histogram smoothing

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In general you can use OpenCV filters also on vectors. In this case GaussianBlur works fine:

cv::Mat hist,hist_smoothed;
...
calcHist(&grayImg, 1, 0, cv::Mat(), hist, 1, &histSize, &histRange, uniform, accumulate);
GaussianBlur(hist, hist_smoothed, Size(smooth_size, smooth_size), 0, 0, BORDER_CONSTANT);

Histograms contain useful information about your image, smoothing it will degrade the information, making the histogram mostly useless.

If you still want to use it, here you are: just replace each value by the mean of its neighbors. e.g. smooth_hist[i]=(hist[i-2]+hist[i-1]+hist[i]+hist[i+1]+hist[i+2])/5. You can use a gaussian weighting, too.