ESP32 Forum

Posted: **Sat Nov 02, 2019 7:52 pm**

Hello, everybody,
I want to call a method in a C++ class from an event in ESP-IDF in an xTask.
Here is the class and method „dispatch“ I want to call:

Code: Display.h Select all

/*

 * Display.h

 *

 *  Created on: 01.11.2019

 *      Author: arne

 */



#ifndef COMPONENTS_HARDWARE_DISPLAY_H_

#define COMPONENTS_HARDWARE_DISPLAY_H_



#include "Modell.h"



class Display {

public:

	Display();

	virtual ~Display();

	/**Analysiert die Befehle des Frontends und ruft entsprechende Methoden auf*/

	void dispatch(char *str);

private:

	Modell theModell;

};



#endif

Code: Display.cpp Select all

/*

 * Display.cpp

 *

 *  Created on: 01.11.2019

 *      Author: arne

 */



#include <Display.h>



Display::Display() {

	// TODO Auto-generated constructor stub



}



Display::~Display() {

	// TODO Auto-generated destructor stub

}



void Display::dispatch(char *str) {



}

To encapsulate the method in an external "C" method I wrote the following wrapper:

Code: DisplayC-Wrapper.h Select all

/*

 * DisplayC-Wrapper.h

 *

 *  Created on: 01.11.2019

 *      Author: arne

 */



#ifndef COMPONENTS_HARDWARE_DISPLAYC_WRAPPER_H_

#define COMPONENTS_HARDWARE_DISPLAYC_WRAPPER_H_



#ifdef __cpluscplus

class Display;

extern "C" {

#else

struct Display;

typedef struct Display Display;

#endif

Display *Display_new();



void Display_dispatch(Display *instance, char *str);



#ifdef __cpluscplus

}

#endif

#endif

Here is the implementation of the wrapper:

Code: DisplayC-Wrapper.c Select all

/*

 * DisplayC-Wrapper.c

 *

 *  Created on: 01.11.2019

 *      Author: arne

 */



#include "DisplayC-Wrapper.h"

#include "Display.h"



Display *Display_new()

{

    return new Display();

}



void Display_dispatch(Display *instance, char *str)

{

    instance->dispatch(str);

}

And here the corresponding call in the xTask:

Code: display.c Select all

void rx_task() {

    Display *instance = Display_new();

    Display_dispatch(instance,"Test");

}

Everything compiles but doesn't link.
I got the following error-message at linkage time:

What do I have to do to make this linking?
Are there suitable compiler flags?
Is it possible to address C++ classes from C methods at all?
Is there more information about this topic somewhere? The best would be to find a working example...

Many greetings
Arne

Posted: **Sun Nov 03, 2019 11:26 am**

Passing the class instance around as 'void *' and then casting is the usual trick.
If the class member dispatch is static then you can just pass the member address as a function pointer.

Code: Select all

extern "C" CToCPlusPlus(void *displayPtr, const chat *str) {
   ((Display *)displayPtr)->dispatch(str);
}

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