HWND hwnd ; // 获取窗口句柄 hwnd = windows.at(ui->spinBox->value());使用hwnd截图

了解了您的问题，您是想通过获取窗口的句柄来实现截屏功能。在 Windows 平台上，可以使用以下方式来实现：

#include <Windows.h>

bool CaptureScreen(HWND hWnd, const char* filename) {
    HDC hDc = GetDC(hWnd);
    HDC hMemDc = CreateCompatibleDC(hDc);
    RECT rcClient;
    GetClientRect(hWnd, &rcClient);
    int nWidth = rcClient.right - rcClient.left;
    int nHeight = rcClient.bottom - rcClient.top;
    HBITMAP hBitmap = CreateCompatibleBitmap(hDc, nWidth, nHeight);
    HBITMAP hOldBitmap = (HBITMAP)SelectObject(hMemDc, hBitmap);
    BitBlt(hMemDc, 0, 0, nWidth, nHeight, hDc, 0, 0, SRCCOPY);
    SelectObject(hMemDc, hOldBitmap);
    DeleteDC(hMemDc);
    ReleaseDC(hWnd, hDc);

    PBITMAPINFO pbi;
    pbi = CreateBitmapInfoStruct(hWnd, hBitmap);

    LPBYTE lpBits;
    lpBits = (LPBYTE)GlobalAlloc(GMEM_FIXED, pbi->bmiHeader.biSizeImage);
    GetDIBits(hMemDc, hBitmap, 0, (UINT)pbi->bmiHeader.biHeight, lpBits, pbi, DIB_RGB_COLORS);

    HANDLE hf;
    BITMAPFILEHEADER hdr;
    PBITMAPINFOHEADER pbih;
    LPBYTE lpData;
    DWORD dwTotal;
    DWORD cb;
    BYTE* hp;
    DWORD dwTmp;

    pbih = (PBITMAPINFOHEADER)pbi;
    lpData = (LPBYTE)(lpBits + pbih->biSize + pbi->bmiHeader.biClrUsed * sizeof(RGBQUAD));
    dwTotal = ((DWORD)(pbih->biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)));
    hdr.bfType = 0x4d42;
    hdr.bfSize = dwTotal;
    hdr.bfReserved1 = 0;
    hdr.bfReserved2 = 0;
    hdr.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) + (DWORD)sizeof(BITMAPINFOHEADER) + pbi->bmiHeader.biClrUsed * sizeof(RGBQUAD);
    hf = CreateFile(filename, GENERIC_READ | GENERIC_WRITE, (DWORD)0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, (HANDLE)NULL);
    if (hf == INVALID_HANDLE_VALUE) return false;
    WriteFile(hf, (LPVOID)&hdr, sizeof(BITMAPFILEHEADER), (LPDWORD)&dwTmp, NULL);
    WriteFile(hf, (LPVOID)pbih, sizeof(BITMAPINFOHEADER) + pbi->bmiHeader.biClrUsed * sizeof(RGBQUAD), (LPDWORD)&dwTmp, (NULL));
    WriteFile(hf, (LPVOID)lpData, (DWORD)pbih->biSizeImage, (LPDWORD)&dwTmp, NULL);
    CloseHandle(hf);
    GlobalFree((HGLOBAL)lpBits);
    DeleteObject(hBitmap);
    return true;
}

上面的代码中，`CaptureScreen` 函数接收两个参数：`hWnd` 表示窗口句柄，`filename` 表示截图保存的文件名。该函数实现了从指定窗口截图，并将截图保存到指定文件中的功能。

注：上述代码中使用了一些辅助函数，如 `CreateBitmapInfoStruct`，请根据实际情况进行实现。