Passing objects by value is almost always a very bad idea in C++, as this means useless copies, and one should pass them by reference instead.
With Eigen, this is even more important: passing \ref FixedSizeVectorizable "fixed-size vectorizable Eigen objects" by value is not only inefficient, it can be illegal or make your program crash! And the reason is that these Eigen objects have alignment modifiers that aren't respected when they are passed by value.
So for example, a function like this, where v is passed by value:
\code
void my_function(Eigen::Vector2d v);
\endcode
needs to be rewritten as follows, passing v by reference:
\code
void my_function(const Eigen::Vector2d& v);
\endcode
Likewise if you have a class having a Eigen object as member:
\code
struct Foo
{
Eigen::Vector2d v;
};
void my_function(Foo v);
\endcode
This function also needs to be rewritten like this:
\code
void my_function(const Foo& v);
\endcode
Note that on the other hand, there is no problem with functions that return objects by value.
