c++ - 为什么这不是 POD 类型？

Question

我在 g++ 4.6.2 (mingw) 上用 g++ -std=c++0x pod_test.cpp 运行了下面的代码。我在 A4 上遇到错误。为什么不是 A4 POD？

#include <iostream>
#include <new>
#include <cstring>

using namespace std;

struct A {
    int a, b;
    char c;
};
struct A2 {
    short buf[1];
};
struct A3:A {
};
struct A4:A {
    short buf[1];
};
static_assert(std::is_pod<A>::value, "Struct must be a POD type");
static_assert(std::is_pod<A2>::value, "Struct must be a POD type");
static_assert(std::is_pod<A3>::value, "Struct must be a POD type");
static_assert(std::is_pod<A4>::value, "Struct must be a POD type");

int main(){}

Answer 1

它不是 POD，因为它违反了标准布局类的规则:

— either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members

继承格中只有一个类可以有非静态数据成员。在这种情况下，两者 A 和 A4 有。