format/printuserdef.cpp

The following code example is taken from the book
C++23 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2026
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <print>
#include <string>
#include <vector>
#include <format>

struct MyInt
{
  int value;
  MyInt(int i) : value{i} {}
};

template<>
struct std::formatter<MyInt> : std::formatter<int>
{
  auto format(const MyInt& obj, auto& ctx) const {
    return std::formatter<int>::format(obj.value, ctx);  // format value
  }
};

int main()
{
  MyInt mi = 42;
  std::println("MyInt:     {}", mi);               // prints: 42
  std::println("MyInt vec: {}", std::vector{mi});  // prints: [42]
}