std::initializer_list in C++ 11

The std::initializer_list class template was added in C++ 11 and contains many built-in functions to perform various operations with the initializer list. It provides member functions like a size(), begin(), end(), and constructor to construct, iterate, and access elements of the initializer list.

To use initializer_list, you need to include the <initializer_list> header in your C++ program.

std::initializer_list in C++

In C++, the std::initializer_list is a class template that allows us to initialize a lightweight object with a list of values. An initializer list is used to set values to variables, arrays, classes, functions, constructors of classes, and standard containers like vectors in a convenient way.

Syntax

initializer_list<T> name_of_list= { };

• Braced Initializer is used to construct the object for initializer_list.
• It is generally implemented as a wrapper over arrays.
• Unlike standard containers like vectors, copying the object of the initializer list does not copy the entire elements to the copied list. But both the original and copied object of the initializer list contain the same elements.

Examples of std::initializer_list in C++

Example 1: The below example demonstrates the use of an initializer list in C++.

Numbers in the list are: 2 4 6 8 10 12 

Note Member initializer list and initializer_list are not the same. Both are different, it should not be confused with each other.

Example 2: Program to illustrate the use of initializer_list to construct the objects.

Elements of Integer type are: 1 2 3 4 5 

Elements of double type are: 1.1 2.2 3.3 4.4 5.5 

Explanation: "std::initializer_list" should be used for immediate consumption, such as iterating in a constructor or passing to a function. It must not be stored as a data member because it does not own its storage and its lifetime is limited to the full expression in which it is created.

Member Functions of std::initializer_list

The following are some of the commonly used member functions of the std::initialzer_list class:

Applications of initializer_list

Apart from the construction of objects, initializer list can be used in the following cases:

1. Variable Function Parameters

<initializer_list> are used to pass a variable number of arguments to a function.

Example: The below example demonstrates the passing of an initializer list to a function.

Size of myList: 5
Elements of myList: 1 2 3 4 5 

2. Store Data in Contigious Memory

The elements of the initializer_list container can be used to store data as it is a lightweight container.

Example: The below example demonstrates the use of range-based loops to access elements of initializer_list.

1 2 3 4 5 

3. Initialize Standard Containers

The initializer_list can be used for initializing the standard containers with a List of Elements like vectors.

Example: The below example demonstrates the use of initializer_list to initialize standard containers.

1 2 3 4 5 

4. Initializer Lists as Return Types

The initializer_list can be used as a return type to return lists from any function. It allows the function to return multiple values.

Example: The below example demonstrates the use of initializer_list as a return type.

1 2 3 4 5 

Limitations of initializer_list

The initializer lists also have some limitations associated with it:

• Size cannot be changed: The size of initializer_list is fixed at compile time. It does not have a dynamic nature as a standard container such as a vector. The size of the initializer cannot be changed once it has been created.
• Cannot access the elements randomly: initializer_list supports only forward iteration. We cannot access the desired or random element using the index as standard containers.
• Immutable elements: The elements within an initializer_list are immutable. Once the list is created, the values cannot be modified. Any attempt to modify the elements through the iterator or by any other means will result in a compilation error.