C ++ - passing references to std::shared_ptr or boost:: shared_ptr

In C++, std::shared_ptr is a smart pointer used to manage dynamically allocated objects with reference counting. When deciding whether to pass std::shared_ptr or boost::shared_ptr by reference, several key considerations must be evaluated:

1. Performance Considerations

Passing std::shared_ptr involves copying the smart pointer, which increments the reference count upon copying and decrements it when the object is destroyed. This process uses atomic operations and can introduce performance overhead. For example:

cpp
void process(std::shared_ptr<MyClass> ptr) {
    // Use ptr
}

Each call to process copies ptr, incrementing the reference count. When the function exits, the reference count is decremented. Frequent calls may create a performance bottleneck.

2. Passing by Reference

To avoid this overhead, consider passing std::shared_ptr by reference:

cpp
void process(const std::shared_ptr<MyClass>& ptr) {
    // Use ptr
}

This approach avoids copying the smart pointer, so the reference count remains unaffected, reducing unnecessary overhead.

3. Function Purpose

• No ownership modification: If your function only reads or uses resources pointed to by the smart pointer without altering ownership, passing by reference is optimal.
• Ownership modification required: If the function needs to change ownership (e.g., storing in another container or passing to threads), pass by value to ensure the reference count updates correctly.

4. Practical Example

Consider a Document class and a DocumentManager class managing Document objects using smart pointers:

cpp
class Document {
public:
    void display() const {
        std::cout << "Display document content." << std::endl;
    }
};

void displayDocument(const std::shared_ptr<Document>& doc) {
    doc->display();
}

class DocumentManager {
private:
    std::vector<std::shared_ptr<Document>> docs;

public:
    void addDocument(const std::shared_ptr<Document>& doc) {
        docs.push_back(doc);
    }

    void showAllDocuments() {
        for (const auto& doc : docs) {
            displayDocument(doc);
        }
    }
};

Here, displayDocument receives std::shared_ptr by reference, avoiding unnecessary reference count operations. DocumentManager's addDocument accepts a reference because it requires shared ownership of Document.

Conclusion

The optimal way to pass std::shared_ptr depends on your specific needs. If ownership isn't modified and performance is a concern, passing by reference is typically better. When ownership changes are required, passing by value is more appropriate.