Are the days of passing const std::string & as a parameter over?

Question

I heard a recent talk by Herb Sutter who suggested that the reasons to pass std::vector and std::string by const & are largely gon

Answer 1

See “Herb Sutter "Back to the Basics! Essentials of Modern C++ Style”. Among other topics, he reviews the parameter passing advice that’s been given in the past, and new ideas that come in with C++11 and specifically looks at the idea of passing strings by value.

The benchmarks show that passing std::strings by value, in cases where the function will copy it in anyway, can be significantly slower!

This is because you are forcing it to always make a full copy (and then move into place), while the const& version will update the old string which may reuse the already-allocated buffer.

See his slide 27: For “set” functions, option 1 is the same as it always was. Option 2 adds an overload for rvalue reference, but this gives a combinatorial explosion if there are multiple parameters.

It is only for “sink” parameters where a string must be created (not have its existing value changed) that the pass-by-value trick is valid. That is, constructors in which the parameter directly initializes the member of the matching type.

If you want to see how deep you can go in worrying about this, watch Nicolai Josuttis’s presentation and good luck with that (“Perfect — Done!” n times after finding fault with the previous version. Ever been there?)

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This is also summarized as ⧺F.15 in the Standard Guidelines.