Currently expr.eval() immediately evaluates the expression and returns a plain object. In some (many?) cases it is desirable to only flag a subexpression that it should be evaluated within a temporary at evaluation time. This was initially discussed in bug 505.
-- GitLab Migration Automatic Message -- This bug has been migrated to gitlab.com's GitLab instance and has been closed from further activity. You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.com/libeigen/eigen/issues/1662.