我正在编写一个模块来将数据写入一个文件,该文件按惯例仅使用行主存储。我希望我的函数能够允许列主要和行主要特征对象作为输入。
目前我首先使用Eigen将列主要对象复制到行主要对象,然后再编写。我的代码适用于大多数情况,但对于Eigen::VectorXi
编译失败的断言我不明白。我该如何解决这个问题?我可以避免创建很多案例吗?
代码(通过输出std::vector
来模仿写作):
#include <vector>
#include <iostream>
#include <Eigen/Eigen>
template <class T, int Rows, int Cols, int Options, int MaxRows, int MaxCols>
std::vector<T> write(const Eigen::Matrix<T,Rows,Cols,Options,MaxRows,MaxCols>& matrix)
{
std::vector<T> data(static_cast<size_t>(matrix.size()));
if (matrix.IsRowMajor) {
std::copy(matrix.data(), matrix.data()+matrix.size(), data.begin());
return data;
} else {
Eigen::Matrix<T, Rows, Cols, Eigen::RowMajor, MaxRows, MaxCols> tmp = matrix;
return write(tmp);
}
}
int main()
{
Eigen::VectorXi matrix = Eigen::VectorXi::LinSpaced(10, 0, 9);
std::vector<int> output = write(matrix);
}
编译错误:
In file included from test.cpp:3:
In file included from /usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/Eigen:1:
In file included from /usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/Dense:1:
In file included from /usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/Core:457:
/usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/src/Core/PlainObjectBase.h:903:7: error: static_assert failed "INVALID_MATRIX_TEMPLATE_PARAMETERS"
EIGEN_STATIC_ASSERT((EIGEN_IMPLIES(MaxRowsAtCompileTime==1 && MaxColsAtCompileTime!=1, (Options&RowMajor)==RowMajor)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/src/Core/util/StaticAssert.h:33:40: note: expanded from macro 'EIGEN_STATIC_ASSERT'
#define EIGEN_STATIC_ASSERT(X,MSG) static_assert(X,#MSG);
^ ~
/usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/src/Core/PlainObjectBase.h:535:7: note: in instantiation of member function 'Eigen::PlainObjectBase<Eigen::Matrix<int, -1, 1, 1, -1, 1>
>::_check_template_params' requested here
_check_template_params();
^
/usr/local/Cellar/eigen/3.3.7/include/eigen3/Eigen/src/Core/Matrix.h:377:9: note: in instantiation of function template specialization 'Eigen::PlainObjectBase<Eigen::Matrix<int, -1, 1, 1, -1, 1>
>::PlainObjectBase<Eigen::Matrix<int, -1, 1, 0, -1, 1> >' requested here
: Base(other.derived())
^
test.cpp:14:79: note: in instantiation of function template specialization 'Eigen::Matrix<int, -1, 1, 1, -1, 1>::Matrix<Eigen::Matrix<int, -1, 1, 0, -1, 1> >' requested here
Eigen::Matrix<T, Rows, Cols, Eigen::RowMajor, MaxRows, MaxCols> tmp = matrix;
^
test.cpp:23:31: note: in instantiation of function template specialization 'write<int, -1, 1, 0, -1, 1>' requested here
std::vector<int> output = write(matrix);
^
1 error generated.
理解静态断言
不幸的是,断言实际上并不是不言自明的,你唯一能从中得到的就是提示,你的模板参数有问题。如果我们查看Eigen的源代码Eigen's source code,我们在第903行找到以下内容:
EIGEN_STATIC_ASSERT((EIGEN_IMPLIES(MaxRowsAtCompileTime==1 && MaxColsAtCompileTime!=1, (Options&RowMajor)==RowMajor)
&& EIGEN_IMPLIES(MaxColsAtCompileTime==1 && MaxRowsAtCompileTime!=1, (Options&RowMajor)==0)
&& ((RowsAtCompileTime == Dynamic) || (RowsAtCompileTime >= 0))
&& ((ColsAtCompileTime == Dynamic) || (ColsAtCompileTime >= 0))
&& ((MaxRowsAtCompileTime == Dynamic) || (MaxRowsAtCompileTime >= 0))
&& ((MaxColsAtCompileTime == Dynamic) || (MaxColsAtCompileTime >= 0))
&& (MaxRowsAtCompileTime == RowsAtCompileTime || RowsAtCompileTime==Dynamic)
&& (MaxColsAtCompileTime == ColsAtCompileTime || ColsAtCompileTime==Dynamic)
&& (Options & (DontAlign|RowMajor)) == Options),
INVALID_MATRIX_TEMPLATE_PARAMETERS)
即使编译器指出了这一点
EIGEN_IMPLIES(MaxRowsAtCompileTime==1 && MaxColsAtCompileTime!=1, (Options&RowMajor)==RowMajor)
导致错误,以下行确实做到了:
EIGEN_IMPLIES(MaxColsAtCompileTime==1 && MaxRowsAtCompileTime!=1, (Options&RowMajor)==0)
了解触发断言的原因
您提供Eigen::VectorXi
作为write
的输入。 Eigen::VectorXi
实际上只是一个类型
Eigen::Matrix<int, Eigen::Dynamic, 1, Eigen::ColMajor, Eigen::Dynamic, 1>
因此行
Eigen::Matrix<T, Rows, Cols, Eigen::RowMajor, MaxRows, MaxCols> tmp = matrix;
在write
扩大到
Eigen::Matrix<int, Eigen::Dynamic, 1, Eigen::RowMajor, Eigen::Dynamic, 1> tmp = matrix;
这会引发断言,因为MaxColsAtCompileTime==1
和MaxRowsAtCompileTime!=1
的矩阵不能是RowMajor
。
解决你的问题
现在的问题是,即使您可以检查输入矩阵是矢量,行主要还是列主要,您也无法声明
Eigen::Matrix<T, Rows, Cols, Eigen::RowMajor, MaxRows, MaxCols>
如果在编译时这样做是不合法的(并且不是由于静态断言)。
您可以使用以下选项来使代码工作:
1. if constexpr
(C ++ 17)
C ++ 17提供了一种在编译时检测是否采用某个条件分支的方法。这种方法的缺点(除了对C ++ 17编译器的要求)是你只能测试常量表达式。
在具体示例中,这看起来像这样:
template <class T, int Rows, int Cols, int Options, int MaxRows, int MaxCols>
std::vector<T> write(const Eigen::Matrix<T, Rows, Cols, Options, MaxRows, MaxCols>& matrix)
{
typedef Eigen::Matrix<T, Rows, Cols, Options, MaxRows, MaxCols> MatrixType;
std::vector<T> data(static_cast<size_t>(matrix.size()));
if constexpr (MatrixType::MaxRowsAtCompileTime == 1 ||
MatrixType::MaxColsAtCompileTime ==1 ||
(MatrixType::Options&Eigen::RowMajor) == Eigen::RowMajor) {
std::copy(matrix.data(), matrix.data() + matrix.size(), data.begin());
return data;
} else {
Eigen::Matrix<T, Rows, Cols, Eigen::RowMajor, MaxRows, MaxCols> tmp = matrix;
return write(tmp);
}
}
2. SFIN
您可以使用write
在编译时使用SFINAE将调用发送到std::enable_if
。以下示例使用原始代码的略微修改版本,但所有内容都应该从上下文中清楚:
// matrix is either a vector or in row-major
template <typename Derived>
std::vector<typename Derived::Scalar> write(const Eigen::MatrixBase<Derived>& matrix,
typename std::enable_if<Derived::MaxRowsAtCompileTime == 1 ||
Derived::MaxColsAtCompileTime == 1 ||
(Derived::Options & Eigen::RowMajor) == Eigen::RowMajor,
Derived>::type* = 0)
{
std::vector<typename Derived::Scalar> data(
static_cast<size_t>(matrix.size()));
std::copy(matrix.derived().data(), matrix.derived().data() + matrix.size(),
data.begin());
return data;
}
// matrix is neither a vector nor in row-major
template <typename Derived>
std::vector<typename Derived::Scalar> write(const Eigen::MatrixBase<Derived>& matrix,
typename std::enable_if<Derived::MaxRowsAtCompileTime != 1 &&
Derived::MaxColsAtCompileTime != 1 &&
(Derived::Options & Eigen::RowMajor) == 0,
Derived>::type* = 0)
{
Eigen::Matrix<typename Derived::Scalar, Derived::RowsAtCompileTime,
Derived::ColsAtCompileTime, Eigen::RowMajor,
Derived::MaxRowsAtCompileTime, Derived::MaxColsAtCompileTime> tmp = matrix;
return write(tmp);
}
这适用于使用C ++ 11编译器。
其他选择是专门化模板,但它将比SFINAE方法更长。
一些测试用例:
Eigen::Matrix<int, 3, 3, Eigen::RowMajor> m;
m << 1, 2, 3,
1, 2, 3,
1, 2, 3;
std::vector<int> output = write(m);
for (const auto& element : output) {
std::cout << element << " ";
}
输出:1 2 3 1 2 3 1 2 3
Eigen::Matrix<int, 3, 3, Eigen::ColMajor> m;
m << 1, 2, 3,
1, 2, 3,
1, 2, 3;
std::vector<int> output = write(m);
for (const auto& element : output) {
std::cout << element << " ";
}
输出:1 2 3 1 2 3 1 2 3
Eigen::VectorXi m = Eigen::VectorXi::LinSpaced(10, 0, 9);
std::vector<int> output = write(m);
for (const auto& element : output) {
std::cout << element << " ";
}
输出:0 1 2 3 4 5 6 7 8 9
Eigen::RowVectorXi m = Eigen::RowVectorXi::LinSpaced(10, 0, 9);
std::vector<int> output = write(m);
for (const auto& element : output) {
std::cout << element << " ";
}
输出:0 1 2 3 4 5 6 7 8 9
更简单的解决方案是让Eigen::Ref
为您完成所有工作:
Ref<const Matrix<T,Rows,Cols,Cols==1?ColMajor:RowMajor,MaxRows,MaxCols>,0, InnerStride<1> > row_maj(matrix);
然后row_maj
将保证按行主顺序顺序存储。如果matrix
兼容,则不会发生复制。没有分公司,没有SFINAE等
在这里,matrix
可以是任何表达,不仅是Matrix<...>
,还有子矩阵,Map
,另一个Ref
等。
要处理任何表达式,只需用Rows
替换XprType::RowsAtCompileTime
等,其中XprType
是matrix
的类型。
template <class XprType>
std::vector<typename XprType::Scalar> write(const Eigen::MatrixBase<XprType>& matrix)
{...}