多重继承歧义基class

Question

考虑代码

struct Base{};
struct Derived: public Base{};

struct A: public Base{};

struct B: public A, public Base{};

struct C: public A, public Derived{}; // why no ambiguity here?

int main() {}

编译器 (g++5.1) 警告说

warning: direct base 'Base' inaccessible in 'B' due to ambiguity struct B: public A, public Base{};

我明白了，Base 在 B 中重复了。

为什么C没有警告？ C 不是同时继承自 A 和 Derived，它们都继承自 Base 吗？
为什么要加virtual
```
struct Derived: virtual Base{};
```

结果现在 B 和 C 发出警告，继续 Wandbox

warning: direct base 'Base' inaccessible in 'B' due to ambiguity struct B: public A, public Base{};

warning: direct base 'Base' inaccessible in 'C' due to ambiguity struct C: public A, public Derived{};

Answer 1

在B中，不可能引用直接继承的Base子对象的成员。考虑：

struct Base {
    int x;
};

struct B: public A, public Base {
    void foo() {
        int& x1 = A::x; // OK
        int& x2 = x; // ambiguous
        // no way to refer to the x in the direct base
    }
};

在 C 这不是问题。两个 x 都可以使用限定名称来引用：

struct C: public A, public Derived {
    void foo() {
        int& x1 = A::x; // OK
        int& x2 = Derived::x; // OK
    }
};

因此，您收到的警告只有在直接基数也通过另一条路径继承时才有意义。

关于你的第二个问题，我无法在 Coliru 上用 g++-5.1 重现 C 的警告。

Answer 2

在 "B" 中无法明确访问 Base 成员，而在 "C" 中是可能的，如以下代码所示：

#include <iostream>

using namespace std;

struct Base
{
    void print()
    {
        cout << "Base" << endl;
    }
};

struct Derived : public Base {};

struct A : public Base
{
    void print()
    {
        cout << "A" << endl;
    }
};

struct B : public A, public Base
{
    void print()
    {
        A::print();

        //error (ambiguous), no way to access to Base::print => warning
        //Base::print();
    }
};

struct C : public A, public Derived
{
    void print()
    {
        A::print();
        Derived::print(); // Not Ambiguous, it's the Base inherited by 'Derived' which is used.
        // Still an error but you can access print indirectly through "Derived" => no warning needed
        //Base::print();
    }
};

int main() 
{
    B b;
    b.print();

    C c;
    c.print();

    return 0; 
}

多重继承歧义基class

Multiple inheritance ambiguous base class

c++

inheritance

multiple-inheritance