C++ API Design – When to Use string_view in an Interface

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I'm using an internal library that was designed to mimic a proposed C++ library, and sometime in the past few years I see its interface changed from using std::string to string_view.

So I dutifully change my code, to conform to the new interface. Unfortunately, what I have to pass in is a std::string parameter, and something that is a std::string return value. So my code changed from something like this:

void one_time_setup(const std::string & p1, int p2) {
   api_class api;
   api.setup (p1, special_number_to_string(p2));
}

to 

void one_time_setup(const std::string & p1, int p2) {
   api_class api;
   const std::string p2_storage(special_number_to_string(p2));
   api.setup (string_view(&p1[0], p1.size()), string_view(&p2_storage[0], p2_storage.size()));
}

I really don't see what this change bought me as the API client, other than more code (to possibly screw up). The API call is less safe (due to the API no longer owning the storage for its parameters), probably saved my program 0 work (due to move optimizations compilers can do now), and even if it did save work, that would only be a couple of allocations that will not and would never be done after startup or in a big loop somewhere. Not for this API.

However, this approach seems to follow advice I see elsewhere, for example this answer:

As an aside, since C++17 you should avoid passing a const std::string&
in favor of a std::string_view:

I find that advice surprising, as it seems to be advocating universally replacing a relatively safe object with a less safe one (basically a glorified pointer and length), primarily for purposes of optimization.

So when should string_view be used, and when should it not?

Best Answer

  1. Does the functionality taking the value need to take ownership of the string? If so use std::string (non-const, non-ref). This option gives you the choice to explicitly move in a value as well if you know that it won't ever be used again in the calling context.
  2. Does the functionality just read the string? If so use std::string_view (const, non-ref) this is because string_view can handle std::string and char* easily without issue and without making a copy. This should replace all const std::string& parameters.

Ultimately you should never need to call the std::string_view constructor like you are. std::string has a conversion operator that handles the conversion automatically.

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