C++ : How to Initialize a map in one line using initialzer_list ?

In this article we will discuss how to initialize a map with std::initialzer_list<T>.

Let’s create a map of string as key and int as value and initialize it with initializer_list i.e.

// Initialize a Map of string & int using initializer_list
std::map<std::string, int> mapOfMarks = {
		{"Riti",2},
		{"Jack",4}
};

Here compiler will create following std::initializer_list<T> object internally,

std::initializer_list<std::pair<const std::string, int> > = {
		{"Riti",2},
		{"Jack",4}
};

Here T is a std::pair<const std::string, int> because map store the elements internally as a pair.

Initializing a map of string & vector

// Initialize a Map of string & vector of int using initializer_list
std::map<std::string, std::vector<int> > mapOfOccur = 	{
							{ "Riti", { 3, 4, 5, 6 } },
							{ "Jack", { 1, 2, 3, 5 } }
							};

Complete Example is as follows,

Output:

#include <iostream>
#include <vector>
#include <map>
#include <list>
int main() {
	// Initialize a Map of string & int using initializer_list
	std::map<std::string, int> mapOfMarks = { { "Riti", 2 }, { "Jack", 4 } };

	for (auto entry : mapOfMarks)
		std::cout << entry.first << " :: " << entry.second << std::endl;

	std::cout << std::endl;

	// Initialize a Map of string & vector of int using initializer_list
	std::map<std::string, std::vector<int> > mapOfOccur = 	{
								{ "Riti", { 3, 4, 5, 6 } },
								{ "Jack", { 1, 2, 3, 5 } }
								};

	// Iterating over the map
	for (auto entry : mapOfOccur) {
		std::cout << entry.first << " :: ";
		for (int i : entry.second)
			std::cout << i << " , ";
		std::cout << std::endl;
	}
	return 0;
}

Output:

Jack :: 4
Riti :: 2

Jack :: 1 , 2 , 3 , 5 , 
Riti :: 3 , 4 , 5 , 6 ,

Initializing a member variable map in constructor with std::initializer_list

#include <iostream>
#include <vector>
#include <map>
#include <initializer_list>

class Course {
	std::map<std::string, int> mMapOfMarks;

public:
	Course(std::initializer_list<std::pair<const std::string, int> > marksMap) :
			mMapOfMarks(marksMap) {
	}
	void display() {
		for (auto entry : mMapOfMarks)
			std::cout << entry.first << " :: " << entry.second << std::endl;
		std::cout << std::endl;
	}

};

int main() {
	// Creating a Course Object and calling the constructor
	// that accepts a initializer_list
	Course mathsCourse { { "Riti", 2 }, { "Jack", 4 } };

	mathsCourse.display();
	return 0;
}

Output:

Jack :: 4
Riti :: 2