C++: VideoWriter& VideoWriter::operator<<(const Mat& image)
C++: void VideoWriter::write(const Mat& image)
Python: cv2.VideoWriter.write(image) → None
C: int cvWriteFrame(CvVideoWriter* writer, const IplImage* image)
Python: cv.WriteFrame(writer, image) → int
So i guess something like
cv::Mat image;
....
cv::VideoWriter writer;
writer.open(...);
if (writer.isOpened())
writer << image;