Why are fixed Eigen Matrix Library types with embedded data not PoD?

Question

Given the fixed size type of Eigen, say Eigen :: Vector3d, why is this type not PoD? The underlying data is an array of 3 doubles, and there is no need for a nontrivial constructor or destructor.

Answer 1

The template is wise, the lot bit (version dependent) continues in the constructor, although at compile time. Although all this is evaluated at compile time and therefore optimized, an empty constructor remains. If you add an empty constructor to the POD type, it will also not be memcpy'ed when used std::copy. Try the following:

#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;
}

:

    POD 0.135008 .
   NotPOD 0.35202 .
Vector3d 0.35302 .