C++ Stl

Can I Have Polymorphic Containers with Value Semantics in C++?

In the world of C++, when dealing with object-oriented programming, you may find yourself needing to store different types of objects that share a common base class. This leads to the intriguing question: Can I have value semantics while benefiting from polymorphic containers?

Understanding the Problem

The Preference for Value Semantics

As many developers, including myself, prefer to use value semantics over pointer semantics, this preference becomes evident when designing efficient and easy-to-manage data structures. For instance, using vector<Class> is often more straightforward than using vector<Class*>, as it alleviates concerns about memory management—specifically, it avoids the need to manually delete dynamically allocated objects.

The Challenge with Polymorphism

However, value collections encounter a significant hurdle when you’re trying to store derived objects that extend a common base class. This situation is typically plagued by slicing, where only the base part of the derived objects is copied into the container. This results in a loss of polymorphic behavior.

Consider this example:

#include <iostream>
#include <vector>

using namespace std;

class Parent {
public:
    Parent() : parent_mem(1) {}
    virtual void write() { cout << "Parent: " << parent_mem << endl; }
    int parent_mem;
};

class Child : public Parent {
public:
    Child() : child_mem(2) { parent_mem = 2; }
    void write() { cout << "Child: " << parent_mem << ", " << child_mem << endl; }
    int child_mem;
};

The Experiment

When attempting to store objects in a vector with value semantics:

vector<Parent> valueVec;
valueVec.push_back(Parent());
valueVec.push_back(Child()); // This gets sliced into a Parent object.

You may expect both objects to retain their identity, but the output will only reflect the base class:

Parent: 1
Parent: 2

The Solution: Using Smart Pointers

One viable solution to this problem is to shift from plain objects to smart pointers, specifically std::shared_ptr or boost::shared_ptr.

Why Smart Pointers?

Smart pointers come with built-in memory management:

  • They automatically manage memory and deallocate resources when they are no longer needed.
  • shared_ptr employs reference counting, ensuring the object stays alive as long as there are references to it.

Implementing the Solution

Here’s how you can correctly implement the solution using shared_ptr:

#include <memory>
#include <vector>

vector<shared_ptr<Parent>> vec;
vec.push_back(shared_ptr<Parent>(new Child()));

Summary

Using polymorphic containers while adhering to value semantics can be challenging due to the slicing issue inherent in C++. However, by adopting smart pointers such as shared_ptr, you can enjoy the benefits of both safety and polymorphism without the direct complications of manual memory management.

Key Takeaways:

  • Avoid plain object containers when dealing with polymorphic objects to prevent slicing.
  • Use smart pointers to manage memory and retain polymorphic behavior.
  • Choosing the right storage type (like shared_ptr) can simplify your code and keep your program safe from memory leaks.

By applying these concepts, you can successfully navigate the complexities of C++ object-oriented programming while maintaining clean and efficient code practices.