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cv::DMatch is the class that holds the results of a matcher.
Given a bunch of training descriptors and another bunch of query descriptors, DMatch tells you what the best match was for each query descriptor.

For each found match, DMatch will tell you the: query descriptor index, the train descriptor index, train image index (each image generates multiple training descriptors), and distance between these query and training descriptors.

See this example for more detailed usage.