c – 出乎意料地能够从基类ctor调用派生类虚函数

前提

我创建了包含成员std :: thread变量的类Thread. Thread的ctor构造成员std :: thread,提供一个指向静态函数的指针,该函数调用纯虚函数(由基类实现).

代码

#include <iostream>
#include <thread>
#include <chrono>

namespace
{

class Thread
{
public:
    Thread()
        : mThread(ThreadStart,this)
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl; // This line commented later in the question.
    }

    virtual ~Thread() { }

    static void ThreadStart(void* pObj)
    {
        ((Thread*)pObj)->Run();
    }

    void join()
    {
        mThread.join();
    }

    virtual void Run() = 0;

protected:
    std::thread mThread;
};

class Verbose
{
public:
    Verbose(int i) { std::cout << __PRETTY_FUNCTION__ << ": " << i << std::endl; }
    ~Verbose() { }
};

class A : public Thread
{
public:
    A(int i)
        : Thread(),mV(i)
    { }

    virtual ~A() { }

    virtual void Run()
    {
        for (unsigned i = 0; i < 5; ++i)
        {
            std::cout << __PRETTY_FUNCTION__ << ": " << i << std::endl;
            std::this_thread::sleep_for(std::chrono::seconds(1));
        }
    }

protected:
    Verbose mV;
};

}

int main(int argc,char* argv[])
{
    A a(42);
    a.join();

    return 0;
}

问题

您可能已经注意到,这里有一个微妙的错误：Thread :: ThreadStart(…)是从Thread ctor上下文调用的,因此调用pure / virtual函数不会调用派生类的实现.运行时错误证实了这一点：

pure virtual method called
terminate called without an active exception
Aborted

但是,如果我在Thread ctor中删除对std :: cout的调用,则会出现意外的运行时行为：

virtual void {anonymous}::A::Run(){anonymous}::Verbose::Verbose(int): : 042

virtual void {anonymous}::A::Run(): 1
virtual void {anonymous}::A::Run(): 2
virtual void {anonymous}::A::Run(): 3
virtual void {anonymous}::A::Run(): 4

即在Thread ctor中删除对std :: cout的调用似乎具有能够从基类`构造函数上下文调用派生类’pure / virtual函数的效果！这与先前的学习和经验不一致.

在Windows 10上的Cygwin x64中构建环境.gcc版本是：

g++ (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation,Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

我对这种观察感到困惑,并且对于正在发生的事情充满了好奇心.谁能摆脱光明？