C 结构实例覆盖先前的实例

Question

我是 C 语言新手，所以答案可能很明显，但我就是无法理解它。

我正在尝试创建 C++ vector kind of structure in C 。我使用 Windows EnumWindows 函数来循环所有窗口。在回调函数中，我为每个窗口创建 windowHandle 结构的新实例。然而，出于某种原因，它似乎并没有创建新实例，而只是覆盖旧实例。或者它可能会创建新实例，但是当我为属性 windowHandle->title 赋予值时，它会将更改应用于 windowHandle 的每个实例？

回调函数:

BOOL CALLBACK delegate(HWND wnd, LPARAM param){
if (filter(wnd)){ //<- just to make sure it's kind of window that we want.
            char windowName[256] = {};
            GetWindowText(wnd, windowName, sizeof(windowName));
            windowHandle *handle = malloc(sizeof(windowHandle)); //<- create new instance of windowHandle for each window
            handle->hWND = wnd;
            handle->title = windowName;
            insert((windowArray*) param, handle); //<- insert each windowHandle in our 'vector'
        }
// but return true here so that we iterate all windows
return TRUE;
}

调用 EnumWindows 函数:

windowArray* array = array_init(20);
EnumWindows(&delegate, (LPARAM) array); //<- not quite sure what that '&'-sign does?

结构:

typedef struct windowHandle{
    HWND hWND;
    char* title;
} windowHandle;

typedef struct windowArray{
    int count;
    int capacity;
    int objectSize;
    windowHandle* windows;
} windowArray;

最后插入:

int insert(windowArray* array, void* handle){
    int index = (array->count * array->objectSize);
    if(index > 0){
        printf("\n%s %s\n", "first element's title is: ", array->windows->title); //<- prints to see if new handle overwrite the previous ones
    }
    printf("%s %s %s %p %s %d\n", "Inserted element ", ((windowHandle*)handle)->title, " with pointer ", handle, " on index ", index);
    fflush(stdout); //<- prints info about first & current window
    memcpy(array->windows + (index), (windowHandle*)handle, array->objectSize);
    array->count++;
}

插入调用期间的输出:

Inserted element  joo - Java - nativeWindowCapture/src/NativeWindowHookImp.c - Eclipse SDK  with pointer  0000000000FB1B90  on index  0

first element's title is:  Komentokehote
Inserted element  Komentokehote  with pointer  0000000000FB1BB0  on index  16

first element's title is:  C struct instance overwrites previous instance - Stack Overflow - Mozilla Firefox
Inserted element  C struct instance overwrites previous instance - Stack Overflow - Mozilla Firefox  with pointer  0000000000FB1BD0  on index  32

first element's title is:  JNI compiler kutsu – Muistio
Inserted element  JNI compiler kutsu – Muistio  with pointer  0000000000FB1BF0  on index  48

first element's title is:  Skype™?
Inserted element  Skype™? with pointer  0000000000FB1C10  on index  64

first element's title is:  eclipse
Inserted element  eclipse  with pointer  0000000000FB1C30  on index  80

first element's title is:  src
Inserted element  src  with pointer  0000000000FBCCE0  on index  96
all added

然后如果我打印 windowArray->windows 的完整内容，我会得到以下结果:

Getting value  src , at index  0  and pointer  0000000000FBBB80
Getting value  src , at index  16  and pointer  0000000000FBBC80
Getting value  src , at index  32  and pointer  0000000000FBBD80
Getting value  src , at index  48  and pointer  0000000000FBBE80
Getting value  src , at index  64  and pointer  0000000000FBBF80
Getting value  src , at index  80  and pointer  0000000000FBC080
Getting value  src , at index  96  and pointer  0000000000FBC180

附注我还想知道为什么 windowArray 的 windows 属性的指针与委托(delegate)中创建的 *handle 对象的指针不同？这看起来内存效率不是很高？

Answer 1

您将handle->title设置为指向windowName。但一旦退出此范围，windowName 将不复存在。因此，您的结构包含一个指向不再存在的数组的指针。

        char windowName[256] = {};
        GetWindowText(wnd, windowName, sizeof(windowName));
        windowHandle *handle = malloc(sizeof(windowHandle)); //<- create new instance of windowHandle for each window
        handle->hWND = wnd;
        handle->title = windowName;
    }
    // when we get here, windowName ceases to exist
    // so why did we store a pointer to it?

退出作用域后，跟随title指针是错误的，因为它不再指向任何仍然存在的对象。也许将 title 从 char * 更改为 char[256]。