std::initializer_list<T> is introduced in C++11 to make initialization uniform in C++.

Before C++11 it was easy to initialize an array with with default elements like,

// Initializing array with default values
int arr[]= {1,2,3,4,5};

But there was no way no to initialize other containers like vector, list and map etc.

For example, there was no way to initialize a vector with default values in a single line before c++11. The only ways was create a vector and push elements in it i.e.

 

// Old Way to initialize a vector
std::vector<int> vec1;
for(int i = 0; i < 5; i++)
	vec1.push_back(i);

But in C++11 we can do this in a single line i.e.
// Initializing a vector using c++11 initializer_list
std::vector<int> vec= {1,2,3,4,5}; // Compile Error before C++ 11

It would have resulted in a compile time error before C++11. It’s possible in C++11 because of Uniform Initialization feature of c++11 i.e.  std::initializer_list<T>

std::initializer_list

We can create light weight object of std::initializer_list<T> that will refer to an array of elements of type T i.e.

std::initialzer_list<int> data = {1,2,4,5};

So, where ever compiler see elements in braces i.e. {a, b, c} it creates a  std::initialzer_list<T>, where T is the type of elements in the list. Now, all containers i.e. vector & list etc. has a parameterized constructor that accepts this std::initializer_list<T> as an argument and insert them i.e.
vector<T>::vector<T>(initializer_list<T>  elements)
{
......
}

Therefore we can directly initialize a vector or any other container with default elements  i.e.
// Initializing a vector using c++11 initializer_list
std::vector<int> vec= {1,2,3,4,5}; // Compile Error before C++ 11

// Initializing a List using c++11 initializer_list
std::list<int> listOfElements= {1,2,3,4,5}; // Compile Error before C++ 11

Here compiler will automatically create a object of std::initialzer_list<T> and make it refer to {1,2,3,4,5} and as vector has a constructor that accepts this as an argument. Hence it will initialize the vector with all the elements in it.

Complete Example is as follows,

#include <iostream>
#include <vector>
#include <list>
int main() {
	// Initializing array with default values
	int arr[] = { 1, 2, 3, 4, 5 };

	// Print the array
	for (int i = 0; i < sizeof(arr) / sizeof(int); i++)
		std::cout << i << " , ";
	std::cout << std::endl;

	// Old Way to initialize a vector
	std::vector<int> vec1;
	for (int i = 0; i < 5; i++)
		vec1.push_back(i);

	// Initializing a vector using c++11 initializer_list
	std::vector<int> vec = { 1, 2, 3, 4, 5 }; // Compile Error before C++ 11

	// Initializing a List using c++11 initializer_list
	std::list<int> listOfElements = { 1, 2, 3, 4, 5 }; // Compile Error before C++ 11

	// Print the Vector
	for (int i : vec)
		std::cout << i << " , ";
	std::cout << std::endl;

	// Print the List
	for (int i : listOfElements)
		std::cout << i << " , ";
	std::cout << std::endl;

	return 0;
}

Output:
0 , 1 , 2 , 3 , 4 , 
1 , 2 , 3 , 4 , 5 , 
1 , 2 , 3 , 4 , 5 ,

std::initialzer_list<T> and Narrowing

 std::initialzer_list<T> also detects the narrowing of data i.e. it will detect the narrowing i.e.

std::vector<int> vec= {1,2,3,4.5,5}; // Will give compile time warnings

Complete Example is as follows,
#include <iostream>
#include <vector>
#include <map>
#include <list>
int main() {
	// Array will not detect narrowing
	int arr[] { 1, 2, 3, 4.5, 5 };

	for (int i = 0; i < sizeof(arr) / sizeof(int); i++)
		std::cout << i << " , ";
	std::cout << std::endl;

	// Initializing a vector using c++11 initializer_list
	// Will detect narrowing
	std::vector<int> vec = { 1, 2, 3, 4.5, 5 };

	for (int i : vec)
		std::cout << i << " , ";
	std::cout << std::endl;

	return 0;
}

Output:
0 , 1 , 2 , 3 , 4 , 
1 , 2 , 3 , 4 , 5 ,

Lifetime of elements in std::initialzer_list<T> is till the object which refers to it.

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