Is it OK to assign matrix expression to auto variable, e.g.

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auto x = A.middleRows(...);

Also what is the difference between Ref and Block types. They seem to serve the same purpose?

Thanks.

Be very careful with Eigen and the auto keyword. In your case, x will be a Block<> object referencing the matrix A, not a copy of of a sub part of A. In this case, this might be ok, but sometimes this can be very dangerous, e.g.:

auto x = (A * y) + z;

in which case x will reference a dead object representing the result of A*y.

Block and Ref are sometimes indeed similar, which is why they share the same MapBase base class, but there are fundamental differences though:
- A Ref<> object always reference a block of an exiting dense matrix.
- A Block<> object is an abstract representation of a sub expression, e.g., it is the return type of (A+B).block(...) whereas assigning '(A+B).block(...)' to a Ref<const MatrixXd> object will allocate memory and evaluate the sub-expression.

Thanks for explaining.
I have another question.

When writing non-template functions that use Ref as it arguments the following is not OK:

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foo(Ref<...>);
foo(A+B);


It's OK only if the Ref is const, for instance: Ref<const MatrixXd>