Should I Pass `shared_ptr` by Reference or Value?

Shared_ptr: Reference vs. Value

When dealing with shared pointers in C , the choice between passing by reference or value can arise. This decision involves considerations of speed, ownership, and potential issues.

Performance

Initially, it may seem that passing by reference (via a const reference) would be faster than passing by value. However, this assumption is not entirely accurate. Modern compilers effectively optimize for both scenarios, making the difference negligible in most cases.

Ownership

Passing by value implies transfer of ownership, while passing by reference retains ownership in the calling function. Unless the intention is to transfer ownership (for example, between data structures or threads), it is preferable to pass by reference.

Additional Considerations

• Move Optimization: As mentioned by Scott Meyers, move optimization may be applicable in certain circumstances and can improve performance.
• Ease of Use: Passing by value allows for direct manipulation of the shared pointer within the function, while passing by reference may require additional dereferencing or assignment.
• Exceptions: In the event of exceptions, passing by reference can be more robust as the original pointer remains intact, while passing by value may result in a dangling pointer.

Conclusion

Based on these factors, the recommended practice is to pass shared_ptrs by reference (specifically, by const reference to prevent accidental modification). This ensures ownership remains with the caller, maintains performance, and simplifies code handling.

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