eigen/doc/Pitfalls.dox

namespace Eigen {

/** \page TopicPitfalls Common pitfalls


\section TopicPitfalls_template_keyword Compilation error with template methods

See this \link TopicTemplateKeyword page \endlink.

\section TopicPitfalls_aliasing Aliasing

Don't miss this \link TopicAliasing page \endlink on aliasing,
especially if you got wrong results in statements where the destination appears on the right hand side of the expression.

\section TopicPitfalls_auto_keyword C++11 and the auto keyword

In short: do not use the auto keywords with Eigen's expressions, unless you are 100% sure about what you are doing. In particular, do not use the auto keyword as a replacement for a Matrix<> type. Here is an example:

\code
MatrixXd A, B;
auto C = A*B;
for(...) { ... w = C * v;  ...}
\endcode

In this example, the type of C is not a MatrixXd but an abstract expression representing a matrix product and storing references to A and B. Therefore, the product of A*B will be carried out multiple times, once per iteration of the for loop. Moreover, if the coefficients of A or B change during the iteration, then C will evaluate to different values.

Here is another example leading to a segfault:
\code
auto C = ((A+B).eval()).transpose();
// do something with C
\endcode
The problem is that eval() returns a temporary object (in this case a MatrixXd) which is then referenced by the Transpose<> expression. However, this temporary is deleted right after the first line, and there the C expression reference a dead object. The same issue might occur when sub expressions are automatically evaluated by Eigen as in the following example:
\code
VectorXd u, v;
auto C = u + (A*v).normalized();
// do something with C
\endcode
where the normalized() method has to evaluate the expensive product A*v to avoid evaluating it twice. On the other hand, the following example is perfectly fine:
\code
auto C = (u + (A*v).normalized()).eval();
\endcode
In this case, C will be a regular VectorXd object.
*/
}