c++ similar function to numpy

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hi there, I have developed my opencv vision algorithm in python. in my algorithm I have used numpy function as follow,

roi=cv2.imread("roi.png") img=cv2.imread("image.png")
img[roi<100] = 0;

basically what I have done is, I have image (roi.png) which contain important area for generate features, so I will consider only the area in img which roi has greater values than 100, so my problem is I need to implement this code in c++, how can I do it without numpy in c++? thanks