[c++] Calling virtual functions inside constructors

As a supplement, calling a virtual function of an object that has not yet completed construction will face the same problem.

For example, start a new thread in the constructor of an object, and pass the object to the new thread, if the new thread calling the virtual function of that object before the object completed construction will cause unexpected result.

For example:

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

class Base
{
public:
  Base()
  {
    std::thread worker([this] {
      // This will print "Base" rather than "Sub".
      this->Print();
    });
    worker.detach();
    // Try comment out this code to see different output.
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  virtual void Print()
  {
    std::cout << "Base" << std::endl;
  }
};

class Sub : public Base
{
public:
  void Print() override
  {
    std::cout << "Sub" << std::endl;
  }
};

int main()
{
  Sub sub;
  sub.Print();
  getchar();
  return 0;
}

This will output:

Base
Sub

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