C++ Handbook

Header Reference: <unordered_map>

Header Reference: <unordered_map>

Hash-based key-value associative container facilities declared in <unordered_map>.

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: <unordered_map>

Main facilities

Main facility groups

What this header is for

Use <unordered_map> when key-value lookup speed matters more than iteration order and hashing is an appropriate fit.

Common patterns

Practical rules

Minimal example

#include <unordered-map>

int main() {
    // Start with the primary facility from <unordered-map>.
    // Then verify lifetime, invalidation, ordering, or error-handling rules.
    return 0;
}

What to verify before relying on this header

  • Which operations invalidate iterators, references, pointers, views, or node handles.
  • Whether ordering, hashing, capacity growth, or allocation behavior affects the surrounding design.
  • Whether a non-owning view or a different container would express the real requirement more clearly.