How to Initialize a Static std::map&lt;int, int&gt; in C++ - Coding

In C++, std::map<int, int> is a commonly used container that stores key-value pairs. There are scenarios where we may want to initialize a static std::map with predefined key-value pairs that remain constant throughout the program’s execution. In this article, we will learn different methods to initialize a static std::map<int, int> in C++.

Initialize std::map<int, int> in C++

There are mainly three ways to initialize a static std::map<int, int> in C++:

With Declaration Using Initializer List
Using a Static Member Function
Using a Static Initialization Block

1. Initialize with Declaration Using Initializer List

The simplest way to initialize a static std::map<int, int> is at the time of declaration by using an initializer list introduced in C++ 11 .

Syntax:

static const std::map<int, int> myMap = {
    {key1, value1},
    {key2, value2},
    ...
    {keyN, valueN}
};

Example:

C++

// C++ program to demonstrate initialization of a static
// std::map<int, int> using initializer list

// Include necessary header files
#include <iostream>
#include <map>

using namespace std;

// Initialize a static const map with some key-value pairs
static const map<int, int> myMap
    = { { 1, 10 }, { 2, 20 }, { 3, 30 }, { 4, 40 } };

int main()
{
    // Iterate through the map and print each key-value pair
    for (const auto& pair : myMap) {
        cout << "Key: " << pair.first
             << ", Value: " << pair.second << endl;
    }
    return 0;
}

Output

Key: 1, Value: 10
Key: 2, Value: 20
Key: 3, Value: 30
Key: 4, Value: 40

2. Using a Static Member Function

Another method is to use a static member function that returns a pre-initialized std::map. This approach is useful when dealing with complex initialization logic.

Syntax:

class MapInitializer {
public:
    static const std::map<int, int>& getMap() {
        static const std::map<int, int> myMap = {
            {key1, value1},
            {key2, value2},
            ...
            {keyN, valueN}
        };
        return myMap;
    }
};

Example:

C++

// C++ program to demonstrate initialization of a static
// std::map<int, int> using static member function

// Include necessary header files
#include <iostream>
#include <map>

using namespace std;

// Class to initialize and return a static map
class MapInitializer {
public:
    // Static function to return a constant reference to the
    // static map
    static const map<int, int>& getMap()
    {
        // Initialize the static map with key-value pairs
        static const map<int, int> myMap = {
            { 1, 10 }, { 2, 20 }, { 3, 30 }, { 4, 40 }
        };
        // Return the map
        return myMap;
    }
};

int main()
{
    // Get the static map from the MapInitializer class
    const map<int, int>& myMap = MapInitializer::getMap();

    // Iterate through the map and print each key-value pair
    for (const auto& pair : myMap) {
        cout << "Key: " << pair.first
             << ", Value: " << pair.second << endl;
    }
    // Return 0 to indicate successful execution
    return 0;
}

Output

Key: 1, Value: 10
Key: 2, Value: 20
Key: 3, Value: 30
Key: 4, Value: 40

3. Using a Static Initialization Block

For older versions of C++ that do not support initializer lists, we can use a static initialization block within a function.

Syntax:

static const std::map<int, int>& getMap() {
    static std::map<int, int> myMap;
    if (myMap.empty()) {
        myMap[key1] = value1;
        myMap[key2] = value2;
        ...
        myMap[keyN] = valueN;
    }
    return myMap;
}

Example:

C++

// C++ program to demonstrate initialization of a static
// std::map<int, int> using static initialization block

// Include necessary header files
#include <iostream>
#include <map>

using namespace std;

// Function to initialize and return a reference to a static
// map
static const map<int, int>& getMap()
{
    // Initialize the static map
    static map<int, int> myMap;
    // Check if the map is empty
    if (myMap.empty()) {
        // Populate the map with key-value pairs
        myMap[1] = 10;
        myMap[2] = 20;
        myMap[3] = 30;
        myMap[4] = 40;
    }
    // Return the map
    return myMap;
}

int main()
{
    // Get the static map from the getMap function
    const map<int, int>& myMap = getMap();

    // Iterate through the map and print each key-value pair
    for (const auto& pair : myMap) {
        cout << "Key: " << pair.first
             << ", Value: " << pair.second << endl;
    }

    return 0;
}

Output

Key: 1, Value: 10
Key: 2, Value: 20
Key: 3, Value: 30
Key: 4, Value: 40

Reffered: https://www.geeksforgeeks.org

C++

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