
};

Now, your creation:

confGlobal_wpr *confGlobal_create(unsigned int device,
                                  char *name,
                                  int number,
                                  char *uid,
                                  char *message,
                                  unsigned char mode)
{
    confGlobal_wpr *cg = new confGlobal_wpr;
    struct confGlobal& cGlobal = cg->instance; //<-- note the reference
    // TODO: populate cGlobal as usual
    return cg;
}

confPartner_wpr *confPartner_create(product_wpr tableProducts_wpr[],
                                    unsigned char numProducts,
                                    category_wpr tableCategories_wpr[],
                                    unsigned char numCategories,
                                    subCategory_wpr tableSubCategories_wpr[],
                                    unsigned char numSubCategories)
{
    confPartner_wpr *cc = new confPartner_wpr;
    struct confPartner& cPartner = cc->instance; //<-- note the reference
    // TODO: populate cPartner as usual
    return cc;
}

my_api_wpr *my_api_create(struct confGlobal_wpr* cGlobal, struct confPartner_wpr* cPartner)
{
    my_api_wpr *my_api = new my_api_wpr;
    my_api->ptr = CT_API::Instance(cGlobal->instance, cPartner->instance);
    return my_api;
}

You should also add corresponding _destroy methods for all the above.

Thanks so much. I've made your modifications in my code and, for the moment, works great. Now, I understand a little bit the wrapper facts and I need to wrap many another struct and methods. One more question: how I destroy the pointer wrapped struct (confGlobal) and pointer wrapper class (ct_api_wpr)? I'm adding destroy methods on my original ask. — crossmax, Jun 06 '19 at 09:56
I didn't realize you could use `extern "C"` on structs. Do you have to? Does it even do anything? I suspect it doesn't. — Omnifarious, Jun 06 '19 at 14:44
@crossmax well, it's weird for the API itself because it's actually a singleton. So technically, `my_api_create` could simply be `my_api_initialize` and return no pointer. For the `confGlobal_wpr` your destroy function would simply be `void confGlobal_destroy(confGlobal_wpr* confGlobal) { delete confGlobal; }` and similar for `confPartner_wpr`. — paddy, Jun 06 '19 at 20:30
@Omnifarious I'm not 100% sure actually, since the structs are already forward-declared as `extern "C"` in the header. What I did was basically the same as wrapping the definitions in `extern "C"`. But it might not be required, and so this might be wrong. Certainly in the traditional use of `extern`, it's only required on the declaration to my knowledge. I haven't needed to write a C++ library wrapper in C for about 20 years ;) If you find the answer, please drop a note back in here. — paddy, Jun 06 '19 at 20:36

Vlad · Answer 2 · 2019-06-06T01:37:47.410

To use C++ code in C project you need define wrapper functions with C calling convention - extern "C"(turning off C++ name mangling/decoration) , and call them and only them in your C project. Inside those C functions you can use C++ code. Pass to C wrapper functions only types that C understands. You can create intermediate structures for passing data to C wrapper functions. Then you need copy data to types that C++ class expects. In you particular case you incorrectly pass confGlobal_wpr wrapper struct but C++ method requires confGlobal, and compiler complains on this directly.

Below is observable snippet how to use C++ code from C code: Foo.h

#include <string>

class Bar
{
public:
    Bar(std::string s) : s_(s)
    {
    }
    std::string s_;
};

class Foo
{
public:
    Foo(Bar) {}
};

CWrappers.h // include this header to C project

struct BarWrapper
{
    char data[100] = "";
};

#ifdef __cplusplus
extern "C" {
#endif

BarWrapper createFoo(char *c);

#ifdef __cplusplus
}
#endif

Wrapper.cpp

#include <algorithm>

#include "Foo.h"
#include "CWrappers.h"

// input and output to this C function should be understandable for C
BarWrapper createFoo(char *c)
{
    // inside you can use C++
    std::string data(c);
    Bar bar(data);
    Foo * foo = new Foo(bar);
    BarWrapper barWrapper;
    std::copy(data.begin(), data.end(), barWrapper.data);
    return barWrapper; // pack data to C struct wrapper (that has no C++ specific)
}

C Project

        #include "CWrappers.h"

        int main()
        {
            BarWrapper barWrapper = createFoo((char *)"test");
        }

score 1 · Answer 3 · answered Jun 06 '19 at 00:02

Structs that contain anything other than primitive data types common to C and C++ largely can't be wrapped in the way you want. In this particular case, you have ::std::strings in your struct. And they definitely can't be accessed reasonably from C at all.

Additionally, the structs contain bool and I don't know if newer versions of the C standard have bool or define it in such a way that it will result in layout compatible structs with C++ implementations on the same platform.

There are solutions to this problem. But they involve using opaque pointers to the struct in C, and always calling functions to access its methods. I will try to whip up an example of how this might work for a really simple struct.

Looking more carefully at your code, it looks like you need a sort of thunk layer that takes the C struct and (in C++) hand-converts it to the C++ struct and returns a pointer to a dynamically allocated C++ struct that you can then pass to other C++ functions that have been exposed to C.

How to create wrapper for using C++ code in C?

3 Answers3