Polymorphism is a powerful concept in object-oriented programming that allows objects of different classes to be treated as objects of a common base class. However, when it comes to serializing and deserializing polymorphic objects in C++, some extra care needs to be taken. In this blog post, we will explore how to properly handle the serialization and deserialization of polymorphic objects in C++.

Serializing Polymorphic Objects

To serialize a polymorphic object, we need to ensure that all the necessary information about its derived classes is properly preserved. One common approach is to use the concept of “type information” to store the actual derived class type along with the object data.

To do this, we can define a base class with virtual functions for serialization and deserialization. Let’s consider the following example:

class Base {
public:
    virtual void serialize(std::ostream& os) const = 0;
    static std::unique_ptr<Base> deserialize(std::istream& is);
};

class Derived1 : public Base {
public:
    void serialize(std::ostream& os) const override {
        os << "Derived1: " << data << std::endl;
    }
    
    static std::unique_ptr<Base> deserialize(std::istream& is) {
        std::string line;
        std::getline(is, line);
        // Parse data from the line
        
        return std::make_unique<Derived1>(data);
    }
    
private:
    int data;
};

class Derived2 : public Base {
public:
    void serialize(std::ostream& os) const override {
        os << "Derived2: " << data << std::endl;
    }
    
    static std::unique_ptr<Base> deserialize(std::istream& is) {
        std::string line;
        std::getline(is, line);
        // Parse data from the line
        
        return std::make_unique<Derived2>(data);
    }
    
private:
    std::string data;
};

In the Base class, we declare a pure virtual serialize function that would be implemented by derived classes to serialize their data. Similarly, we define a deserialize function that returns a std::unique_ptr<Base> to handle the dynamic polymorphism during deserialization.

The serialize function of each derived class appends the type and data to the output stream os. The deserialize functions read the data from the input stream is, parse it to retrieve the type and data, and then construct the appropriate derived class object using std::make_unique and return it.

Deserializing Polymorphic Objects

Deserializing polymorphic objects involves reading the serialized data, determining the type of the object, and then reconstructing the appropriate derived class object.

Here’s an example of how you can deserialize a polymorphic object:

std::unique_ptr<Base> obj = Base::deserialize(input_stream);
if (obj) {
    obj->someBaseClassFunction();
    // ...
}

The deserialize function of the Base class reads the serialized data from the input stream and uses it to select the appropriate derived class to construct and return.

Conclusion

Serializing and deserializing polymorphic objects in C++ can be challenging, but by properly preserving type information and using virtual functions, we can overcome these challenges. By implementing the serialize and deserialize functions in the base and derived classes, we can ensure that the object data is correctly serialized and deserialized while preserving the polymorphic behavior.

#C++ #Polymorphism