了解虚拟基类和构造函数调用

问题描述

对于虚拟基类如何工作有点困惑。特别是，我想知道如何基类的构造函数被调用。我写了一个例子来理解它：

I'm a bit confused about how virtual base classes work. In particular, I was wondering how the constructor of the base class gets called. I wrote an example to understand it:

#include <cstdio>
#include <string>
using std::string;

struct A{
    string s;
    A() {}
    A(string t): s(t) {}
};

struct B: virtual public A{
    B(): A("B"){}
};

struct C: virtual public A {};

struct D: public B, public C {};

struct E: public C, public B {};

struct F: public B {};

int main(){
    D d;
    printf("\"%s\"\n",d.s.c_str());
    E e;
    printf("\"%s\"\n",e.s.c_str());
    F f;
    printf("\"%s\"\n",f.s.c_str());
    B b;
    printf("\"%s\"\n",b.s.c_str());
}

哪些输出

""
""
""
"B"

我不知道在前两种情况会发生什么，但对于第三个，至少我期望输出是B。所以现在我只是困惑。

I wasn't sure what would happen in the first two cases, but for the third one at least I was expecting the output to be "B". So now I'm just confused. What are the rules for understanding how the constructor of A gets called?

推荐答案

总是只有一个构造函数调用，并且总是实例化的具体类。如果你在 B '中，你需要赋予每个派生类一个构造函数来调用基类的构造函数，这是

There is always just one constructor call, and always of the actual, concrete class that you instantiate. It is your responsibility to endow each derived class with a constructor which calls the base classes' constructors if and as necessary, as you did in B's constructor.

更新：很抱歉，缺少您的主要观点！感谢ildjarn。

Update: Sorry for missing your main point! Thanks to ildjarn.

但是， B 几乎从 A 。根据标准（FIDS中的10.1.4），对于每个指定为virtual的不同基本类，最大派生对象应包含该类型的单个基本类子对象。在你的情况下，这意味着当构建基础时，你的类 F 立即调用 A 的默认构造函数， code> B 的。

However, your B inherits virtually from A. According to the standard (10.1.4 in the FIDS), "for each distinct baseclass that is specified virtual, the most derived object shall contain a single base class subobject of that type". In your case this means that when constructing the base, your class F immediately calls A's default constructor, not B's.

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