为什么本征矩阵到C数组转换给垃圾值的前两个指标

我首先要说的是，我不是100%熟悉Eigen库(只是出于好奇下载了它来看看)，它的文档有点缺乏，但你的问题是一个基本的C问题，可以通过几种方式补救。

首先，我们将解释在你的代码中是如何给出垃圾值的:

int *test()
{
    /* create an auto scoped variable on the stack;
       this variable is only "visible" to this function
       and any references to it or it's underlying data
       outside the scope of this function will result
       in "undefined behaviour" */
    Eigen::MatrixXi arr = Eigen::MatrixXi::Ones(6,1);
    arr(4)=3;
    arr(5)=19;
    /* arr.data() is defined as returning a pointer to the scalar underlying type (or
    a C-style array in other words). Regardless of the type being returned, it is pointer based
    and you are returning a pointer to a location in memory, not the actual data being held in
    the memory. */
    return arr.data();
} /* the variable arr is destroyed here since we left function scope and the return value (the pointer location)
is put in the return register and "program flow" is returned back to the main function where the pointer being
returned now points to "invalid" memory */
int main()
{
    int *c_arr; // create a pointer type that can reference int types
    c_arr = test(); // point it to the result of the test function (see notes above)
    /* c_arr now points to a memory location returned from test, but since the
    arr variable no longer exists here, when you go through and print the values pointed
    to at those memory locations you will get what is at those locations and could be "anything"
    except a valid reference to the original arr variable and it's underlying data. */
    for (int i=0; i<6;++i)
    {
        std::cout << c_arr[i] << std::endl;
    }
    return 0;
}

这就是为什么要，至于如何解决这个问题有几种方法;一种是将返回数组作为变量传递给test函数(例如void test(int*& val))，然后您可以选择在测试函数中为变量分配新内存，或者假设用户已经这样做了，并且必须假设用户将在自己之后清理并调用delete[](不仅仅是delete，因为您正在操作数据数组)。

但是这有很多需要注意的地方，需要知道要分配多少空间，并确保在分配完成后重新分配。我不确定为什么你特别需要一个C风格的数组，但因为你使用c++，它可能会更谨慎，如果你使用一些STL和容器函数可用来帮助你，例如:

#include <iostream>
#include <vector>
#include <Eigen/Core>
std::vector<int> test()
{
    Eigen::MatrixXi arr = Eigen::MatrixXi::Ones(6,1);
    arr(4)=3;
    arr(5)=19;
    // we need the size so we know how big of a container to allocate
    std::size_t sz = arr.innerSize() * arr.outerSize();
    std::vector<int> ret(sz);
    // get a temporary C array pointer so we can reference the data
    int* tmp = arr.data();
    // copy from tmp[0] to tmp[sz] and insert the data into the first element of ret
    std::copy(tmp, tmp+sz, ret.begin());
    // return the (copied) data
    return ret;
}
int main()
{
    std::vector<int> c_arr = test();
    // c_arr now points to valid data it holds and can be iterated on
    for (std::size_t i = 0; i < c_arr.size(); ++i) {
        std::cout << c_arr[i] << std::endl;
    }
    // if you need a C-style array from here, you can easily copy the data
    // from the vector to your C-array
    return 0;
}

我研究了使用类的cast()函数，但是不能完全弄清楚语法，使它比上面的方式复制它更少痛苦，因为看起来你必须调用cast函数到不同的Eigen类型，然后从那里再次强制转换，但知道有一个cast函数和其他方法来获得MatrixX类的底层数据，如果你需要访问它。