C++ Handbook

Header Reference: <expected>

Header Reference: <expected>

The value-or-error result type std::expected and the main patterns it supports in modern C++ APIs.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

What header pages are for

Header reference pages are meant to answer a practical question quickly: what this header provides, when to reach for it, and which usage rules are easiest to get wrong.

  • Start here when you already know roughly which header you need but want a fast operational summary.
  • Use the example section below as a minimal pattern, then adapt it to your real container, ownership, text, or concurrency workflow.
  • Jump to broader index pages when you need exhaustive coverage rather than a header-focused summary.

Header Reference: <expected>

Main types

What it provides

When to use it

When not to use it

Nearby pages

Minimal example

#include <expected>
#include <string>

std::expected<int, std::string> parse(bool ok) {
    if (!ok) {
        return std::unexpected("parse failure");
    }
    return 42;
}

What to verify before relying on this header

  • Whether the type models ownership, borrowing, type erasure, value transport, or a compile-time constraint.
  • Which operations are constant time versus potentially allocating, dispatching, or instantiating more template machinery than expected.
  • How the facility composes with nearby headers like ``, ``, ``, or ``.