用WindowsAPI写一个c++程序，弹出一个空窗口，大小300*300像素。写出一个示例程序

首先，在 Windows API 中，创建一个窗口需要用到 `CreateWindow` 函数。下面是一个使用这个函数创建一个空窗口的 C 程序的示例：

#include <windows.h>

// 窗口过程函数
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
        case WM_CLOSE:
            DestroyWindow(hwnd);
            break;
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        default:
            return DefWindowProc(hwnd, message, wParam, lParam);
    }
    return 0;
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wc;
    HWND hwnd;
    MSG msg;

    // 注册窗口类
    wc.cbSize        = sizeof(WNDCLASSEX);
    wc.style         = 0;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = "WindowClass";
    wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);
    if(!RegisterClassEx(&wc))
    {
        MessageBox(NULL, "Window Registration Failed!", "Error!",
            MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    // 创建窗口
    hwnd = CreateWindowEx(
        WS_EX_CLIENTEDGE,
        "WindowClass",
        "窗口标题",
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, 300, 300,
        NULL, NULL, hInstance, NULL);
    if(hwnd == NULL)
    {
        MessageBox(NULL, "窗口创建失败!", "Error!", MB_ICONEXCLAMATION | MB_OK);
        return 0;
    }

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    // 消息循