c++ - 从两个模板参数多重继承的奇怪？

Question

#include <iostream>

using namespace std;

template <typename E1, typename E2>
class Mix : public E1, public E2
{
public:
    Mix() : E1(1), E2(2)
    {  
        // Set nothing here
        cerr << "This is " << this << " in Mix" << endl;
        print(cerr);
    }

    void print(ostream& os)
    {
        os << "E1: " << E1::e1 << ", E2: " << E2::e2 << endl;
        // os << "E1: " << e1 << ", E2: " << e2 << endl; won't compile
    }
};

class Element1
{
public:
    Element1(unsigned int e) : e1(e)
    {
        cerr << "This is " << this << " in Element1" << endl;
    }

    unsigned int e1;
};

class Element2
{
public:
    Element2(unsigned int e) : e2(e)
    {
        cerr << "This is " << this << " in Element2" << endl;
    }

    unsigned int e2;
};


int main(int argc, char** argv)
{
   Mix<Element1, Element2> m; 
}

现在，由于我们从两个模板参数类继承，我希望 this 在两个构造函数中相同，但事实并非如此。这是运行日志:

This is 0x7fff6c04aa70 in Element1
This is 0x7fff6c04aa74 in Element2
This is 0x7fff6c04aa70 in Mix
E1: 1, E2: 2

如您所见，虽然 this 在 Element1 和 Mix 中是相同的，但在 Element2 中却不是这样。这是为什么？此外，我希望能够从基类访问 e1 和 e2。你能解释一下这种行为吗？

Answer 1

元素 Mix 包含一个 Element1 和一个 Element2。这些是——也许实现特别对齐——一个接一个地写在内存中。如果您将 Mix 用作 Element1，这将指向两者中的第一个(大小为 Element1)，如果您将它用作 Element2 它将指向第二个(大小为 Element2)，如果您将它用作 Mix 它将指向基地址，即与 Element1 的基址相同，但大小不同(至少为 Element1 的大小 + Element2 的大小)。

编辑:您也可以通过输出大小来验证这一点:

#包含

using namespace std;

template <typename E1, typename E2>
class Mix : public E1, public E2
{
public:
    Mix() : E1(1), E2(2)
    {  
        // Set nothing here
        cerr << "This is " << this << " + " << sizeof(*this) << " in Mix" << endl;
        print(cerr);
    }

    void print(ostream& os)
    {
        os << "E1: " << E1::e1 << ", E2: " << E2::e2 << endl;
        // os << "E1: " << e1 << ", E2: " << e2 << endl; won't compile
    }
};

class Element1
{
public:
    Element1(unsigned int e) : e1(e)
    {
        cerr << "This is " << this << " + " << sizeof(*this) << " in Element1" << endl;
    }

    unsigned int e1;
};

class Element2
{
public:
    Element2(unsigned int e) : e2(e)
    {
        cerr << "This is " << this << " + " << sizeof(*this) << " in Element2" << endl;
    }

    unsigned int e2;
};


int main(int argc, char** argv)
{
   Mix<Element1, Element2> m; 
}

输出:

This is 0x7fffc9cad310 + 4 in Element1
This is 0x7fffc9cad314 + 4 in Element2
This is 0x7fffc9cad310 + 8 in Mix
E1: 1, E2: 2