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_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.
*/
}