Question

class A {
        int i;
public: 
        A() {cout<<"in A s def const
";};
        A(int k) {cout<<"In A const
";  i = k; }
        };

class B : virtual public A {
public:
        B(){cout<<"in B s def const
";};
        B(int i) : A(i) {cout<<"in B const
";}
        };

class C :   public B {
public:
        C() {cout<<"in C def cstr
";}
        C(int i) : B(i) {cout<<"in C const
";}
        };

int main()
{
        C c(2);
        return 0;
}

The output in this case is

in A s def const
in B const
in C const

Why is this not entering into in A const

`It should follow the order of 1 arg constructor call. But what actually is happening on deriving B from A using virtual keyword.

还有几个问题:

即使我删除上述方案中的虚拟关键词,删除所有违约构造,也会造成错误。因此,它为什么需要造物

Answer 1

The constructors for virtual base classes are always called from the most derived class, using any arguments it might pass in. In your case, the most derived class doesn t specify an initializer for A, so the default constructor is used.

Answer 2

http://stackoverflow.com/a/10534300/241631” 因此,如果你想要<代码>A,则需要打字的构造者需要添加到<代码>上。 C 初始化清单。

C(int i) : A(i), B(i) {cout<<"in C const
";}

For the second part of your question, default constructors are not required, but then the derived class must call the non-default constructor explicitly, since the compiler is unable to do that for you in the absence of a non-default constructor.

#include <iostream>
using namespace std;

class A {
  int i;
public:
  // A() {cout<<"in A s def const
";};
  A(int k) {cout<<"In A const
";  i = k; }
};

class B : virtual public A {
public:
  // B(){cout<<"in B s def const
";};
  B(int i) : A(i) {cout<<"in B const
";}
};

class C :   public B {
public:
  C() : A(42), B(42) {cout<<"in C def cstr
";}
  C(int i) : A(i), B(i) {cout<<"in C const
";}
};

int main()
{
  C c(2), c2;
  return 0;
}

This prints out

In A const
in B const
in C const
In A const
in B const
in C def cstr

Answer 3

这里有两个问题。

为什么不卷入冲突?

因为你正在使用虚拟遗产。

http://www.paragia.com/c++-faq-lite/multiple-inheritance.html#faq-25.11”rel=“nofollow” 顶级主子的初始化名单直接援引虚拟基级主子。在此情况下,这意味着<代码> C s Constructionor calls A s Constructionor directly。由于您没有具体规定<代码>A,在<代码>上打字 C 初始化清单,即违约构造。

具体做法是将<代码>C:C(int)的实施改为:

C(int i) : A(i), B(i) {cout<<"in C const
";}

If I remove the virtual keyword in above program and remove all the default constructor it gives error. So, why it needs the def constructor?

编码。此外,还注明了<代码>A的电梯,因此使用了违约构件。请删除<代码>。 A s def ctor,B, t可加以汇编。

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