I am trying to compile an example problem from a book.

I got errors when I compiled on my system, so I tried an online compiler and it worked. I updated g++ to version 9 and tried again, but it still won't compile. 
I get the error `'std::string_view' has not been declared`.

```cpp
// Sorting words recursively
#include <iostream>
#include <iomanip>
#include <memory>
#include <string>
#include <string_view>
#include <vector>

using Words = std::vector<std::shared_ptr<std::string>>;

// Function prototypes
void swap(Words& words, size_t first, size_t second);
void sort(Words& words);
void sort(Words& words, size_t start, size_t end);
void extract_words(Words& words, std::string_view text, std::string_view separators);
void show_words(const Words& words);
size_t max_word_length(const Words& words);

```

First error occurs in the `extract_words` prototype, and all attempts after that to use the `text` parameter, or any use of `std::string_view`, causes an error.