Based on the 'c++' tag you have in your question:
You should not use it, you should use cv::Mat. Every IplImage is a cv::Mat internally.
IplImage is only used in the C API. It is a kind of CvMat and holds image data. Its name originates from OpenCV's roots (Intel IPL).
It is not relevant anymore if you use the C++ API of OpenCV.
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