Should Pass-by-Value Be the Default for Value Types in C 11?

Pass-by-Value in C 11: A Reassessment

In C 11, the introduction of rvalue references and move constructors has sparked a debate: should pass-by-value become the default for value types?

Origins of Pass-by-Reference

Historically, passing large objects by value in C was inefficient due to unnecessary copying. Reference parameters were preferred to avoid the costly duplication. However, references introduced complexities in ownership and memory management, particularly for heap-allocated objects.

The New Paradigm in C 11

With rvalue references and move constructors, it is now feasible to implement value types like std::vector and std::string that are inexpensive to pass by value. This raises the question: should pass-by-value be the new standard for these types?

Recommendations

According to Dave Abrahams, passing by value is recommended if a copy is required within the function. This allows the compiler to optimize the copy, as seen in the following example:

void foo(T t)
{
    // ...
}

This can be used flexibly to pass lvalues, prvalues, or xvalues:

T lval;
foo(lval); // copy from lvalue
foo(T {}); // (potential) move from prvalue
foo(std::move(lval)); // (potential) move from xvalue

Exceptions

Pass-by-reference to const remains a reasonable option for certain scenarios where a copy is unnecessary or undesirable. In such cases, the following syntax can be employed:

class T {
    U u;
    V v;
public:
    T(U u, V v)
        : u(std::move(u))
        , v(std::move(v))
    {}
};

Conclusion

In C 11, pass-by-value is a compelling option for value types that require copying within functions. It simplifies parameter handling and allows for efficient move operations. However, pass-by-reference to const remains a suitable alternative in cases where a copy is not required.

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