从构造函数调用虚函数 [英] calling virtual function from constructor

问题描述

您好，我正在阅读Effective C ++，并有项目9：在构建或销毁期间不要调用虚拟函数。我想知道我的代码是否罚款，即使它违反了这条规则：

Hello I'm reading Effective C++ and there is the "Item 9:Never call virtual functions during construction or destruction". And I'm wondering if mine code is fine even if it breaks this rule:

using namespace std;

class A{
public:
  A(bool doLog){
    if(doLog)
      log();
  }

  virtual void log(){
    cout << "logging A\n";
  }
};


class B: public A{
public:
  B(bool doLog) : A(false){
    if(doLog)
      log();
  }

  virtual void log(){
    cout << "logging B\n";
  }
};


int main() {
  A a(true);
  B b(true);
}

这种方法有什么问题吗？

Is there something wrong with this approach? May I get in trouble when I do something more complicated?

编辑：

它接缝了我，大多数答案没有得到我在那里做，他们只是解释了为什么调用虚拟函数从构造函数可能有危险。

It seams to me that most answers didn't get what I did there and they simply explained again why is calling virtual function from constructor potentially dangerous.

我想强调我的程序的输出看起来像这样：

I would like to stress out that output of my program looks like this:

logging A
logging B

所以我得到A记录，当它被构造和B记录。这就是我想要的！但是我问，如果你发现任何错误（潜在的危险）与我的黑客克服在构造函数中调用虚函数的问题。

So I get A logged when it is constructed and B logged when it is constructed. And that is what I want! But I'm asking if You find anything wrong(potentially dangerous) with my "hack" to overcome the problem with calling virtual function in constructor.

推荐答案

我想知道我的代码是否正常，即使它违反了这条规则：

And I'm wondering if mine code is fine even if it breaks this rule:

这取决于你的意思是罚款。你的程序格式良好，它的行为是明确定义的，所以它不会调用未定义的行为和这样的东西。

It depends on what you mean by "fine". Your program is well-formed, and its behavior is well-defined, so it won't invoke undefined behavior and stuff like that.

但是，看到对虚函数的调用，调用是通过调用由覆盖该函数的最派生类型提供的实现来解决的。

However, one may expect, when seeing a call to a virtual function, that the call is resolved by invoking the implementation provided by the most derived type which overrides that function.

除了在构造期间，对应的子对象尚未构造，因此最导出的子对象是当前正在构造的子对象。结果：调用的调度就像函数不是虚拟的一样。

Except that during construction, the corresponding sub-object has not been constructed yet, so the most derived subobject is the one currently being constructed. Result: the call is dispatched as if the function were not virtual.

这是反直觉的，你的程序不应该依赖这种行为。因此，作为一个文化程序员，你应该习惯避免这种模式，并遵循Scott Meyer的指导方针。

This is counter-intuitive, and your program should not rely on this behavior. Therefore, as a literate programmer, you should get used to avoid such a pattern and follow Scott Meyer's guideline.

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