Code for my blog post <https://superkogito.github.io/blog/CaptureScreenUsingOpenCv.html> A snippet for capturing the screen content using OpenCV

CaptureSceenshotUsingOpenCV.cpp

/*****************************************************************************************************************
 * \file   CaptureSceenshotUsingOpenCV.cpp
 * \brief  capture screenshot using OpenCV and save it as a png.
 *
 * \author SuperKogito
 * \date   July 2020
 *
 * @note:
 * references and sources:
 * - http://msdn.microsoft.com/en-us/library/windows/window/dd183402%28v=vs.85%29.aspx
 * - https://stackoverflow.com/questions/34466993/opencv-desktop-capture
 * - https://stackoverflow.com/questions/14148758/how-to-capture-the-desktop-in-opencv-ie-turn-a-bitmap-into-a-mat
 *
 *****************************************************************************************************************/
#pragma once
#include <Windows.h>
#include <opencv2/opencv.hpp>

using namespace cv;

/**
 * Create a Bitmap file header..
 *
 * @param hwindowDC : window handle.
 * @param widht	    : image width.
 * @param height    : image height.
 *
 * @return Bitmap header.
 */
BITMAPINFOHEADER createBitmapHeader(int width, int height)
{
	BITMAPINFOHEADER  bi;

	// create a bitmap
	bi.biSize = sizeof(BITMAPINFOHEADER);
	bi.biWidth = width;
	bi.biHeight = -height;  //this is the line that makes it draw upside down or not
	bi.biPlanes = 1;
	bi.biBitCount = 32;
	bi.biCompression = BI_RGB;
	bi.biSizeImage = 0;
	bi.biXPelsPerMeter = 0;
	bi.biYPelsPerMeter = 0;
	bi.biClrUsed = 0;
	bi.biClrImportant = 0;

	return bi;
}

/**
 * Capture a screen window as a matrix.
 *
 * @param hwnd : window handle.
 *
 * @return Mat (Mat of the captured image)
 */
Mat captureScreenMat(HWND hwnd)
{
	Mat src;

	// get handles to a device context (DC)
	HDC hwindowDC = GetDC(hwnd);
	HDC hwindowCompatibleDC = CreateCompatibleDC(hwindowDC);
	SetStretchBltMode(hwindowCompatibleDC, COLORONCOLOR);

	// define scale, height and width
	int scale = 1;
	int screenx = GetSystemMetrics(SM_XVIRTUALSCREEN);
	int screeny = GetSystemMetrics(SM_YVIRTUALSCREEN);
	int width = GetSystemMetrics(SM_CXVIRTUALSCREEN);
	int height = GetSystemMetrics(SM_CYVIRTUALSCREEN);

	// create mat object
	src.create(height, width, CV_8UC4);

	// create a bitmap
	HBITMAP hbwindow = CreateCompatibleBitmap(hwindowDC, width, height);
	BITMAPINFOHEADER bi = createBitmapHeader(width, height);

	// use the previously created device context with the bitmap
	SelectObject(hwindowCompatibleDC, hbwindow);

	// copy from the window device context to the bitmap device context
	StretchBlt(hwindowCompatibleDC, 0, 0, width, height, hwindowDC, screenx, screeny, width, height, SRCCOPY);  //change SRCCOPY to NOTSRCCOPY for wacky colors !
	GetDIBits(hwindowCompatibleDC, hbwindow, 0, height, src.data, (BITMAPINFO*)&bi, DIB_RGB_COLORS);            //copy from hwindowCompatibleDC to hbwindow

	// avoid memory leak
	DeleteObject(hbwindow);
	DeleteDC(hwindowCompatibleDC);
	ReleaseDC(hwnd, hwindowDC);

	return src;
}

int main()
{
	// capture image
	HWND hwnd = GetDesktopWindow();
	Mat src = captureScreenMat(hwnd);

	// encode result
	std::vector<uchar> buf;
	cv::imencode(".png", src, buf);

	// save img
	cv::imwrite("Screenshot.png", src);

	buf.clear();
	return 0;
}

Idk why you get this error since .clear() is definitely part of the vector class https://en.cppreference.com/w/cpp/container/vector/clear
make sure that you are defining your vector variable above the clearing and it is not inside some sort of a loop/if condition so it can be seen