Here is an interesting proposal for matrix assembly via horizontal and vertical stacking: https://forum.kde.org/viewtopic.php?f=74&t=140012
It works with C++11.
Still need to debate on final API, implement proper unit tests, docs, etc.
and for the record, here is the first attempt from the same author: https://forum.kde.org/viewtopic.php?f=74&t=123881&p=375217&hilit=initializer_list#p375217
Very interesting indeed. And probably the closest we can get to Matlab-style [,;] matrix stacking.
I always found the << operator inconvenient, as it requires to resize the target in advance, and makes inline usage of stacked matrices very ugly.
Maybe it would even be possible to support C++03 (for a limited number of concatenated matrices). OTOH, maybe Eigen3.4 will be a good time to start giving up C++03 support (at least not force new features into it).