Should I Pass `shared_ptr` by Value or by Reference?

Passing Shared Pointers: By Reference or By Value

When passing a shared_ptr (from boost or C 11 STL) to a function, the choice between passing it by const reference (void foo(const shared_ptr& p)) or by value (void foo(shared_ptr p)) has been a topic of debate.

Traditionally, it was believed that passing by reference would be faster as it avoids unnecessary copy operations. However, as discussed by experts at C and Beyond 2011 (Scott, Andrei, and Herb), this is no longer the case.

Should You Pass by Value or by Reference?

Unless there is an explicit need to share ownership of an object (such as between data structures or threads), there is no compelling reason to pass a shared_ptr by value. Passing by const reference is preferred for the following reasons:

• Performance: Passing by const reference is typically faster than copying the shared_ptr by value, especially for large objects. This is because only the pointer itself is passed, rather than the entire object.
• Correctness: Passing by const reference ensures that the shared_ptr cannot be modified within the function. This helps prevent accidental changes to the shared object.

When to Pass by Value

The only time it is advisable to pass a shared_ptr by value is when the intention is to transfer ownership of the object to the function. This is typically the case when the function needs to take responsibility for managing the object's lifetime.

Additional Considerations

For optimal performance, it is important to consider whether the shared_ptr can be move-optimized. As explained by Scott Meyers, move-optimization can eliminate the need for unnecessary copy operations. However, this optimization is only available in certain versions of C .

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