std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

node-handle (C++17)

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

hive (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

Iterator invalidation

Member function table

Non-member function table

 

std::unordered_map

Member types
Member functions
unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
Non-member functions
operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
Deduction guides(C++17)

 

iterator begin()noexcept;	(1)	(since C++11) (constexpr since C++26)
const_iterator begin()constnoexcept;	(2)	(since C++11) (constexpr since C++26)
const_iterator cbegin()constnoexcept;	(3)	(since C++11) (constexpr since C++26)

Returns an iterator to the first element of *this.

If *this is empty, the returned iterator will be equal to end().

Return value

Iterator to the first element.

Complexity

Constant.

Example

Run this code

#include <cmath>#include <iostream>#include <unordered_map>   struct Node {double x, y;};   int main(){ Node nodes[3]={{1, 0}, {2, 0}, {3, 0}};   // mag is a map mapping the address of a Node to its magnitude in the planestd::unordered_map<Node*, double> mag ={{ nodes +0, 1}, { nodes +1, 2}, { nodes +2, 3}};   // Change each y-coordinate from 0 to the magnitudefor(auto iter = mag.begin(); iter != mag.end();++iter){auto cur = iter->first;// pointer to Node cur->y = mag[cur];// could also have used cur->y = iter->second;}   // Update and print the magnitude of each nodefor(auto iter = mag.begin(); iter != mag.end();++iter){auto cur = iter->first; mag[cur]=std::hypot(cur->x, cur->y);std::cout<<"The magnitude of ("<< cur->x <<", "<< cur->y <<") is ";std::cout<< iter->second <<'\n';}   // Repeat the above with the range-based for loopfor(auto i : mag){auto cur = i.first; cur->y = i.second; mag[cur]=std::hypot(cur->x, cur->y);std::cout<<"The magnitude of ("<< cur->x <<", "<< cur->y <<") is ";std::cout<< mag[cur]<<'\n';// Note that in contrast to std::cout << iter->second << '\n'; above, // std::cout << i.second << '\n'; will NOT print the updated magnitude}}

Possible output:

The magnitude of (3, 3) is 4.24264 The magnitude of (1, 1) is 1.41421 The magnitude of (2, 2) is 2.82843 The magnitude of (3, 4.24264) is 5.19615 The magnitude of (1, 1.41421) is 1.73205 The magnitude of (2, 2.82843) is 3.4641

See also

endcend	returns an iterator to the end (public member function)
begincbegin (C++11)(C++14)	returns an iterator to the beginning of a container or array (function template)