How do I cast a parent class as the child class

Question

It's been a while since I have had to write C++ code and I'm feeling kind of stupid. I've written code that is similar to, but is not exactly, the code below:

class Parent {     ... };  class Child : public Parent {     ... };  class Factory {     static Parent GetThing() { Child c; return c; } };  int main() {     Parent p = Factory::GetThing();     Child c1 = p; // Fails with "Cannot convert 'Parent' to 'Child'"     Child c2 = (Child)p; // Fails with "Could not find a match for 'TCardReadMessage::TCardReadMessage(TCageMessage)'" } 

I know this is supposed to be simple but I'm not sure what I'm doing wrong.

Answer 1

A Parent object returned by value cannot possibly contain any Child information. You have to work with pointers, preferably smart pointers, so you don't have to clean up after yourself:

#include <memory>  class Factory {     // ...  public:      static std::unique_ptr<Parent> GetThing()     {         return std::make_unique<Child>();     } };  int main() {     std::unique_ptr<Parent> p = Factory::GetThing();     if (Child* c = dynamic_cast<Child*>(p.get()))     {         // do Child specific stuff     } } 

Answer 2

Refer to the code snippet below:

Child* c = dynamic_cast<Child*>(parentObject); 

where, parentObject is of type Parent*

Ensure, that the "parentObject" is actually of "Child" type, otherwise undefined-behavior.

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