I'm using a dense matrix as a vertex list, so I have something like:

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template< typename T > class foo {
   Eigen::Matrix<T,4,Eigen::Dynamic> _points;
public:
   Eigen::Matrix<T,4,1> point( long index ) const {
      return _points.col( index );
   }
   ?????& point( long index ) {
      return _points.col( index );
   }
}

The first (const) getter is easy; I convert the column reference to a normal matrix and return it. But I have no idea what return type to use in the second case. I want to be able to do code like:

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myFoo.point( 3 ).normalize( );

(ok so normalize makes no sense here but you get the idea). I've tried this but it causes a compiler crash (!)

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decltype( _points.col( 0 ) )& point( long index ) {
   return _points.col( index );
}

Is there a particular (explicit) class / typedef I can use?

Nevermind; I just found this thread viewtopic.php?f=74&t=95125 which answers this.

The working solution is:

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template< typename T > class foo {
   typedef Eigen::Matrix<T,4,Eigen::Dynamic> points_t;
   points_t _points;
public:
   typename points_t::ConstColXpr point( long index ) const {
      return _points.col( index );
   }
   typename points_t::ColXpr point( long index ) {
      return _points.col( index );
   }
}