lang/maxvalue.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>

// concept for pointer-like objects:
template<typename T>
concept IsPointer = requires(T p) {
                      *p;             // operator * has to be valid
                      p == nullptr;   // can compare with nullptr
                      {p < p} -> std::convertible_to<bool>;  // < yields bool
                    };

// maxValue() for plain values:
auto maxValue(auto a, auto b)
{
  return b < a ? a : b;
}

// maxValue() for pointers:
auto maxValue(IsPointer auto a, IsPointer auto b)
requires std::totally_ordered_with<decltype(*a), decltype(*b)>  
{
  return maxValue(*a, *b);  // return maximum value of where the pointers refer to
}

int main()
{
  int x = 42;
  int y = 77;
  std::cout << maxValue(x, y) << '\n';      // maximum value of ints
  std::cout << maxValue(&x, &y) << '\n';    // maximum value of where the pointers point to

  int* xp = &x;
  int* yp = &y;
  std::cout << maxValue(&xp, &yp) << '\n';  // maximum value of pointer to pointer
  
  double d = 49.9;
  std::cout << maxValue(xp, &d) << '\n';    // maximum value of int and double pointer
}