调用构造函数的顺序

问题描述

#include <iostream>
using namespace std;
struct A{
  A() {cout << "A" << endl;}
  A(int a) {cout << "A+" << endl;}
};

struct B : virtual A{
  B() : A(1) {cout << "B" << endl;}
};
struct C : virtual A{
  C() : A(1) {cout << "C" << endl;}
};
struct D : virtual A{
  D() : A() {cout << "D" << endl;}
};
struct E : B, virtual C, D{
  E(){cout << "E" << endl;}
};
struct F : D, virtual C{
  F(){cout << "F" << endl;}
};
struct G : E, F{
  G() {cout << "G" << endl;}
};

int main(){
  G g;
  return 0;
}

计划打印：

A
C
B
D
E
D
F
G

我想知道应该使用什么规则来确定构造函数的调用顺序。感谢。

I would like to know what rules should I use to determine in what order constructors get called. Thanks.

推荐答案

虚拟基础子对象首先由最大派生类构造，这是有意义的唯一方式，因为在运行时（因此虚拟）直到对象构造，虚拟基础与最大导出对象的关系是未知的。

Virtual base subobjects are constructed first, by the most-derived class, before any other bases. This is the only way that makes sense, since the relation of the virtual bases to tbe most-derived object is not known until object construction, at runtime (hence "virtual"). All intermediate initializers for virtual bases are ignored.

那么，你的虚拟基础是什么？ G 源自 E 和 F 。 E 从 C 中实际派生，后者又从 A ，因此 A ， C 是第一个。接下来， F 不会添加任何其他虚拟库。接下来， E 有非虚基数 B 和 D ，按顺序构建，然后 E 完成。然后 F 的非虚拟基础 D 和 F 完成。最后， G 已完成。

So, what are your virtual bases? G derives from E and F. E derives virtually from C, which in turn derives virtually from A, so A, C are first. Next, F doesn't add any further virtual bases. Next, E has non-virtual bases B and D, in that order, which are constructed next, and then E is complete. Then comes F's non-virtual base D, and F is complete. Finally, G is complete.

总而言之，它是虚拟基础 code>， C ，则非虚拟基础 B ， code>， E 和 D ，然后 G 本身。

All in all, it's virtual bases A, C, then non-virtual bases B, D, E and D, F, and then G itself.

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