How to Store Polymorphic Objects in C++ STL Containers?

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Introduction


Can we store polymorphic objects by value in C++ STL containers? This topic often arises among C++ developers, particularly with the challenges posed by object slicing when dealing with base and derived classes. The crux of the issue lies in the behavior of STL containers and how they handle objects.

In C++, when we store derived class instances in a container that holds base class objects by value, object slicing occurs, which results in the loss of the derived class-specific behaviour. This article aims to explore this concept specifically within the context of C++11 and later revisions, dissecting the challenges and potential approaches.

Understanding the Slicing Problem


When you store a derived class object in a container designed to hold objects of its base class by value, only the base class part of the object is retained. This phenomenon is known as object slicing. It typically occurs because the container copies the objects, and the copy operation only retains the base class attributes and behaviours.

Here’s an example to illustrate the problem:

Sample Code: Polymorphic Containers with Value Semantics


#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() override { cout << "Child: " << parent_mem << ", " << child_mem << endl; }
int child_mem;
};

int main() {
// Using pointer semantics, we can hold polymorphic objects
vector<Parent*> pointerVec;
pointerVec.push_back(new Parent());
pointerVec.push_back(new Child());

pointerVec[0]->write(); // Outputs: Parent: 1
pointerVec[1]->write(); // Outputs: Child: 2, 2

// But using value semantics leads to slicing
vector<Parent> valueVec;
valueVec.push_back(Parent());
valueVec.push_back(Child()); // Slicing occurs

valueVec[0].write(); // Outputs: Parent: 1
valueVec[1].write(); // Outputs: Parent: 2
}

Explanation of the Example

1. 
   Pointer Semantics: The first vector, pointerVec, demonstrates that polymorphic behavior is preserved when using pointers. We can add both Parent and Child objects to the vector. When we call write(), each object’s respective implementation is invoked.
   
   
2. 
   Value Semantics: The second vector, valueVec, attempts to store objects by value. Here, the Child object is sliced down to a Parent object when added to valueVec. This results in a loss of the derived class's functionality, evident in the output. The write method of Child is never called, as the object is treated purely as a Parent.
   

Using Smart Pointers for Flexible Memory Management


While creating objects on the stack is preferable for stack-based management, if you're okay storing pointers, consider using smart pointers, such as std::unique_ptr or std::shared_ptr. This circumvents the slicing problem entirely, as you can store derived class pointers that maintain their polymorphic behavior while ensuring proper memory management.

For example:

#include <iostream>
#include <vector>
#include <memory>

using namespace std;

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

class Child : public Parent {
public:
void write() override { cout << "Child" << endl; }
};

int main() {
vector<unique_ptr<Parent>> vec;
vec.push_back(make_unique<Parent>());
vec.push_back(make_unique<Child>());

for (const auto& p : vec) {
p->write(); // Both Parent and Child output will be called correctly
}
}

Why Use Smart Pointers?

• Automatic Memory Management: They automatically handle memory deallocation, preventing leaks.
• Polymorphic Behavior: They allow you to maintain polymorphic behavior while working with base class pointers.

Conclusion


In conclusion, storing polymorphic objects by value in STL containers in C++ is not feasible without running into slicing issues. However, by leveraging pointer semantics, particularly smart pointers, we can effectively manage memory while still achieving the desired polymorphic behavior. Using std::unique_ptr or std::shared_ptr offers a robust solution to enhance your C++ coding practices.

Frequently Asked Questions

Can I use raw pointers instead of smart pointers?


Using raw pointers is possible, but it's risky as it requires you to manually manage memory, increasing the chances of memory leaks and undefined behavior. Smart pointers automate this process and are recommended in modern C++.

Are there any specific STL containers better suited for polymorphism?


While any STL container can technically hold pointers to polymorphic objects, containers like std::vector or std::list are commonly used due to their flexibility with dynamic sizes and efficient memory management.

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