当作为与结构相同类型的参数传递时，包装在 C++ 类中的 C 结构如何交互？

Question

Say I have the code below using the WINDOW struct from NCurses.

#include "ncurses.h"

class Window : public WINDOW {
    Window(int winHeight, int winWidth, int startX, int startY) {
        newwin(winHeight, winWidth, startX, startY);
    }
    void refresh() {
        wrefresh(this);
    }
};

int main() {
    initscr();

    Window win = Window(5, 5, 5, 5);
    win.refresh();

    endwin();
    return 0;
}

Is there a proper way to have C++ classes inherit C structs so that they can be passed as them, similar to how C++ classes that have inherited each other can?

Answer 1

One way to create C++ 'wrappers' for structures defined in a C library, so that you can easily pass pointers to API calls in such a library is to emulate the way the MFC (Microsoft Foundation Class) library wraps GDI objects, such as the RECT structure, into classes like CRect .

MFC uses straightforward inheritance of the 'base' structure, and provides operators in each class that return pointers to the base structure (which will actually be a class instance's this pointer).

In the code below, I show the definition of the RECT structure and some excerpts of the CRect class, from the relevant Windows headers and MFC headers/source.

// The "C" base structure, from "windef.h"
typedef struct tagRECT
{
    LONG    left;
    LONG    top;
    LONG    right;
    LONG    bottom;
} RECT, *LPRECT;

typedef const RECT *LPCRECT;

class CRect : public tagRECT
{
public:
    CRect(int l, int t, int r, int b) {
        // Assign given parameters to base structure members...
        left = l; top = t; right = r; bottom = b;
    }
//...
    // Utility routines...
    int Width() const {
        return right - left;
    }
//...
    // convert between CRect and LPRECT/LPCRECT (no need for &)
    operator LPRECT() {
        return this;
    }
    operator LPCRECT() const {
        return this;
    }
//...
};

With wrapper classes defined like this, you can pass a CRect object (as in the comment above, no need for & ) to a call from your C Library that expects a pointer to a RECT . For example, the GetWindowRect() function takes an LPRECT parameter and is called, from C, like this:

HWND hWnd;
RECT rect;
BOOL answer = GetWindowRect(hWnd, &rect); // Pass pointer to C structure

Using the CRect class, you can just pass the object, and the LPRECT operator will take care of the rest (but you can add the & if you really want to):

HWND hWnd;
CRect rc;
BOOL answer = GetWindowRect(hWnd, rc);

There are limitations and caveats involved in this approach (eg horrid things may happen if the C library expects a RECT to be passed by value ), but it may be an approach you find helpful.