StereoBm Opencv function gives an error "object of an abstract class cannot be defined"

Question

When I intialise StereoBM object I get an error "object of an abstract class cannot be defined". I checked calib3d.hpp and I found that StereoBM is an abstract class. Please provide me a solution how to define all those functions in StereoBM class

cv::Mat leftimg =cv::imread("leftimage.jpg");
cv::Mat rightimg = cv::imread("rightimage.jpg");
cv::Size imagesize = leftimg.size();

cv::Mat g1,g2,disp,disp8;
cv::cvtColor(leftimg,g1,cv::COLOR_BGR2GRAY);
cv::cvtColor(rightimg,g2,cv::COLOR_BGR2GRAY);

cv::StereoBM *sbm;    
sbm->setDisp12MaxDiff(1);
sbm->setSpeckleRange(8);
sbm->setSpeckleWindowSize(0);
sbm->setUniquenessRatio(0);
sbm->setTextureThreshold(507);
sbm->setMinDisparity(-39);
sbm->setPreFilterCap(61);
sbm->setPreFilterSize(5);
sbm->compute(g1,g2,disp);
normalize(disp, disp8, 0, 255, CV_MINMAX, CV_8U);

cv::imshow("left", leftimg);
cv::imshow("right", rightimg);
cv::imshow("disp", disp8);

Answer 1

The suggestion in the comment fixed the error so I'm posting the working code after replacing it with the suggestion so it can help others in case of similar problem.

cv::Mat leftimg =cv::imread("leftimage.jpg");
cv::Mat rightimg = cv::imread("rightimage.jpg");
cv::Size imagesize = leftimg.size();
cv::Mat g1,g2,disp,disp8;
cv::cvtColor(leftimg,g1,cv::COLOR_BGR2GRAY);
cv::cvtColor(rightimg,g2,cv::COLOR_BGR2GRAY);

cv::StereoBM *sbm=cv::createStereoBM(16,2);    
sbm->setDisp12MaxDiff(1);
sbm->setSpeckleRange(8);
sbm->setSpeckleWindowSize(0);
sbm->setUniquenessRatio(0);
sbm->setTextureThreshold(507);
sbm->setMinDisparity(-39);
sbm->setPreFilterCap(61);
sbm->setPreFilterSize(5);
sbm->compute(g1,g2,disp);
normalize(disp, disp8, 0, 255, CV_MINMAX, CV_8U);

cv::imshow("left", leftimg);
cv::imshow("right", rightimg);
cv::imshow("disp", disp8);