c++ - Should methods returning const std::string& return const std::string_view instead?

Question

Assume we have a simple getter method in a class that returns a const reference to a std::string member:

const std::string& getString() const noexcept { return someString; }

With the advent of std::string_view in C++17, I wonder whether it has any advantages of writing this instead:

const std::string_view getString() const noexcept { return someString; }

Does one method have advantages/disadvantages over the other? Clearly (correct me if I'm wrong) both solutions will definitely be better than this:

const char* getString() const noexcept { return someString.c_str(); }

I've seen this related question, but I'm asking for something slightly different.

Answer 1

Yes, you should write:

const std::string& getString() const noexcept { return someString; }

Instead of (note: not const, because never return const values):

std::string_view getString() const noexcept { return someString; }

The reason is - you already have a string. So it's not like you have to pay anything extra to get a string out of it. And string has one notable semantic difference to an arbitrary string_view: it's null-terminated by guarantee. We know this. Maybe some downstream user needs to rely on that information. If they need null-termination (e.g. they need to pass to some C API that requires it) and you give a string_view, they have to make a string out of it themselves. You save nothing, but potentially make downstream users do more work.

If, however, you had a vector<char> instead... then I would suggest to return a span<char const> or the equivalent thereof. Since there is no semantic difference and you're just providing a view.

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There also the separate argument of what:

auto x = obj.getString();

should do. This either takes a copy of the string (expensive, but safe) or effectively a reference to it (cheap, but potentially dangling). But it doesn't entirely look like a reference, it looks like a value. This is a broad issue with reference-semantic types in general (things like reference_wrapper, string_view, span, tuple<T&...>, optional<T&> if it existed, etc.).

I don't have an answer for this case, but it's something to be aware of.