fig07_12.cpp

// fig07_12.cpp
// C++20 spans: Creating views into containers.
#include <array>
#include <fmt/format.h>
#include <iostream>
#include <numeric>
#include <span>
#include <vector>

// items parameter is treated as a const int* so we also need the size to
// know how to iterate over items with counter-controlled iteration
void displayArray(const int items[], size_t size) {
   for (size_t i{0}; i < size; ++i) {
      std::cout << fmt::format("{} ", items[i]);
   }
}

// span parameter contains both the location of the first item
// and the number of elements, so we can iterate using range-based for
void displaySpan(std::span<const int> items) {
   for (const auto& item : items) { // spans are iterable
      std::cout << fmt::format("{} ", item);
   }
}

// spans can be used to modify elements in the original data structure
void times2(std::span<int> items) {
   for (int& item : items) {
      item *= 2;
   }
}

int main() {
   int values1[]{1, 2, 3, 4, 5};
   std::array values2{6, 7, 8, 9, 10};
   std::vector values3{11, 12, 13, 14, 15};

   // must specify size because the compiler treats displayArray's items 
   // parameter as a pointer to the first element of the argument
   std::cout << "values1 via displayArray: ";
   displayArray(values1, 5);

   // compiler knows values1's size and automatically creates a span
   // representing &values1[0] and the array's length
   std::cout << "\nvalues1 via displaySpan: ";
   displaySpan(values1);

   // compiler also can create spans from std::arrays and std::vectors
   std::cout << "\nvalues2 via displaySpan: ";
   displaySpan(values2);
   std::cout << "\nvalues3 via displaySpan: ";
   displaySpan(values3);

   // changing a span's contents modifies the original data
   times2(values1);
   std::cout << "\n\nvalues1 after times2 modifies its span argument: ";
   displaySpan(values1);

   // spans have various array-and-vector-like capabilities
   std::span mySpan{values1}; // span<int>
   std::cout << "\n\nmySpan's first element: " << mySpan.front()
      << "\nmySpan's last element: " << mySpan.back();

   // spans can be used with standard library algorithms
   std::cout << "\n\nSum of mySpan's elements: "
      << std::accumulate(std::begin(mySpan), std::end(mySpan), 0);

   // spans can be used to create subviews of a container
   std::cout << "\n\nFirst three elements of mySpan: ";
   displaySpan(mySpan.first(3));
   std::cout << "\nLast three elements of mySpan: ";
   displaySpan(mySpan.last(3));
   std::cout << "\nMiddle three elements of mySpan: ";
   displaySpan(mySpan.subspan(1, 3));

   // changing a subview's contents modifies the original data
   times2(mySpan.subspan(1, 3));
   std::cout << "\n\nvalues1 after modifying elements via span: ";
   displaySpan(values1);

   // access a span element via []
   std::cout << "\n\nThe element at index 2 is: " << mySpan[2];
}



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