c++ - 从两个模板参数具有多重继承的可能性？

#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，由于我们同样继承自两个模板参数类，因此我希望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

可以看到，虽然Element1和Mix中的this是相同的，但对于Element2而言并非如此。这是为什么？另外，我希望可以从基类访问e1和e2。你能解释这种行为吗？

最佳答案

元素Mix包含Element1和Element2。这些-也许是专门对齐的实现-依次写在内存中。如果您将Mix用作Element1，则它将指向两者中的第一个(尺寸为Element1)，如果将其用作Element2，它将指向第二个(尺寸为Element2)，如果您将其用作Mix，它将指向基址，该基址与Element1的基址相同，但具有不同的大小(至少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:

＃包括

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

关于c++ - 从两个模板参数具有多重继承的可能性？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/17596171/