C++菱形继承问题：子类如何分别继承不同父类的属性值？

问题描述 #

在菱形继承场景下：

• 基类A的带参构造函数初始化_hp=10、_ep=10、_ad=0
• 类B、C均虚继承A，B构造函数将_hp设为100、_ep设为50、_ad设为20；C构造函数将_hp设为100、_ep设为100、_ad设为30
• 类D同时继承B和C，期望D继承B的_ep(50)、C的_hp(100)和_ad(30)，但实际输出中_ep始终为100（来自C），结果受D的继承顺序影响，需要实现自由访问任意父类属性值的效果。

最小可复现代码 #

#include <iostream>
#include <string>

class A
{
public :
    A(){
        return ;
    };
    A(std::string name) : _hp(10), _ep(10), _ad(0){
        std::cout << "A object created !" << std::endl;
        return ;
    };
    ~A(){
        std::cout << "A object " << this->_name << " destroyed." << std::endl;
        return ;
    };

protected:
    std::string _name;
    int         _hp;
    int         _ep;
    int         _ad;
};

class B : virtual public A
{
public :
    B(){
        return ;
    };
    B(std::string name) : A(name){
        std::cout << "B object created !" << std::endl;
        this->_hp = 100;
        this->_ep = 50;
        this->_ad = 20;
        return ;
    };
    ~B(){
        std::cout << "B object " << this->_name << " destroyed." << std::endl;
        return ;
    };
};

class C : virtual public A
{
public :
    C(){
        return ;
    };
    C(std::string name) : A(name){
        std::cout << "C object created !" << std::endl;
        this->_hp = 100;
        this->_ep = 100;
        this->_ad = 30;
        return ;
    };
    ~C(){
         std::cout << "C object " << this->_name << " destroyed." << std::endl;
         return ;
    };
};

class D : public B, public C
{
public :
    D(){
        return ;
    };
    D(std::string name) : A(name + "_comes_from_a"){
        this->_name = name;
        std::cout << "D object created !" << std::endl;
        this->_hp = C::_hp;
        this->_ep = B::_ep;
        this->_ad = C::_ad;
        std::cout << "HIT POINTS " << this->_hp << std::endl;
        std::cout << "ENERGY POINTS " << this->_ep << std::endl;
        std::cout << "ATTACK DAMAGE " << this->_ad << std::endl;
        return;
    }
    ~D(){
        std::cout << "D object " << this->_name << " destroyed." << std::endl;
        return ;
    };
};

int main(void)
{
     D  obj_D("TEST");
     return (0);
}

问题根源 #

虚继承机制下，基类A的实例在D对象中仅存在一份。构造顺序为A → B → C → D：B构造时修改了A的_ep为50，但后续C构造时又将_ep覆盖为100。因此D中尝试访问B::_ep时，实际指向的是被C修改后的共享A成员，而非B最初设置的50。

解决方案 #

方案1：为B、C新增独立成员存储自身属性值 #

让B和C各自保存自己的目标属性值，而非直接修改共享的基类成员，这样D可以直接访问这些独立变量，不受构造顺序影响：

修改B和C的定义：

class B : virtual public A
{
public:
    B() {};
    B(std::string name) : A(name), _b_hp(100), _b_ep(50), _b_ad(20) {
        std::cout << "B object created !" << std::endl;
    };
    ~B(){
        std::cout << "B object " << this->_name << " destroyed." << std::endl;
    };
protected:
    int _b_hp;
    int _b_ep;
    int _b_ad;
};

class C : virtual public A
{
public:
    C() {};
    C(std::string name) : A(name), _c_hp(100), _c_ep(100), _c_ad(30) {
        std::cout << "C object created !" << std::endl;
    };
    ~C(){
        std::cout << "C object " << this->_name << " destroyed." << std::endl;
    };
protected:
    int _c_hp;
    int _c_ep;
    int _c_ad;
};

调整D的构造函数：

D(std::string name) : A(name + "_comes_from_a"), B(name), C(name) {
    this->_name = name;
    std::cout << "D object created !" << std::endl;
    this->_hp = C::_c_hp;
    this->_ep = B::_b_ep;
    this->_ad = C::_c_ad;
    std::cout << "HIT POINTS " << this->_hp << std::endl;
    std::cout << "ENERGY POINTS " << this->_ep << std::endl;
    std::cout << "ATTACK DAMAGE " << this->_ad << std::endl;
}

方案2：在D中直接显式设置目标值 #

如果明确知道每个属性的期望数值，可跳过父类的属性覆盖逻辑，在D构造函数中直接设置最终值：

D(std::string name) : A(name + "_comes_from_a"), B(name), C(name) {
    this->_name = name;
    std::cout << "D object created !" << std::endl;
    // 直接设置目标属性值
    this->_hp = 100;
    this->_ep = 50;
    this->_ad = 30;
    std::cout << "HIT POINTS " << this->_hp << std::endl;
    std::cout << "ENERGY POINTS " << this->_ep << std::endl;
    std::cout << "ATTACK DAMAGE " << this->_ad << std::endl;
}

方案3：在D构造函数中覆盖父类的修改结果 #

利用构造顺序的特性，在D构造阶段重新设置被C覆盖的_ep值，确保最终结果符合预期：

D(std::string name) : A(name + "_comes_from_a"), B(name), C(name) {
    this->_name = name;
    std::cout << "D object created !" << std::endl;
    // 覆盖C设置的_ep值
    this->_ep = 50;
    this->_hp = 100;
    this->_ad = 30;
    std::cout << "HIT POINTS " << this->_hp << std::endl;
    std::cout << "ENERGY POINTS " << this->_ep << std::endl;
    std::cout << "ATTACK DAMAGE " << this->_ad << std::endl;
}

内容的提问来源于stack exchange，提问作者Mitsun0bu