lib/spanfix.cpp

The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <iostream>
#include <string>
#include <vector>
#include <ranges>
#include <algorithm>
#include <span>

template<typename T, std::size_t Sz>
void printSpan(std::span<T, Sz> sp)
{
  for (const auto& elem : sp) {
    std::cout << '"' << elem << "\" ";
  }
  std::cout << '\n';
}

int main()
{
  std::vector<std::string> vec{"New York", "Tokyo", "Rio", "Berlin", "Sydney"};

  // define view to first 3 elements:
  std::span<const std::string, 3> sp3{vec.data(), 3};
  std::cout << "first 3:              ";
  printSpan(sp3);

  // sort referenced elements:
  std::ranges::sort(vec);
  std::cout << "first 3 after sort(): ";
  printSpan(sp3);

  // insert a new element:
  // - must reassign the internal array of the vector if it reallocated new memory
  auto oldCapa = vec.capacity();
  vec.push_back("Cairo");
  if (oldCapa != vec.capacity()) {
    sp3 = std::span<std::string, 3>{vec.data(), 3};
  }
  std::cout << "first 3: ";
  printSpan(sp3);

  // let span refer to the last three elements:
  sp3 = std::span<const std::string, 3>{vec.end()-3, vec.end()};
  std::cout << "last 3:  ";
  printSpan(sp3);

  // let span refer to the last three elements:
  sp3 = std::span{vec}.last<3>();
  std::cout << "last 3:  ";
  printSpan(sp3);
}