mirror of
https://gitlab.com/libeigen/eigen.git
synced 2024-12-21 07:19:46 +08:00
74 lines
3.8 KiB
Plaintext
74 lines
3.8 KiB
Plaintext
namespace Eigen {
|
|
|
|
/** \eigenManualPage TopicStlContainers Using STL Containers with Eigen
|
|
|
|
\eigenAutoToc
|
|
|
|
\section StlContainers_summary Executive summary
|
|
|
|
If you're compiling in \cpp17 mode only with a sufficiently recent compiler (e.g., GCC>=7, clang>=5, MSVC>=19.12), then everything is taken care by the compiler and you can stop reading.
|
|
|
|
Otherwise, using STL containers on \ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen types", or classes having members of such types, requires the use of an over-aligned allocator.
|
|
That is, an allocator capable of allocating buffers with 16, 32, or even 64 bytes alignment.
|
|
%Eigen does provide one ready for use: aligned_allocator.
|
|
|
|
Prior to \cpp11, if you want to use the `std::vector` container, then you also have to <code> \#include <Eigen/StdVector> </code>.
|
|
|
|
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.
|
|
|
|
\section allocator Using an aligned allocator
|
|
|
|
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 (or more) locations. Fortunately, %Eigen does provide such an allocator: Eigen::aligned_allocator.
|
|
|
|
For example, instead of
|
|
\code
|
|
std::map<int, Eigen::Vector4d>
|
|
\endcode
|
|
you need to use
|
|
\code
|
|
std::map<int, Eigen::Vector4d, std::less<int>,
|
|
Eigen::aligned_allocator<std::pair<const int, Eigen::Vector4d> > >
|
|
\endcode
|
|
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.
|
|
|
|
\section StlContainers_vector The case of std::vector
|
|
|
|
This section is for c++98/03 users only. \cpp11 (or above) users can stop reading here.
|
|
|
|
So in c++98/03, the situation with `std::vector` is more complicated because of a bug in the standard (explanation below).
|
|
To workaround the issue, 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>.
|
|
|
|
Here is an example:
|
|
\code
|
|
#include<Eigen/StdVector>
|
|
/* ... */
|
|
std::vector<Eigen::Vector4f,Eigen::aligned_allocator<Eigen::Vector4f> >
|
|
\endcode
|
|
|
|
<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::Vector4d>`, some Eigen::Vector4d objects will be passed by value, which discards any alignment modifiers, so a Eigen::Vector4d 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::Vector4d, that is able to deal properly with this situation.
|
|
</span>
|
|
|
|
\subsection vector_spec An alternative - specializing std::vector for Eigen types
|
|
|
|
As an alternative to the recommended approach described above, you have the option to specialize std::vector for Eigen types requiring alignment.
|
|
The advantage is that you won't need to declare std::vector all over with Eigen::aligned_allocator. One drawback on the other hand side is that
|
|
the specialization needs to be defined before all code pieces in which e.g. `std::vector<Vector2d>` is used. Otherwise, without knowing the specialization
|
|
the compiler will compile that particular instance with the default `std::allocator` and you program is most likely to crash.
|
|
|
|
Here is an example:
|
|
\code
|
|
#include<Eigen/StdVector>
|
|
/* ... */
|
|
EIGEN_DEFINE_STL_VECTOR_SPECIALIZATION(Matrix2d)
|
|
std::vector<Eigen::Vector2d>
|
|
\endcode
|
|
|
|
|
|
|
|
*/
|
|
|
|
}
|