#include "iostream.h" class CBase { public: CBase(int a) :a(a) { } int a; }; class CDerive1 : public CBase { public: CDerive1(int a) :CBase(a) { } }; class CDerive2 : public CBase { public: CDerive2(int a) :CBase(a) { } }; class CDerive : public CDerive1,public CDerive2 { public: CDerive(int a,int b) :CDerive1(a),CDerive2(b) { } }; void main() { CDerive d(1,2); cout<<d.a<<endl; } 问题一：在不改变原有程序意图的前提下，分别用三种方法改正以上程序，并使程序正确输出。

该程序存在二义性，因为 CDerive 类同时继承了 CDerive1 和 CDerive2，而这两个类都继承自 CBase，因此在 CDerive 中会有两份 CBase 类的成员变量 a，这样在访问 d.a 时就会存在二义性，导致编译错误。

为了解决这个问题，需要通过虚继承来消除二义性。具体实现方法如下：

方法一：使用虚继承

#include <iostream.h>

class CBase {
public:
    CBase(int a) :a(a) { }
    int a;
};

class CDerive1 : virtual public CBase {
public:
    CDerive1(int a) :CBase(a) { }
};

class CDerive2 : virtual public CBase {
public:
    CDerive2(int a) :CBase(a) { }
};

class CDerive : public CDerive1, public CDerive2 {
public:
    CDerive(int a, int b) :CBase(a), CDerive1(a), CDerive2(b) { }
};

void main() {
    CDerive d(1, 2);
    cout << d.a << endl;
}

方法二：将 CDerive1 和 CDerive2 中的 CBase 改为 public 继承

#include <iostream.h>

class CBase {
public:
    CBase(int a) :a(a) { }
    int a;
};

class CDerive1 : public CBase {
public:
    CDerive1(int a) :CBase(a) { }
};

class CDerive2 : public CBase {
public:
    CDerive2(int a) :CBase(a) { }
};

class CDerive : public CDerive1, public CDerive2 {
public:
    CDerive(int a, int b) :CBase(a), CDerive1(a), CDerive2(b) { }
};

void main() {
    CDerive d(1, 2);
    cout << d.CDerive1::a << endl;
}

方法三：在 CDerive 类中定义一个新的成员变量 a，并在构造函数中将 CDerive1 和 CDerive2 中的 a 赋值给这个新变量。

#include <iostream.h>

class CBase {
public:
    CBase(int a) :a(a) { }
    int a;
};

class CDerive1 : public CBase {
public:
    CDerive1(int a) :CBase(a) { }
};

class CDerive2 : public CBase {
public:
    CDerive2(int a) :CBase(a) { }
};

class CDerive : public CDerive1, public CDerive2 {
public:
    CDerive(int a, int b) :CDerive1(a), CDerive2(b), a(a) { }
    int a;
};

void main() {
    CDerive d(1, 2);
    cout << d.a << endl;
}

无论使用哪种方法，都可以消除二义性，使程序正确输出。