How to create an unordered_set of user defined class or struct in C++? - GeeksforGeeks

22 Nov, 2021
The unordered_set internally implements a hash table to store elements. By default we can store only predefined type as int, string, float etc. If we want to store the element of user defined type as structure then compiler will show an error because before storing elements into unordered_set compiler performs some checking. And while comparing two user defined type compiler can not compare them hence it generate an error. So, in order to store a structure in a unordered_set, some comparison function need to be designed. Since unordered_set also store implements hash table to store elements we should also have to implement hash function to perform hashing related work. Below method explains its implementation. Implementation: We create a structure type and define a comparison function inside that structure that will used to compare two structure type objects. Since unordered_set internally implements hash function so we should have also implement the hash function for user defined type objects. Syntax To store user defined type elements unordered_set should follow following syntax  
unordered_set(elementType, MyHashType) us;
// element type is user defined type and MyHashType is class implementing hash function
Below code explains it.  
CPP
// CPP implementation to use// user-defined data type in// structures#include <bits/stdc++.h>using namespace std; // Structure definitionstruct Test {     int id;     // This function is used by unordered_set to compare    // elements of Test.    bool operator==(const Test& t) const    {        return (this->id == t.id);    }}; // class for hash functionclass MyHashFunction {public:    // id is returned as hash function    size_t operator()(const Test& t) const    {        return t.id;    }}; // Driver methodint main(){    // put values in each    // structure define below.    Test t1 = { 110 }, t2 = { 102 },         t3 = { 101 }, t4 = { 115 };     // define a unordered_set having    // structure as its elements.    unordered_set<Test, MyHashFunction> us;     // insert structure in unordered_set    us.insert(t1);    us.insert(t2);    us.insert(t3);    us.insert(t4);     // printing the elements of unordered_set    for (auto e : us) {        cout << e.id << " ";    }     return 0;}
115 101 110 102
 
115 101 110 102
 
Below is another example where we use predefined hash functions to make overall hash function of our defined class. 
CPP
// CPP program to demonstrate working of unordered_set// for user defined data types.#include <bits/stdc++.h>using namespace std; struct Person {    string first, last;     Person(string f, string l)    {        first = f;        last = l;    }     bool operator==(const Person& p) const    {        return first == p.first && last == p.last;    }}; class MyHashFunction {public:     // We use predefined hash functions of strings    // and define our hash function as XOR of the    // hash values.    size_t operator()(const Person& p) const    {        return (hash<string>()(p.first)) ^ (hash<string>()(p.last));    }}; // Driver codeint main(){    unordered_set<Person, MyHashFunction> us;    Person p1("kartik", "kapoor");    Person p2("Ram", "Singh");    Person p3("Laxman", "Prasad");     us.insert(p1);    us.insert(p2);    us.insert(p3);     for (auto e : us) {        cout << "[" << e.first << ", "             << e.last << "]\n";    }     return 0;}
[Laxman, Prasad]
[kartik, kapoor]
[Ram, Singh]
 
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