The std::string class is a really useful class because it simplifies dealing with strings so much. A flaw is that if we want to pass around a substring of it, we need to pass a pointer and a length variable, two iterators, or a copy of the substring. We did that in the previous recipe, where we removed the surrounding whitespace from a string by taking a copy of the substring range that does not contain the surrounding whitespace.

If we want to pass a string or a substring to a library that does not even support std::string, we can only provide a raw string pointer, which is a bit disappointing, because it sets us back to the old C days. Just as with the substring problem, a raw pointer does not carry information about the string length with it. This way, one would have to implement a bundle of a pointer and a string length.

In a simplified way, this is exactly what std::string_view is. It is available since C++17 and provides a way to pair a pointer to some string together with that string's size. It embodies the idea of having a reference type for arrays of data.

If we design functions which formerly accepted std::string instances as parameters, but did not change them in a way that would require the string instances to reallocate the memory that holds the actual string payload, we could now use std::string_view and be more compatible with libraries that are STL-agnostic. We could let other libraries provide a string_view view on the payload strings behind their complex string implementations and then use that in our STL code. This way, the string_view class acts as a minimal and useful interface, which can be shared among different libraries.

Another cool thing is that string_view can be used as a non-copy reference to substrings of larger string objects. There are a lot of possibilities to use it profitably. In this section, we will play around with string_view in order to get a feeling for its ups and downs. We will also see how we can hide the surrounding whitespace from strings by adapting string views instead of modifying or copying the actual string. This method avoids unnecessary copying or data modification.