In C++, std::map is a commonly used associative container that stores elements in key-value pairs, allowing for efficient data retrieval based on keys. One common question that arises is what requirements must be met for a class to be used as a key in std::map.

In this article, we will learn the requirements for key classes in std::map, understand the reasoning behind these requirements, and provide examples to illustrate their importance.

Requirements for std::map Key Classes

The std::map is part of the Standard Template Library (STL) in C++ and is implemented as a balanced binary tree. The keys in a std::map must be unique and must follow certain requirements to ensure proper functionality. To be used as a key in std::map, a class must meet the following requirements:

1. Copyable and Assignable

The key class must be copyable and assignable. This ensures that the keys can be duplicated and assigned as needed when managing the elements within the map.

2. Comparison Operator

The key class must have a valid comparison operator to define an ordering among the keys. By default, std::map uses std::less<KeyType>, which relies on the < operator. However, we can define a custom comparison operator to specify a different ordering.

If we choose not to use the default < operator, we can create a custom comparison operator. This operator must define a strict weak ordering, meaning if CmpMyType()(a, b) returns true, then CmpMyType()(b, a) must return false. Additionally, if both comparisons return false, the elements are considered equal.

Example:

The below example demonstrates the Custom Comparison Operator:

struct MyType {
    int a;
    std::string b;
};

struct CmpMyType {
    bool operator()(const MyType& lhs, const MyType& rhs) const {
        if (lhs.a < rhs.a) return true;
        if (lhs.a > rhs.a) return false;
        return lhs.b < rhs.b;
    }
};

C++ Program to Demonstate the Valid Key Class for std::map

The below program illustrates an example of a custom class that meets the criteria for being a key in std::map to demonstrate the requirements.

// C++ program to demonstrate the use of std::map with a
// custom key type (Person)

#include <iostream>
#include <map>
#include <string>

using namespace std;

// Structure to represent a person
struct Person {
    int id;
    string name;

    // Comparison operator for strict weak ordering
    bool operator<(const Person& other) const
    {
        if (id < other.id)
            return true;
        if (id > other.id)
            return false;
        return name < other.name;
    }
};

int main()
{
    // Declare a map with Person as key and int as value
    map<Person, int> personMap;

    // Inserting elements into the map
    personMap[Person{ 1, "Alice" }] = 100;
    personMap[Person{ 2, "Bob" }] = 200;
    personMap[Person{ 3, "Charlie" }] = 300;

    // Accessing and printing elements in the map
    for (const auto& pair : personMap) {
        cout << pair.first.name << " : " << pair.second
             << endl;
    }

    return 0;
}

Alice : 100
Bob : 200
Charlie : 300

Note: The strict weak ordering is very important here because it allows std::map to maintain the elements in a sorted order. This ordering enables efficient operations like O(log n) lookup and insertion time by key value.

Conclusion

To be used as a key in std::map, a class must meet several requirements, including being copyable and assignable, and having a valid comparison operator to define a strict weak ordering. These requirements ensure that the keys can be ordered, managed efficiently, and integrated seamlessly with the map’s underlying data structure. By understanding and meeting these requirements, we can create custom key classes that work effectively with std::map.