Difference Between Map and HashMap in C++ STL

Difference Between Map and HashMap in STL C++

Hi Coders, Writing reliable and effective programming requires a better understanding of data structures. The Standard Template Library (STL) offers two main essential containers for associative mapping: 'map' and 'unordered_map'. unordered_map is also called 'HashMap'. 

It is essential to know the differences between these two containers in order to select the appropriate one. We'll examine the main distinctions between 'map' and 'HashMap' in the C++ STL in this post, along with some examples to help you understand. Both serve the purpose of storing key-value pairs, they differ significantly in their underlying mechanisms and usage scenarios.

Understanding Map:

std::map is a sorted associative container that maintains its elements in an increasing order based on the keys. Internally, it typically employs a balanced binary search tree (such as a Red-Black Tree) to achieve this ordering. This structure ensures efficient insertion,retrieval and deletion operations in the worst-case scenario of time complexity O(log n) .

Key Characteristics of Map:

1. Ordered Structure: Elements in a map are arranged in a sorted order according to their keys.

2. Balanced Tree: The underlying data structure is usually a balanced binary search tree, facilitating logarithmic time complexity for most of its operations.

3. Unique Keys: In order to enforce distinct associations between keys and values, each key in a map must be unique.

`std::map` is an ordered associative container that maintains elements in sorted order based on their keys. Consider the following example:

#include <iostream>
#include <map>

int main() {
    std::map<<int, std::string> myMap;
    // Inserting elements into the map
    myMap[1] = "apple";
    myMap[3] = "banana";
    myMap[2] = "orange";
    // Printing elements of the map
    for(const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    return 0;


1: apple
2: orange
3: banana

In the above example, the elements are automatically sorted by keys in ascending order.

Unveiling HashMap:

std::unordered_map, also known as HashMap, offers a different approach to storing key-value pairs. It uses a hash table implementation in which elements are stored in an unordered manner based on the hash value of the keys. This leads to a constant-time complexity for average-case operations (typically O(1)). It makes it highly efficient for large datasets.

Key Characteristics of HashMap:

1. Unordered Storage: Elements are not stored in any particular order. They are arranged according to their hash values.

2. Hash Table: HashMap employs a hash table data structure which offers constant-time complexity for average-case operations like insertion, deletion, and retrieval.

3. Hashing Function Dependency: To distribute elements evenly across the hash table, an effective hashing function is essential for efficient performance.

`std::unordered_map`, or `HashMap`, is an unordered associative container that stores elements based on their hash values. Let's see an example:

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<int, std::string> myHashMap;
    // Inserting elements into the HashMap
    myHashMap[1] = "apple";
    myHashMap[3] = "banana";
    myHashMap[2] = "orange";
    // Printing elements of the HashMap
    for(const auto& pair : myHashMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    return 0;


1: apple
2: orange
3: banana

In the above example, the order in which elements are inserted appears to be random, but their hash values allow for efficient storing and retrieval.

Choosing the Right Container:

When deciding between `map` and `HashMap` in C++, consider the following factors:

1. Performance Requirements: If your application involves frequent lookups and inserts, especially with large datasets. `HashMap` offers better performance due to its constant-time complexity for most operations.

2. Ordered Access vs. Unordered Access: If maintaining the order of elements based on keys is crucial for your application. `map` provides this guarantee. However, if the order is not a concern and you prioritize performance, `HashMap` is the better choice.

3. Key Uniqueness: `map` enforces unique keys, while `HashMap` allows duplicate keys but overwrites existing values. You should choose based on your specific requirement for key uniqueness.


By considering factors such as performance requirements, ordered access, and key uniqueness, you can choose the appropriate container for your application needs. Whether you opt for the ordered structure of `map` or the efficiency of `HashMap`, using these containers effectively will enhance the performance and scalability of your C++ programs.


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