lang/designated.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 <format>

struct Value {
  double amount = 0;
  int precision = 2;
  std::string unit = "Dollar";
};

int main()
{
  Value v1{100};                               // OK
  Value v2{.amount = 100, .unit = "Euro"};     // OK
  Value v3{.precision = 8, .unit = "$"};       // OK

  Value v4{100, .unit = "Euro"};               // ERROR: all or none designated
  Value v5{.unit = "$", .amount = 20};         // ERROR: invalid order
  Value v6(.amount = 29.9, .unit = "Euro");    // ERROR: only supported for curly braces

  for (Value val : {v1, v2, v3}) {
    std::cout << std::format("{:.{}f} {}\n", val.amount, val.precision, val.unit);
  }
}