third_party/eigen/doc/PassingByValue.dox

namespace Eigen {

/** \eigenManualPage TopicPassingByValue Passing Eigen objects by value to functions

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 TopicFixedSizeVectorizable "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.

For example, a function like this, where \c v is passed by value:

\code
void my_function(Eigen::Vector2d v);
\endcode

needs to be rewritten as follows, passing \c v by const reference:

\code
void my_function(const Eigen::Vector2d& v);
\endcode

Likewise if you have a class having an %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.

*/

}