C++ runtime error with shared_ptr and builder pattern - c++

Question

I was studying c++ language with shared pointer and builder pattern.
I have written following code that is not working but I don't understand why it emits run-time error.
Could you tell me why it is not working well and how can I solve this problem to work well?
#include <iostream>
#include <memory>
#include <string>
using namespace std;
class Popup
{
public:
Popup(int value, string str){
this->v = value;
this->str = str;
}
virtual void print() = 0;
int v;
string str;
};
typedef shared_ptr<Popup> PopupPtr;
class PopupA : public Popup
{
public:
PopupA(int v, string str) : Popup(v, str) { }
virtual void print() {
cout << "PopupA" << endl;
}
};
typedef shared_ptr<PopupA> PopupAPtr;
class PopupB : public Popup
{
public:
PopupB(int v, string str) : Popup(v, str) { }
virtual void print() {
cout << "PopupB" << endl;
}
};
typedef shared_ptr<PopupB> PopupBPtr;
class Builder
{
public:
PopupPtr popupPtr;
Builder() { };
shared_ptr<Builder> init(int value, string str) {
shared_ptr<Builder> builder;
switch (value)
{
case 1:
popupPtr = PopupAPtr(new PopupA(value, str));
break;
case 2:
popupPtr = PopupBPtr(new PopupB(value, str));
break;
default:
cout << "default error" << endl;
break;
}
if (popupPtr) {
builder = shared_ptr<Builder>(this);
}
else {
cout << "popup is null" << endl;
}
if (!builder) {
cout << "builder is null" << endl;
}
return builder;
}
PopupPtr build()
{
if (!popupPtr) {
cout << "popup is null" << endl;
}
return PopupPtr(popupPtr);
}
};
typedef shared_ptr<Builder> BuilderPtr;
int main()
{
BuilderPtr builderPtr = BuilderPtr(new Builder());
PopupPtr popupPtr1 = builderPtr->init(1, "111111111111")->build();
popupPtr1->print();
PopupPtr popupPtr2 = builderPtr->init(2, "222222222222")->build();
popupPtr2->print();
return 0;
}
Thanks in advance for your answers and sorry for my poor english. If you don't understand my question please make a comment.

Answer 1

Your problem is this line:
builder = shared_ptr<Builder>(this);
This will not create a copy of the std::shared_ptr already tracking this, nor will it affect the reference count of it. This creates an entirely new shared pointer which will track this independently, causing a double-delete when both of the reference counts hit zero.
Fortunately, the standard library provides a solution to this problem in the form of std::shared_from_this.
First you need to enable this for your class:
class Builder : std::enable_shared_from_this<Builder>
{
//...
};
Then instead of creating a new std::shared_ptr from this, call std::shared_from_this:
builder = std::shared_from_this();