使用虚拟继承时初始化基类

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问题描述

在 OS X 上使用 Xcode 5.1 编译以下代码时出现意外错误.Apple LLVM 5.1 版 (clang-503.0.40)(基于 LLVM 3.4svn)

I'm getting unexpected errors when compiling the following code using Xcode 5.1 on OS X.Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)

class GrandParent
{
public:
    GrandParent(int age) : m_age(age)
    {
    }

    virtual ~GrandParent() {}

private:
    GrandParent();
    GrandParent(const GrandParent&);
    const GrandParent& operator=(const GrandParent&);

    int m_age;
};

class Parent1 : public virtual GrandParent
{
public:
    Parent1(int age) : m_age(age)
    {
    }

    virtual ~Parent1() {}

private:
    Parent1();
    Parent1(const Parent1&);
    const Parent1& operator=(const Parent1&);

    int m_age;
};

class Parent2 : public virtual GrandParent
{
public:
    Parent2(int age) : m_age(age)
    {
    }

    virtual ~Parent2() {}

private:
    Parent2();
    Parent2(const Parent2&);
    const Parent2& operator=(const Parent2&);

    int m_age;
};

class Child : public Parent1, public Parent2
{
public:
    Child(int grandParentAge, int parent1Age, int parent2Age, int childAge) :
        GrandParent(grandParentAge),
        Parent1(parent1Age),
        Parent2(parent2Age),
        m_age(childAge)
    {
    }

    virtual ~Child() {}

private:
    Child();
    Child(const Child&);
    const Child& operator=(const Child&);

    int m_age;
};

报告的错误是:

error: inherited virtual base class 'GrandParent' has private default constructor
    Parent1(int age) : m_age(age)
    ^
note: declared private here
    GrandParent();
    ^
error: inherited virtual base class 'GrandParent' has private default constructor
    Parent2(int age) : m_age(age)
    ^
note: declared private here
    GrandParent();

我的理解是，从它继承的类(Parent1 或 Parent2)不会调用虚拟基类(GrandParent)的构造函数.相反，构造函数由具体类(Child)的构造函数调用.

My understanding is that the constructor for the virtual base class (GrandParent) is not called by the class that inherits from it (Parent1 or Parent2). Instead, the constructor is called by the constructor of the concrete class (Child).

这是正确的吗?

如果我为 GrandParent 提供默认构造函数，它编译正常.但是，如果我构造一个子对象:

If I supply a default constructor for GrandParent it compiles OK. However, if I construct a child object:

Child child(80, 50, 49, 20);

并检查它我可以看到:

Child) child = {
  Parent1 = {
    GrandParent = (m_age = 49)
    m_age = 50
  }
  Parent2 = {
    GrandParent = (m_age = 80)
    m_age = 49
  }
  m_age = 20

所以使用 Parent1 时 GrandParent 的年龄是不正确的，但对于 Parent2 是正确的.

So the age of GrandParent when using Parent1 is incorrect but for Parent2 it is correct.

我是不是误会了什么?或者错误可能是编译器错误?

Have I misunderstood something? Or could the error be a compiler bug?

更新

如果我将 Parent1 的构造函数更新为:

If I update the ctor of Parent1 (and do the same for Parent2)to:

Parent1(int age) : GrandParent(100), m_age(age){}

现在可以编译了.检查值显示:

it now compiles. Inspecting the values reveals:

(Child) child = {
  Parent1 = {
    GrandParent = (m_age = 49)
    m_age = 50
  }
  Parent2 = {
    GrandParent = (m_age = 80)
    m_age = 49
  }
  m_age = 20

这显然是不对的.此外，修改后的代码在 Windows 上使用 VS 2013 Express 编译，检查结果正确.

This is clearly not right. Furthermore, the modified code compiles on Windows using VS 2013 Express and the values on inspection are correct.

使用虚拟继承时初始化基类

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