The associated container unordered_map in c++ stores elements that are generated by combining a key value and a mapped value. The mapped value is the material associated with the key, and the key value is used to uniquely identify the element. Both the key and the value may be of any predefined or custom kind. However, the cost of searching, inserting, and deleting from the hash table is on average O. (1).

Unordered_map Vs. Unordered_set

You only have a key and no value in an unordered set, which is often used to see whether anything is present or not in a list. Remember the issue of determining the frequency of individual terms. You can't use an unordered_map c++ (or set) because you don't have any counts to store.

A map (like a set) is an ordered sequence that consists of the unique keys, while you can store keys in unordered_map in any order, hence the name.

Since it applies the map as a structured tree structure, it is easy to keep the elements in order (by specific tree traversal). Map operations have an average time complexity of O(Log n), while an unordered_map in C++ has an average time complexity of O(1).

There are several functions that operate with unordered maps. Operator =, operator [], empty and size for power, begin and end for the iterator, find and count for lookup, insert and delete for alteration are the most useful among them.

The C++11 library has functions to see the number of buckets used internally, and the height of the buckets. You can also see the hash function, and the different hash policies used, but these are less usable in real-world applications.

The iterator iterates over all elements of unordered_map in c++. 

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Unordered_map-class Template

In an unordered_map in c++, an unordered map is a dictionary-type data structure that stores elements. It has a (key, value) pair sequence that allows for quick retrieval of individual elements based on their specific key.

Each unique key has only one value associated with it, and you use key-value to uniquely identify an element. An associative array is another name for it. The mapped value in a (key, value) pair is an entity that has content associated with each unique key. 

The type of key and mapped value for each pair can be different.

The elements in an unordered map are not sorted in any particular order based on their key-value or mapped value. Instead, it groups values into buckets based on their hash values, allowing for quick access to individual elements through their key values.

When you access an individual entity by key, the unordered_map c++, the container outperforms the map container. 

Unordered_map in C++ implements the direct access operator (subscript operator []), which allows direct access to mapped values using their key value as the argument.

The unordered_map class template is specified in the C++ Standard Template Library (STL); it is a compilation of class templates to use in basic programming tasks such as data structures and functions such as those found in a list, stack, or array. It's usually a set of container types, algorithms, and iterators. Since it is a generic library, its elements are parameterized.

Unordered_map Public Types

Size Type Implementation-Defined typedef;

Integral form with no symbol.

Any non-negative difference type value can be represented by ‘size type.'

Implementation-Defined Difference Type typedef;

The different forms of iterator and ‘const_iterator' are all signed integral types.

Implementation-Defined Iterator

  • The value type is the value type of this iterator.
  • Except for the output iterator.
  • Iterator that can be converted to const iterator.

Implementation-Defined Const Iterator typedef;

  • The value type is the value type of a constant iterator.
  • Except for the output iterator.

Implementation-Defined Local Iterator typedef;

  • An iterator that has the same value, variance, pointer, and relation types as an iterator.
  • To iterate over a single bucket, use a local iterator object.

Const Local Iterator typedef Implementation-Defined;

  • The const iterator has a value type, difference type, pointer, and reference type that is identical to this constant iterator.
  • To iterate over a single bucket, use a const local iterator object.

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Unordered_map Iterators

Iterators to an unordered_map in c++ container elements provide access to both the key and the mapped attribute. The class identifies a pair class with its first value corresponding to the const variant of the key type (template parameter Key) and its second value corresponding to the mapped value (template parameter T) for this purpose.

typedef pair<const Key, T> value_type;

iterator begin();

const_iterator begin() const;


An iterator is the first element of the container or, if the box is empty, the past-the-end value for the container.

iterator end();

const_iterator end() const;


An iterator referring to the past-the-end value for the container.

const_iterator cbegin() const;


A constant iterator refers to the first element of the container, or if the container is empty, the past-the-end value for the container.

const_iterator cend() const;


A constant iterator referring to the past-the-end value for the container.

Member Types

The following aliases are member types of unordered_map in C++. They are widely used as parameters and return types by member functions:

member type




the first template parameter (Key)


the second template parameter (T)


pair<const key_type,mapped_type>


the third template parameter (Hash)

defaults to: hash<key_type>


the fourth template parameter (Pred)

defaults to: equal_to<key_type>


the fifth template parameter (Alloc)

defaults to: allocator<value_type>




const value_type&



for the default allocator: value_type*



for the default allocator: const value_type*


a forward iterator to value_type


a forward iterator to const value_type


a forward iterator to value_type


a forward iterator to const value_type


an unsigned integral type

usually the same as size_t


a signed integral type

usually the same as ptrdiff_t

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What are the Methods of an Unordered_map in C++?

The at() function of unordered_map in C++  returns a reference to the value with the variable as the main k (public member function) operator.

  • · [] Begin with the factor of access (public member function)
  • () Returns an iterator to the beginning of the unordered map container's container (public member function) end
  • () In the unordered map container, it returns an iterator pointing to the final element in the container's end location (public member function) 
  • () Finds the bucket number in the map where the unit with the key k is placed. 
  • cbegin () returns to the beginning of the const iterator (public member function) cend () Return the const iterator to the end of the bucket (public member function)
  • · bucket count () is a function that counts the number of buckets in an unordered map. There are no settings required for this functionality (a role reserved for elected officials)
  • capacity of a bucket () The unordered map's number of entries in each bucket is returned (a role reserved for elected officials)
  • count() returns the total number of entries in a keyed unordered_map
  • · (function for elected members)
  • · range equal () Return the limits of a sequence that contains all of the elements in the container and has a key that corresponds to k (function for elected members)
  • · find() returns the element's iterator (public member function)
  • · set up () creates and places an element (public member function)
  • · hint to place () creates and inserts the ‘hint' part (public member function)
  • · put in () adds elements to the mix (public member function)
  • · remove () deletes the elements (public member function)
  • · lucid () is a material that is easy to understand (public member function)
  • · switch () is a material that is easy to understand (public member function)
  • · load factor is a metric that measures how much load is there.  
  • · load factor maximum () The full load factor can be obtained or set (public member function)
  • · rephrase () decides on the number of buckets to use (public member function)
  • · keep in mind () changes in capacity are requested (public member function)
  • · hash function is a function that generates a hash in your program.


This concludes this write-up of the C++ definition of unordered_map in c++. Join the Simplilearn's Post Graduate Program in Full Stack Web Development course and enjoy learning the concept in detail. You can also learn about the course in a detailed manner with the help of a skill-up course. Get hired by the right company and enjoy a flourishing career. 

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About the Author

Ravikiran A SRavikiran A S

Ravikiran A S works with Simplilearn as a Research Analyst. He an enthusiastic geek always in the hunt to learn the latest technologies. He is proficient with Java Programming Language, Big Data, and powerful Big Data Frameworks like Apache Hadoop and Apache Spark.

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