Howto pass funcptr from C++ Class to a C API?

Question

How can I pass a dynamic generated C++ function as *funcprt to a C API?

The API exports this:

DllImport void api_register_func(char *func_name, void (*funcptr)(char *, char *));

I have to create the function while runtime because I don't know about it before. So I used a class:

class JsFunc
{
    public:
        char * JsFuncName;
        char * JsParameter;

    void RunFunc(char * val1, char * val2)
    {
        printf("\nJsFunc.runFunc executed, JsParameter=%s passed\n",JsParameter);
    }
};

And call it like this:

JsFunc * jsm = new JsFunc ();
jsm->JsFuncName = external_parameter1; 
jsm->JsParameter = external_parameter2; 
api_register_func(external_parameter1, jsm->RunFunc);

But VisualStudio 2015 tells me:

Error C3867 'JsFunc::runFunc': non-standard syntax; use '&' to create a pointer to member VJCS C:\Users\astrauss\Source\Repos\VCJS\VCJS\VCJS.cpp 54

Sorry if the code is bad, I'm not a C/C++ programmer but need to get this running for my daily work. Thanks!

Unfortunately most of this makes no sense at all. It seems like you're asking about the solution instead of asking about the problem ([XY Problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)). Try editing your question — Amit, Aug 04 '15 at 20:12
You can't do precisely that, since RunFunc has a "hidden" `this` parameter that C doesn't know anything about (which holds a pointer to your `jsm` object. You'll have to find some way that you can "stow" the address of `jsm` somewhere. — Mats Petersson, Aug 04 '15 at 20:13
Hmm, it is new to me that you can create functions at run-time in C++. They seem to have made a great step forward ... — too honest for this site, Aug 04 '15 at 20:22
Just because you're using C++ doesn't mean you can't create a function outside of a class you can use. — dbush, Aug 04 '15 at 20:24
There is no runMacro member in your class, if you show how you declared it, I might help. — Cem Kalyoncu, Aug 04 '15 at 20:25
Can you write a **complete** and **verifiable** example of what it is you want to do? You have inconsistent names throughout, it's unclear what you're trying to accomplish. — Barry, Aug 04 '15 at 20:32
@Andreas Try again please. Your variable references `ctx` and `runFunc`, your class has `Parameter` `and `RunFunc`. And does the API really not allow you to provide some sort of `void* context`? — Barry, Aug 04 '15 at 20:37
What do the parameters of the registered function actually represent? Are any of them user-defined values? Clearly this example does not allow passing a user-defined value to `api_register_func()` itself, but maybe there is a separate function for assigning context data? If so, you can pass your object pointer as the context data so your registered function can access the object. — Remy Lebeau, Aug 04 '15 at 20:47
The Paramters come from a JS-Intepreter(duktape) that I have embedded to my dll. The intepreter reads a script that defines the functions i want to execute through the main Program/API. — Andreas, Aug 04 '15 at 20:51

score 1 · Accepted Answer · answered Aug 04 '15 at 20:54

Unfortunately, you don't have very many options here. The API:

DllImport void api_register_func(char *func_name, 
                                 void (*funcptr)(char *, char *));

only lets you provide a function pointer, without any context. So you can pass in a free function or a static member function, but you have no way of providing a member function. If you had a context:

DllImport void better_api_register_func(char *func_name, 
                                        void *context,
                                        void (*funcptr)(void *, char *, char *));

Then you could write a simple lambda:

api_register_func(external_parameter1,
                  jsm, //context
                  [](void *ctxt, char* arg1, char* arg2) {
                      static_cast<JsFunc*>(ctxt)->runFunc(arg1, arg2);
                  });

and you're effectively using a member function as the callback. However, this isn't an option. Instead, you're stuck with using a global variable:

std::unique_ptr<JsFunc> the_global_func;

Which you can allocate at runtime:

the_global_func.reset(new JsFunc);
// set stuff

And then use the global in a lambda. Since it's global, we don't need to (in fact, can't) capture it - which means our lambda can be convertible to a function pointer:

api_register_func(external_paramter1,
                  [](char* arg1, char* arg2) {
                      the_global_func->runFunc(arg1, arg2);
                  });

Thanks a lot! That should work for me. At the moment I can't try it with the API but it compiles :) — Andreas, Aug 04 '15 at 21:28
Barry, your Code works for one "the_global_func". But I have not found a way to to this for more than one. Is there another possibility than define more than one "the_global_func" (the_global_func1,the_global_func1,...)? I tried to put the_global_func into a vector, but can't pass the index to api_register_func. When I change [](char... to [=](char... I can pass the index but then the lambda returns another type. Thanks again! — Andreas, Aug 05 '15 at 09:34

Howto pass funcptr from C++ Class to a C API?

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