Question

A little background:
I’ve got a library of C code that is part of larger system (all C). For this particular part of the C library a GUI has to be created which allows users to play around with the options. For the GUI I’ve chosen QT, since cross-platform support is desired.
I’m using Eclipse and MinGW as IDE and compiler (but I think the question is more language than compiler specific?).

With QT I’ve created a widget that holds a pointer to a struct implemented in C that contains pointers to several functions that perform the logic of the C library.

//QTWidget.cpp
extern "C" {
#include "c-src/CLogic.h"
//extern char* textHelper;
}

QTWidget::QTWidget(QWidget *parent)
{
    //rtw is a struct that contains a function pointer to a member of QTWidget
    this->rtw.displayText = &QTWidget::displayText;
    this->clogic = CLogic_getInstance(&rtw);
}

//Public SLOT, connected to a button's clicked SIGNAL
void QTWidget::buttonClicked()
{
    this->clogic->buttonClicked();
}

void QTWidget::displayText(char *text, int position)
{
    //I've tried creating a QString from the char*, but this does not work at all.
    //ui.textItem->setText(textHelper);
    ui.textItem->setText(text);
}

When the user presses a button in the GUI, the method QTWidget::buttonClicked() is called, which tells the C library to do something. Note the the CLogic struct has a reference to the the QTWidget in the form of a struct RefToWidget which holds a function pointer.

//CLogic.c
static CLogic instance;

void CLogic_buttonClicked()
{
    //I've tried several variants here, such as making a global 
    //char* textHelper = "Hello World"; 
    //that is referenced by using the "extern" keyword in the CPP file above.
    instance.rtw->displayText("Hello World", 1);
}
CLogic* CLogic_getInstance(RefToWidget *rtw)
{
    instance.rtw = rtw;
    instance.buttonClicked = &CLogic_buttonClicked();
}

When debugging this program, I find that all the function calls are executed as intended (when I press a button, the QT slot buttonClicked() is called, the CLogic_buttonClicked() is called, which calls the QTWidget::displayText() as planned, but in this last call the parameters are invalid. The char* text points to 0x1 and claims to be pointing to memory out of bounds, while the int position looks like some random number (uninitialized).

How do I pass this data from C to CPP?

EDIT @Luccas Matteis:

#ifdef __cplusplus
#include "QTWidget.h"

extern "C" {
#endif

struct RefToWidget{
#ifdef __cplusplus
    void (QTWidget::*displayLine)(char* text, int lineNumber);
#else
    void (*displayLine)(char* text, int lineNumber);
#endif
};

typedef struct RefToWidget RefToWidget;
#ifdef __cplusplus
}
#endif

As said above the function calls behave as expected, but the data is not passed “correctly” (even though that when I look at it, the code seems a bit… weird… ;))

Answer 1

Your problem is not passing the char * from C to C++ but calling a C++ function from C. I presume that CLogic.c is compiled as a C library? If not, can you rename it to CLogic.cpp or use a compiler switch to force it to be compiled as C++ even though it has a C file extension?

If you want a C++ GUI on a C library/system, you need to use proper Model-View-Controller logic. Here the C++ code is the View and the Controller while the C code is the Model (as best as I can tell from your description). You need to make it so you set and get data from the model but the model never calls the View or Controller as you are trying to do.

Think about what you are REALLY trying to do. If you just want to display a static string when the button is pressed, why go to the bother of calling into CLogic.c? If you want to display a string that depends upon the state of the CLogic instance then instead do something like:

void QTWidget::buttonClicked()
{
    char *display_text = this->clogic->get_button_click_text();
    ui.textItem->setText(display_text);
}