类型转换二义性

class A { };
class B { };
class C : public A, public B { };

void print(A & a) {}
void print(B & a) {}

int main() {
    C c;
    print(c);       // 二义性
    return system("pause");
}

基类的构造顺序与派生列表中基类的顺序一致

代码

class A {
public:
    A() { std::cout << "A()" << std::endl; }
};

class B {
public:
    B() { std::cout << "B()" << std::endl; }
};

class C : public A, public B {
public:
    C() = default;
};

class D : public B, public A {
public:
    D() = default;
};

int main() {
    C c;
    std::cout << "--------------" << std::endl;
    D d;
    return system("pause");
}

输出结果

A()
B()
--------------
B()
A()

继承基类构造函数

class A {
public:
    A() { std::cout << "A()" << std::endl; }
    A(int a) { std::cout << "A(int a)" << std::endl; }
    A(char a) { std::cout << "A(char a)" << std::endl; }
};

class B {
public:
    B() { std::cout << "B()" << std::endl; }
    B(double a) { std::cout << "B(double a)" << std::endl; }
    B(char a) { std::cout << "B(char a)" << std::endl; }
};


class C : public A, public B {
public:
    using A::A;
    using B::B;

    C() { std::cout << "C()" << std::endl; }
    C(char a) { std::cout << "C(char a)" << std::endl; }
};

// 多重继承中继承构造函数时，派生类的其他成员执行默认初始化
int main() {
    std::cout << "--------------" << std::endl;
    C a(0);
    std::cout << "--------------" << std::endl;
    C b(0.0);
    std::cout << "--------------" << std::endl;
    C c('0');
    std::cout << "--------------" << std::endl;
    return system("pause");
}

名字查找二义性问题

注意：编译器并不检查可取用性，即便实际上只有一个可取用

class A {
public:
    void doSomething1() {}
    void doSomething2() {}
    void doSomething3() {}
    int a;
};

class B {
public:
    void doSomething1() {}
    void doSomething2(int) {}

private:
    void doSomething3() {}
    int a;
};

class C : public A, public B {
};

int main() {
    C c;
    c.doSomething1();      // 错误
    c.doSomething2();      // 错误
    c.doSomething3();      // 错误
    c.a;
    return system("pause");
}