Why is value taking setter member functions not recommended in Herb Sutter's CppCon 2014 talk (Back to Basics: Modern C++ Style)?

Question

In Herb Sutter\'s CppCon 2014 talk Back to Basics: Modern C++ Style he refers on slide 28 (a web copy of the slides are here) to this pattern:

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Answer 1

There were two reasons considered for why passing by value might be better than passing by const reference.

1. more efficient
2. noexcept

In the case of setters for members of type std::string, he debunked the claim that passing by value was more efficient by showing that passing by const reference typically produced fewer allocations (at least for std::string).

He also debunked the claim that it allows the setter be noexcept by showing that the noexcept declaration is misleading, since an exception can still occur in the process of copying the parameter.

He thus concluded that passing by const reference was to be preferred over passing by value, at least in this case. However, he did mention that passing by value was a potentially good approach for constructors.

I do think that the example for std::string alone is not sufficient to generalize to all types, but it does call into question the practice of passing expensive-to-copy but cheap-to-move parameters by value, at least for efficiency and exception reasons.