将自动与Eigen结合使用会导致错误的结果 [英] Wrong results using auto with Eigen

问题描述

当使用两个向量求和时，使用auto和 Vector 会得到不同的结果。

我的代码：

  #include stdafx.h 
 #include< iostream> 
 #include D：\externals\eigen_3_1_2\include\Eigen\Geometry 
 
 typedef Eigen :: Matrix< double，3，1> Vector3; 
 
 void foo（const Vector3& Ha，volatile int j）
 {
 const auto resAuto = Ha + Vector3（0.，0.，j * 2.567）; 
 const Vector3 resVector3 = Ha + Vector3（0.，0.，j * 2.567）; 
 
 std :: cout<<  resAuto =<< resAuto<< std :: endl; 
 std :: cout<<  resVector3 =<< resVector3<< std :: endl; 
} 
 
 int main（int argc，_TCHAR * argv []）
 {
 Vector3 Ha（-24.9536，-29.3876,65.801）; 
 Vector3 z（0.，0.，2.567）; 
 
 int j = 7; 
 
 foo（Ha，j）; 
返回0； 
} 
  

结果：

< blockquote>

resAuto = -24.9536，-29.3876,65.801

resVector3 = -24.9536，-29.3876,83.77

按任意键继续。 。

我知道Eigen会进行内部优化，从而产生不同的结果。但这看起来像是Eigen和C ++ 11中的错误。

解决方案

自动关键字告诉编译器根据 = 的右侧猜测最佳对象。您可以通过添加

  std :: cout<< typeid（resAuto）.name（）<< std :: endl; 
 std :: cout<< typeid（resVector3）.name（）<< std :: endl; 
  

到 foo （不要忘记包括< typeinfo> ）。

在这种情况下，构造临时 Vector3之后，将调用 operator + 方法，该方法将创建 CwiseBinaryOp 对象。此对象是Eigens 惰性评估的一部分（可以提高性能）。如果要强制进行急切的评估（并因此确定类型），则可以使用

  const auto resAuto =（Ha + Vector3（ 0.，0.，j * 2.567））。eval（）; 
  

代替您在 foo 中的行。 / p>

一些注意事项：

• Vector3 与Eigen中定义的 Vector3d 类相同


• 您可以使用 #include< ; Eigen / Core> 而不是 #include< Eigen / Geometry> 来包含大多数Eigen标头，再加上某些定义，应该是。

I got different results using auto and using Vector when summing two vectors.

My code:

#include "stdafx.h"
#include <iostream>
#include "D:\externals\eigen_3_1_2\include\Eigen\Geometry"

typedef Eigen::Matrix<double, 3, 1>       Vector3;

void foo(const Vector3& Ha, volatile int j) 
{
    const auto resAuto = Ha + Vector3(0.,0.,j * 2.567);
    const Vector3 resVector3 = Ha + Vector3(0.,0.,j * 2.567);

    std::cout << "resAuto = " << resAuto <<std::endl;
    std::cout << "resVector3 = " << resVector3 <<std::endl;
}

int main(int argc, _TCHAR* argv[])
{
    Vector3 Ha(-24.9536,-29.3876,65.801);
    Vector3 z(0.,0.,2.567);

    int j = 7;

    foo(Ha,j);
    return 0;
}

The results:

resAuto = -24.9536, -29.3876,65.801

resVector3 = -24.9536,-29.3876,83.77

Press any key to continue . . .

I understand that Eigen does internal optimization that generate different results. But it looks like a bug in Eigen and C++11.

解决方案

The auto keyword tells the compiler to "guess" the best object based on the right hand side of the =. You can check the results by adding

std::cout << typeid(resAuto).name() <<std::endl;
std::cout << typeid(resVector3).name() <<std::endl;

to foo (don't forget to include <typeinfo>).

In this case, after constructing the temporary Vector3, the operator+ method is called, which creates a CwiseBinaryOp object. This object is part of Eigens lazy evaluation (can increase performance). If you want to force eager evaluation (and therefore type determination), you could use

const auto resAuto = (Ha + Vector3(0.,0.,j * 2.567)).eval();

instead of your line in foo.

A few side notes:

• Vector3 is identical to the Vector3d class defined in Eigen
• You can use #include <Eigen/Core> instead of #include <Eigen/Geometry> to include most of the Eigen headers, plus certain things get defined there that should be.

这篇关于将自动与Eigen结合使用会导致错误的结果的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！