C++ compilation error: Virtual base classes are defined in different symbol tables in the same hierarchy. How to modify it?

C Compilation error: In the same hierarchy, virtual base classes are defined in different symbol tables. How to modify it?

When using C programming, we may encounter such an error: virtual base classes are defined in different symbol tables in the same hierarchy. So, how to solve this error?

First, let us understand what a virtual base class is. In C, a virtual base class refers to an inherited base class, which is only inherited once and not repeated in the inheritance relationship. It is mainly used to solve problems caused by multiple inheritance, such as diamond inheritance, etc.

Next, let’s look at a piece of code:

class A{
public:
    A(int a):m_a(a){}
    int m_a;
};

class B: virtual public A{
public:
    B(int b):A(b),m_b(b){}
    int m_b;
};

class C: virtual public A{
public:
    C(int c):A(c),m_c(c){}
    int m_c;
};

class D: public B,public C{
public:
    D(int b,int c,int d):A(0),B(b),C(c),m_d(d){}
    int m_d;
};

In the above code, we defined 4 classes, among which class B and class C both inherit object A through virtual inheritance. . Finally, class D inherits classes B and C. In this inheritance relationship, we can find that A is only inherited once, and it is a virtual inheritance. This avoids problems caused by diamond inheritance and also ensures that only one A object is created.

However, when we try to compile the above code, we may encounter the following error:

error: 'A' is an ambiguous base of 'D'

This error message tells us that in the same hierarchy, it is defined in different symbol tables The virtual base class A.

So, to solve this problem, we can take the following two ways:

1. By using the scope interpreter to specify the base class: we can in class B and class C Use the scope interpreter A:: to specify the scope of base class A respectively, avoiding the problem of defining virtual base class A in different symbol tables. The modified code is as follows:

class B: virtual public A{
public:
    B(int b):A(b),m_b(b){}
    int m_b;
};

class C: virtual public A{
public:
    C(int c):A(c),m_c(c){}
    int m_c;
};

class D: public B,public C{
public:
    D(int b,int c,int d):A(0),B(b),C(c),m_d(d){}
    int m_d;
};

2. By using virtual inheritance to specify the base class: We can change the inheritance method in class B and class C to virtual inheritance, which can avoid Problem defining virtual base class A in different symbol tables. The modified code is as follows:

class B: virtual public A{
public:
    B(int b):A(b),m_b(b){}
    int m_b;
};

class C: virtual public A{
public:
    C(int c):A(c),m_c(c){}
    int m_c;
};

class D: virtual public B,virtual public C{
public:
    D(int b,int c,int d):A(0),B(b),C(c),m_d(d){}
    int m_d;
};

Whether you use the scope interpreter or virtual inheritance, you can avoid the problem of defining virtual base classes in different symbol tables in the same hierarchy. , thereby solving compilation errors.

In short, virtual inheritance is very commonly used in C programming, especially when dealing with multiple inheritance, it can avoid some problems. But at the same time, we need to pay attention to some problems that may be caused by virtual inheritance. In actual use, we need to use it flexibly and choose the appropriate inheritance method according to our needs.

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