1 namespace Eigen { 2 3 /** \page TopicStlContainers Using STL Containers with Eigen 4 5 \b Table \b of \b contents 6 - \ref summary 7 - \ref allocator 8 - \ref vector 9 10 \section summary Executive summary 11 12 Using STL containers on \ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen types", or classes having members of such types, requires taking the following two steps: 13 14 \li A 16-byte-aligned allocator must be used. Eigen does provide one ready for use: aligned_allocator. 15 \li If you want to use the std::vector container, you need to \#include <Eigen/StdVector>. 16 17 These issues arise only with \ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen types" and \ref TopicStructHavingEigenMembers "structures having such Eigen objects as member". For other Eigen types, such as Vector3f or MatrixXd, no special care is needed when using STL containers. 18 19 \section allocator Using an aligned allocator 20 21 STL containers take an optional template parameter, the allocator type. When using STL containers on \ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen types", you need tell the container to use an allocator that will always allocate memory at 16-byte-aligned locations. Fortunately, Eigen does provide such an allocator: Eigen::aligned_allocator. 22 23 For example, instead of 24 \code 25 std::map<int, Eigen::Vector4f> 26 \endcode 27 you need to use 28 \code 29 std::map<int, Eigen::Vector4f, std::less<int>, 30 Eigen::aligned_allocator<std::pair<const int, Eigen::Vector4f> > > 31 \endcode 32 Note that the third parameter "std::less<int>" is just the default value, but we have to include it because we want to specify the fourth parameter, which is the allocator type. 33 34 \section vector The case of std::vector 35 36 The situation with std::vector was even worse (explanation below) so we had to specialize it for the Eigen::aligned_allocator type. In practice you \b must use the Eigen::aligned_allocator (not another aligned allocator), \b and \#include <Eigen/StdVector>. 37 38 Here is an example: 39 \code 40 #include<Eigen/StdVector> 41 \/* ... *\/ 42 std::vector<Eigen::Vector4f,Eigen::aligned_allocator<Eigen::Vector4f> > 43 \endcode 44 45 \subsection vector_spec An alternative - specializing std::vector for Eigen types 46 47 As an alternative to the recommended approach described above, you have the option to specialize std::vector for Eigen types requiring alignment. 48 The advantage is that you won't need to declare std::vector all over with Eigen::allocator. One drawback on the other hand side is that 49 the specialization needs to be defined before all code pieces in which e.g. std::vector<Vector2d> is used. Otherwise, without knowing the specialization 50 the compiler will compile that particular instance with the default std::allocator and you program is most likely to crash. 51 52 Here is an example: 53 \code 54 #include<Eigen/StdVector> 55 \/* ... *\/ 56 EIGEN_DEFINE_STL_VECTOR_SPECIALIZATION(Matrix2d) 57 std::vector<Eigen::Vector2d> 58 \endcode 59 60 <span class="note">\b Explanation: The resize() method of std::vector takes a value_type argument (defaulting to value_type()). So with std::vector<Eigen::Vector4f>, some Eigen::Vector4f objects will be passed by value, which discards any alignment modifiers, so a Eigen::Vector4f can be created at an unaligned location. In order to avoid that, the only solution we saw was to specialize std::vector to make it work on a slight modification of, here, Eigen::Vector4f, that is able to deal properly with this situation. 61 </span> 62 63 */ 64 65 } 66
