1 namespace Eigen { 2 3 /** \page TopicPitfalls Common pitfalls 4 5 \section TopicPitfalls_template_keyword Compilation error with template methods 6 7 See this \link TopicTemplateKeyword page \endlink. 8 9 \section TopicPitfalls_auto_keyword C++11 and the auto keyword 10 11 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: 12 13 \code 14 MatrixXd A, B; 15 auto C = A*B; 16 for(...) { ... w = C * v; ...} 17 \endcode 18 19 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. 20 21 Here is another example leading to a segfault: 22 \code 23 auto C = ((A+B).eval()).transpose(); 24 // do something with C 25 \endcode 26 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: 27 \code 28 VectorXd u, v; 29 auto C = u + (A*v).normalized(); 30 // do something with C 31 \endcode 32 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: 33 \code 34 auto C = (u + (A*v).normalized()).eval(); 35 \endcode 36 In this case, C will be a regular VectorXd object. 37 */ 38 } 39
