What is the purpose of cv::MatExpr?

Question

OpenCV arithmetic operations produce a cv::MatExpr, for example:

MatExproperator+(const Mat & a, const Mat &b);

I see that this is used to represent an expression before it is evaluated. What is the purpose of doing this?

Answer 1

Let's say you have this expression:
Mat A = 3 + B * 5;
where B is also a Mat. If the operators + and * where to return a Mat, B * 5 would have create a temporary Mat and then the + operator would have create another Mat. Instead, B * 5 returns a MatExpr that doesn't actually creates a Mat, it just "remembers" the operation it needs to peform. Then, the + operator creates another MatExpr and only the = operator creates a Mat, thus avoiding the temporary Mat.
See https://en.wikipedia.org/wiki/Lazy_evaluation