Calling a C++ class-method

[dasarne]

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: [Select all] [Expand/Collapse]

1. /*
2.  * Display.h
3.  *
4.  *  Created on: 01.11.2019
5.  *      Author: arne
6.  */
7.  
8. #ifndef COMPONENTS_HARDWARE_DISPLAY_H_
9. #define COMPONENTS_HARDWARE_DISPLAY_H_
10.  
11. #include "Modell.h"
12.  
13. class Display {
14. public:
15.     Display();
16.     virtual ~Display();
17.     /**Analysiert die Befehle des Frontends und ruft entsprechende Methoden auf*/
18.     void dispatch(char *str);
19. private:
20.     Modell theModell;
21. };
22.  
23. #endif

GeSHi © Codebox Plus Extension

Code: [Select all] [Expand/Collapse]

1. /*
2.  * Display.cpp
3.  *
4.  *  Created on: 01.11.2019
5.  *      Author: arne
6.  */
7.  
8. #include <Display.h>
9.  
10. Display::Display() {
11.     // TODO Auto-generated constructor stub
12.  
13. }
14.  
15. Display::~Display() {
16.     // TODO Auto-generated destructor stub
17. }
18.  
19. void Display::dispatch(char *str) {
20.  
21. }

GeSHi © Codebox Plus Extension

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

Code: [Select all] [Expand/Collapse]

1. /*
2.  * DisplayC-Wrapper.h
3.  *
4.  *  Created on: 01.11.2019
5.  *      Author: arne
6.  */
7.  
8. #ifndef COMPONENTS_HARDWARE_DISPLAYC_WRAPPER_H_
9. #define COMPONENTS_HARDWARE_DISPLAYC_WRAPPER_H_
10.  
11. #ifdef __cpluscplus
12. class Display;
13. extern "C" {
14. #else
15. struct Display;
16. typedef struct Display Display;
17. #endif
18. Display *Display_new();
19.  
20. void Display_dispatch(Display *instance, char *str);
21.  
22. #ifdef __cpluscplus
23. }
24. #endif
25. #endif

GeSHi © Codebox Plus Extension

Here is the implementation of the wrapper:

Code: [Select all] [Expand/Collapse]

1. /*
2.  * DisplayC-Wrapper.c
3.  *
4.  *  Created on: 01.11.2019
5.  *      Author: arne
6.  */
7.  
8. #include "DisplayC-Wrapper.h"
9. #include "Display.h"
10.  
11. Display *Display_new()
12. {
13.     return new Display();
14. }
15.  
16. void Display_dispatch(Display *instance, char *str)
17. {
18.     instance->dispatch(str);
19. }

GeSHi © Codebox Plus Extension

And here the corresponding call in the xTask:

Code: [Select all] [Expand/Collapse]

1. void rx_task() {
2.     Display *instance = Display_new();
3.     Display_dispatch(instance,"Test");
4. }

GeSHi © Codebox Plus Extension

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

[PeterR]

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);
}