39 lines
1.7 KiB
Plaintext
39 lines
1.7 KiB
Plaintext
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.
|
|
*/
|
|
}
|