Why Use Pointers to Objects in C++?

The primary reasons to use pointers to objects instead of objects themselves in C++ include polymorphism support, memory management flexibility, performance optimization, and avoiding object slicing. While efficiency and direct memory access are factors, the most critical reasons relate to C++'s manual memory management system and object-oriented programming paradigms that differ significantly from Java’s approach.

Contents

• Memory Management Differences in C++ vs Java
• Polymorphism and Object Slicing
• Performance and Memory Efficiency
• Dynamic Allocation and Object Lifetime
• Practical Examples and Best Practices
• Modern C++ Solutions

Memory Management Differences in C++ vs Java

In C++, you have explicit control over where objects are stored, which fundamentally differs from Java’s approach. When you declare Object myObject;, the object is created on the stack and automatically destroyed when it goes out of scope. This is similar to local variables in Java, but with one crucial difference: C++ objects can also be created on the heap using new.

According to ICS 45C Spring 2022 documentation, “When the function ends, its local variables and parameters (i, n, and p) — which are stored on the run-time stack — are destroyed automatically. But the dynamically-allocated integers remain on the heap.”

The key distinction is that:

• Stack objects: Automatically managed, fast allocation/deallocation, limited size
• Heap objects: Require manual deletion (or smart pointers), slower allocation, larger capacity, persistent lifetime

This means that as a Java developer, you’re accustomed to all objects being heap-allocated with automatic garbage collection, whereas C++ gives you the choice between stack and heap allocation with different performance characteristics.

Polymorphism and Object Slicing

The most important reason to use pointers for objects in C++ is polymorphism. When working with inheritance and base/derived classes, storing objects by value (without pointers) causes object slicing.

As explained in Stack Overflow discussions, “In languages like Java, all the objects are created on the heap (i.e. dynamic memory), and an ‘object’ is actually a ‘reference’ to the object.” This is why Java doesn’t have the same object slicing issues - it’s designed around heap allocation with references.

Consider this example:

class Base {
public:
    virtual void print() { cout << "Base"; }
};

class Derived : public Base {
public:
    virtual void print() override { cout << "Derived"; }
    void derivedOnly() { cout << "Special"; }
};

// Without pointers - causes slicing
void withoutPointers() {
    Base obj = Derived();  // Object slicing!
    obj.print();           // Outputs "Base" instead of "Derived"
    // obj.derivedOnly();   // Error: no member named 'derivedOnly'
}

// With pointers - works correctly
void withPointers() {
    Base* obj = new Derived();  // Full object preserved
    obj->print();               // Outputs "Derived"
    // obj->derivedOnly();     // Error: Base has no derivedOnly
    delete obj;
}

The C++ documentation confirms that “objects allocated have been declared having the derived class type directly” when using pointers, preserving the complete object hierarchy.

Performance and Memory Efficiency

While not the primary reason, performance considerations do favor pointers in certain scenarios:

1. Large object passing: Passing large objects by pointer/reference avoids expensive copying operations
2. Container efficiency: Storing pointers in containers like std::vector<Base*> is more memory-efficient than storing objects by value

As noted in Software Engineering Stack Exchange, “If it should be able to live longer, we need a heap-allocated object. Are we trying to write exception-safe code? (Yes!) If so, handling ownership of more than one naked pointer is extremely tedious and rather bug-prone.”

However, the performance benefit must be weighed against the complexity of manual memory management.

Dynamic Allocation and Object Lifetime

Pointers allow objects to exist beyond the scope where they were created:

Object* createObject() {
    Object* obj = new Object();  // Created on heap
    return obj;  // Can be returned from function
}

int main() {
    Object* myObject = createObject();  // Still valid!
    myObject->testFunc();               // Works fine
    delete myObject;                    // Must remember to delete
    return 0;
}

This is impossible with stack objects since they’re automatically destroyed when the function ends. As Reddit discussions explain, “if the vector contains pointers to some object type, allocation and deallocation of the objects-pointed-to is not automatic.”

Practical Examples and Best Practices

Here’s why the arrow operator -> is used instead of the dot operator .:

• Dot operator .: Used for objects (e.g., myObject.testFunc())
• Arrow operator ->: Used for pointers (e.g., myObject->testFunc() is equivalent to (*myObject).testFunc())

The arrow operator combines dereferencing and member access into a single operation, making pointer-based code more readable.

Common use cases for pointers:

1. Polymorphic containers:

std::vector<Base*> objects;
objects.push_back(new Derived());
objects.push_back(new AnotherDerived());
for (auto obj : objects) {
    obj->virtualMethod();  // Calls appropriate implementation
}

2. Avoiding expensive copies:

void processLargeObject(const LargeObject& obj);  // Pass by reference
// vs
void processLargeObject(LargeObject obj);          // Expensive copy

3. Optional objects:

Object* myObject = nullptr;  // Can be null
if (condition) {
    myObject = new Object();
}
if (myObject) {
    myObject->testFunc();  // Safe to use
}

Modern C++ Solutions

Modern C++ provides safer alternatives to raw pointers:

1. Smart pointers (std::unique_ptr, std::shared_ptr): Automatic memory management
2. References: Safer alternative to pointers when you know the object will exist
3. Value semantics: Often preferred over pointers for simple objects

As Stack Overflow answers suggest, “The better approach is usually to use a ‘smart pointer’, which is an object that holds a pointer and has a destructor that releases it.”

For a Java developer transitioning to C++, it’s helpful to think of C++ pointers as similar to Java references, but with more explicit control and different memory management semantics. The key is understanding when stack allocation is appropriate (for small, short-lived objects) and when heap allocation with pointers is necessary (for polymorphism, large objects, or objects that need to persist).

Conclusion

1. Polymorphism is the primary reason: Pointers are essential for proper object-oriented programming with inheritance in C++, preventing object slicing that occurs with value-based storage.

2. Memory management flexibility: C++ gives you explicit control over object lifetime and storage location, unlike Java’s automatic garbage collection.

3. Performance considerations matter: While not always the main factor, pointers can improve performance by avoiding expensive copies and enabling efficient container storage.

4. Modern C++ provides safer alternatives: Smart pointers offer the benefits of pointers with automatic memory management, making them preferable to raw pointers in most cases.

5. Java vs C++ mental model: Think of C++ stack objects as similar to Java local variables, while heap-allocated objects with pointers are similar to Java objects, but with explicit memory management required.

As you transition from Java to C++, focus on understanding when each approach is appropriate, and leverage modern C++ features like smart pointers to write safer, more maintainable code while still taking advantage of the performance and flexibility that pointers provide.