C++ - expected and Result Patterns (C++23)

Overview

Estimated time: 45–60 minutes

Model operations that can fail without exceptions using std::expected (C++23) or similar result types. Improve clarity and testability for predictable errors.

Learning Objectives

• Use std::expected to return values or errors.
• Design error enums/structs and propagate errors ergonomically.

Prerequisites

• C++ - Error Handling & Exceptions

Basic usage

#include 
#include 
#include 
#include 

std::expected to_int(std::string_view s){
  int out{};
  auto first = s.data();
  auto last  = s.data() + s.size();
  auto res = std::from_chars(first, last, out);
  if (res.ec == std::errc{}) return out;
  return std::unexpected(res.ec);
}

Consuming results

#include 
#include 

void print_result(const std::expected& r){
  if (r) std::cout << *r << "\n";
  else   std::cout << std::make_error_code(r.error()).message() << "\n";
}

Common Pitfalls

• Overusing exceptions and expected together—pick a dominant strategy for layers.
• Ignoring unexpected construction for errors; be explicit about error type.

Checks for Understanding

1. When is expected preferable to exceptions?
2. How do you access the error?

Exercises

1. Write expected parse_double that returns a message on failure.
2. Chain two expected-returning functions and propagate errors early.