在构造函数中调用虚函数 [英] Calling virtual functions inside constructors

问题描述

假设我有两个 C++ 类:

Suppose I have two C++ classes:

class A
{
public:
  A() { fn(); }

  virtual void fn() { _n = 1; }
  int getn() { return _n; }

protected:
  int _n;
};

class B : public A
{
public:
  B() : A() {}

  virtual void fn() { _n = 2; }
};

如果我写如下代码:

int main()
{
  B b;
  int n = b.getn();
}

人们可能期望 n 设置为 2.

One might expect that n is set to 2.

结果 n 被设置为 1.为什么?

It turns out that n is set to 1. Why?

推荐答案

从构造函数或析构函数调用虚函数是危险的，应尽可能避免.所有 C++ 实现都应调用在当前构造函数的层次结构级别定义的函数版本，而不是进一步调用.

Calling virtual functions from a constructor or destructor is dangerous and should be avoided whenever possible. All C++ implementations should call the version of the function defined at the level of the hierarchy in the current constructor and no further.

C++ FAQ Lite 部分涵盖了这一点23.7 非常详细.我建议阅读该内容(以及常见问题解答的其余部分)以进行跟进.

The C++ FAQ Lite covers this in section 23.7 in pretty good detail. I suggest reading that (and the rest of the FAQ) for a followup.

摘录:

[...] 在构造函数中，虚拟调用机制被禁用，因为尚未发生从派生类的覆盖.对象是从基础向上构造的，派生前的基础".

[...] In a constructor, the virtual call mechanism is disabled because overriding from derived classes hasn’t yet happened. Objects are constructed from the base up, "base before derived".

[...]

销毁是在基类之前的派生类"完成的，所以虚函数的行为就像在构造函数中一样:只使用本地定义——并且不调用覆盖函数以避免触及(现在被销毁的)派生类部分对象.

Destruction is done "derived class before base class", so virtual functions behave as in constructors: Only the local definitions are used – and no calls are made to overriding functions to avoid touching the (now destroyed) derived class part of the object.

EDIT 大部分更正(感谢 litb)

EDIT Corrected Most to All (thanks litb)

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