How to return smart pointers ( shared_ptr ), by reference or by value?

In C++, smart pointers such as std::shared_ptr are used to manage dynamically allocated memory, prevent memory leaks, and simplify memory management.

When returning std::shared_ptr from a function, there are typically two approaches: returning by value and returning by reference. Below, I will explain both approaches and provide recommended practices.

1. Returning std::shared_ptr by Value

This is the most common and recommended approach. When returning std::shared_ptr by value, C++'s move semantics are utilized, meaning no unnecessary reference counting increments or decrements occur. Compiler optimizations, such as Return Value Optimization (RVO), can further improve performance. This avoids additional performance overhead and maintains code simplicity and safety.

Example code:

cpp
#include <memory>

std::shared_ptr<int> createInt() {
    std::shared_ptr<int> myIntPtr = std::make_shared<int>(10);
    return myIntPtr;  // Returning by value utilizes move semantics
}

int main() {
    std::shared_ptr<int> receivedPtr = createInt();
    return 0;
}

In this example, createInt() returns a std::shared_ptr<int> by value. Due to move semantics, no extra reference counting operations occur.

2. Returning std::shared_ptr by Reference

Generally, it is not recommended to return std::shared_ptr by reference. This can lead to unintended operations on internal resources by external entities, such as modification or deallocation, which may cause program instability or errors. If returning by reference is necessary, ensure that the lifetime of the returned reference is properly managed.

Example code:

cpp
#include <memory>

std::shared_ptr<int> globalIntPtr = std::make_shared<int>(20);

const std::shared_ptr<int>& getInt() {
    return globalIntPtr;  // Returning a global `std::shared_ptr` by reference
}

int main() {
    const std::shared_ptr<int>& receivedPtr = getInt();
    return 0;
}

In this example, a global std::shared_ptr is returned by reference, but this approach restricts the function's usage context and may lead to difficult-to-trace errors.

Conclusion

In summary, it is generally recommended to return std::shared_ptr by value. This approach not only leverages modern C++ features (such as move semantics) but also maintains code safety and clarity. Returning by reference is generally not recommended unless there is a compelling reason and the lifetime management of the smart pointer is fully understood.