Should We Still Pass `std::string` and `std::vector` by `const &`?

Overhauling Parameter Passing Conventions for Optimization

In a recent talk, Herb Sutter suggested that passing std::vector and std::string objects by const & is becoming obsolete. This claim has sparked discussions among programmers.

Sutter's Rationale

Sutter argues that in cases where the function returns an rvalue (i.e., a temporary object), passing the input parameter by value allows for more efficient handling using move semantics. This avoids unnecessary copies or memory allocations.

Consider the following code snippet:

std::string do_something ( std::string inval )
{
   std::string return_val;
   // ... do stuff ...
   return return_val;
}

Passing inval by value allows the function to directly modify and return the value, eliminating the need for a copy constructor.

Concerns about Efficiency

Critics argue that passing large objects like std::string by value can still be inefficient due to the overhead of moving the object. A std::string instance consists of several components, including a heap pointer and a member char[] for short string optimization.

Arguments for Reference Parameters

Despite the potential inefficiencies, proponents of reference parameters argue that they offer several benefits:

• Encapsulation: Functions that take references make it clear that they do not modify the original object.
• Reduced complexity: Reference parameters simplify code by eliminating the need to consider ownership and lifetime management.

Conclusion

The choice of whether to pass objects by const & or by value depends on the specific use case. In situations where efficiency is critical and the function operates on rvalues, passing by value with move semantics can provide performance benefits. However, in cases where encapsulation, simplicity, or ownership management is important, reference parameters remain valuable.

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