c++ - 为什么 Eigen Matrix Library 的具有整数数据的固定大小类型不是 PoD？

Question

给定一个固定大小的 Eigen 类型，比如说 Eigen::Vector3d，为什么这个类型不是 PoD？底层数据是一个包含 3 个 double 的数组，不需要非平凡的构造函数或析构函数。

Answer 1

在模板方面，很多位(取决于版本)在构造函数中进行，尽管是在编译时。虽然所有这些都在编译时进行评估并因此进行了优化，但仍然存在一个空的构造函数。如果您向 POD 类型添加一个空的构造函数，那么在使用 std::copy 时它也不会被 memcpy。试试这个:

#include <chrono>
#include <Eigen/Core>
#include <vector>
#include <iostream>

struct notpod
{
    notpod() {}
    double d[3];
};

struct pod
{
    double d[3];
};

using Eigen::Vector3d;
int main(int argc, char** argv)
{
    std::chrono::time_point<std::chrono::high_resolution_clock > start, end;
    int sz = 20000000;

    {
        std::vector<pod> a(sz), b(sz);
        start = std::chrono::high_resolution_clock ::now();
        std::copy(a.begin(), a.end(), b.begin());
        end = std::chrono::high_resolution_clock ::now();
        std::cout << "     POD vector copy took " << (std::chrono::duration<double>(end - start)).count() << " seconds.\n";
    }

    {
        std::vector<notpod> na(sz), nb(sz);
        start = std::chrono::high_resolution_clock ::now();
        std::copy(na.begin(), na.end(), nb.begin());
        end = std::chrono::high_resolution_clock ::now();
        std::cout << "  NotPOD vector copy took " << (std::chrono::duration<double>(end - start)).count() << " seconds.\n";
    }

    {
        std::vector<Vector3d> a3(sz), b3(sz);
        start = std::chrono::high_resolution_clock ::now();
        std::copy(a3.begin(), a3.end(), b3.begin());
        end = std::chrono::high_resolution_clock ::now();
        std::cout << "Vector3d vector copy took " << (std::chrono::duration<double>(end - start)).count() << " seconds.\n";
    }


    return 0;
}

我机器上的输出:

     POD vector copy took 0.135008 seconds.
NotPOD vector copy took 0.35202 seconds.
Vector3d vector copy took 0.35302 seconds.