Pure Virtual Function Called: An Explanation

Summary

"Pure virtual function called" is the dying message of the occasional crashed C++ program. What does it mean? You can find a couple of simple, well-documented explanations out there that apply to problems easy to diagnose during postmortem debugging. There is also another rather subtle bug that generates the same message. If you have a mysterious crash associated with that message, it might well mean your program went indirect on a dangling pointer. This article covers all these explanations.

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Build 'em Up, Tear 'em Down

When you construct an instance of a derived class, what happens, exactly? If the class has avtbl, the process goes something like the following.

Step 1: Construct the top-level base part:.

1. Make the instance point to the base class's vtbl.
2. Construct the base class instance member variables.
3. Execute the body of the base class constructor.

Step 2: Construct the derived part(s) (recursively):

1. Make the instance point to the derived class's vtbl.
2. Construct the derived class instance member variables.
3. Execute the body of the derived class constructor.

Destruction happens in reverse order, something like this:

Step 1: Destruct the derived part:

1. (The instance already points to the derived class's vtbl.)
2. Execute the body of the derived class destructor.
3. Destruct the derived class instance member variables.

Step 2: Destruct the base part(s) (recursively):

1. Make the instance point to the base class's vtbl.
2. Execute the body of the base class destructor.
3. Destruct the base class instance member variables.

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附实例：

#include <iostream>class Base{public:virtual ~Base(){std::cout<<"Base::~Base()"<<std::endl;sleep(30);}virtual void do_sth() = 0;};class Derived: public Base{public:virtual ~Derived(){std::cout<<"Derived::~Derived()"<<std::endl;}virtual void do_sth(){std::cout<<"Derived::do_sth()"<<std::endl;}};void* mydo(void *p){sleep(10);Base * _p = (Base*) p;_p->do_sth();}int main(){Base * pb = new Derived();pthread_t thread;pthread_create(&thread,NULL,mydo,pb);delete pb;sleep(10000);return 0;}

原文链接："Pure Virtual Function Called": An Explanation