16.7 — std::initializer_list

By Alex on March 9th, 2017 | last modified by Alex on December 21st, 2020

Consider a fixed array of integers in C++:

1	int array[5];

If we want to initialize this array with values, we can do so directly via the initializer list syntax:

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#include <iostream>   int main() { int array[] { 5, 4, 3, 2, 1 }; // initializer list for (auto i : array) std::cout << i << ' ';   return 0; }

This prints:

5 4 3 2 1

This also works for dynamically allocated arrays:

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#include <iostream>   int main() { auto *array{ new int[5]{ 5, 4, 3, 2, 1 } }; // initializer list for (int count{ 0 }; count < 5; ++count) std::cout << array[count] << ' '; delete[] array;   return 0; }

In the previous lesson, we introduced the concept of container classes, and showed an example of an IntArray class that holds an array of integers:

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#include <cassert> // for assert() #include <iostream> class IntArray { private:     int m_length{};     int *m_data{}; public:     IntArray() = default;     IntArray(int length):         m_length{ length },         m_data{ new int[length]{} }     {     }     ~IntArray()     {         delete[] m_data;         // we don't need to set m_data to null or m_length to 0 here, since the object will be destroyed immediately after this function anyway     }     int& operator[](int index)     {         assert(index >= 0 && index < m_length);         return m_data[index];     }     int getLength() const { return m_length; } };   int main() { // What happens if we try to use an initializer list with this container class? IntArray array { 5, 4, 3, 2, 1 }; // this line doesn't compile for (int count{ 0 }; count < 5; ++count) std::cout << array[count] << ' ';   return 0; }

This code won’t compile, because the IntArray class doesn’t have a constructor that knows what to do with an initializer list. As a result, we’re left initializing our array elements individually:

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int main() { IntArray array(5); array[0] = 5; array[1] = 4; array[2] = 3; array[3] = 2; array[4] = 1;   for (int count{ 0 }; count < 5; ++count) std::cout << array[count] << ' ';   return 0; }

That’s not so great.

Class initialization using std::initializer_list

When a compiler sees an initializer list, it automatically converts it into an object of type std::initializer_list. Therefore, if we create a constructor that takes a std::initializer_list parameter, we can create objects using the initializer list as an input.

std::initializer_list lives in the <initializer_list> header.

There are a few things to know about std::initializer_list. Much like std::array or std::vector, you have to tell std::initializer_list what type of data the list holds using angled brackets, unless you initialize the std::initializer_list right away. Therefore, you’ll almost never see a plain std::initializer_list. Instead, you’ll see something like std::initializer_list<int> or std::initializer_list<std::string>.

Second, std::initializer_list has a (misnamed) size() function which returns the number of elements in the list. This is useful when we need to know the length of the list passed in.

Let’s take a look at updating our IntArray class with a constructor that takes a std::initializer_list.

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#include <cassert> // for assert() #include <initializer_list> // for std::initializer_list #include <iostream>   class IntArray { private: int m_length{}; int *m_data{};   public: IntArray() = default;   IntArray(int length) : m_length{ length }, m_data{ new int[length]{} } {   }   IntArray(std::initializer_list<int> list) : // allow IntArray to be initialized via list initialization IntArray(static_cast<int>(list.size())) // use delegating constructor to set up initial array { // Now initialize our array from the list int count{ 0 }; for (auto element : list) { m_data[count] = element; ++count; } }   ~IntArray() { delete[] m_data; // we don't need to set m_data to null or m_length to 0 here, since the object will be destroyed immediately after this function anyway }   IntArray(const IntArray&) = delete; // to avoid shallow copies IntArray& operator=(const IntArray& list) = delete; // to avoid shallow copies   int& operator[](int index) { assert(index >= 0 && index < m_length); return m_data[index]; }   int getLength() const { return m_length; } };   int main() { IntArray array{ 5, 4, 3, 2, 1 }; // initializer list for (int count{ 0 }; count < array.getLength(); ++count) std::cout << array[count] << ' ';   return 0; }

This produces the expected result:

5 4 3 2 1

It works! Now, let’s explore this in more detail.

Here’s our IntArray constructor that takes a std::initializer_list<int>.

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IntArray(std::initializer_list<int> list): // allow IntArray to be initialized via list initialization IntArray(static_cast<int>(list.size())) // use delegating constructor to set up initial array { // Now initialize our array from the list int count{ 0 }; for (int element : list) { m_data[count] = element; ++count; } }

On line 1: As noted above, we have to use angled brackets to denote what type of element we expect inside the list. In this case, because this is an IntArray, we’d expect the list to be filled with int. Note that we don’t pass the list by const reference. Much like std::string_view, std::initializer_list is very lightweight and copies tend to be cheaper than an indirection.

On line 2: We delegate allocating memory for the IntArray to the other constructor via a delegating constructor (to reduce redundant code). This other constructor needs to know the length of the array, so we pass it list.size(), which contains the number of elements in the list. Note that list.size() returns a size_t (which is unsigned) so we need to cast to a signed int here. We use direct initialization, rather than brace initialization, because brace initialization prefers list constructors. Although the constructor would get resolved correctly, it’s safer to use direct initialization to initialize classes with list constructors if we don’t want to use the list constructor.

The body of the constructor is reserved for copying the elements from the list into our IntArray class. For some inexplicable reason, std::initializer_list does not provide access to the elements of the list via subscripting (operator[]). The omission has been noted many times to the standards committee and never addressed.

However, there are easy ways to work around the lack of subscripts. The easiest way is to use a for-each loop here. The for-each loops steps through each element of the initialization list, and we can manually copy the elements into our internal array.

One caveat: Initializer lists will always favor a matching initializer_list constructor over other potentially matching constructors. Thus, this variable definition:

1	IntArray array { 5 };

would match to IntArray(std::initializer_list<int>), not IntArray(int). If you want to match to IntArray(int) once a list constructor has been defined, you’ll need to use copy initialization or direct initialization. The same happens to std::vector and other container classes that have both a list constructor and a constructor with a similar type of parameter

1 2	std::vector<int> array(5); // Calls std::vector::vector(std::vector::size_type), 5 value-initialized elements: 0 0 0 0 0 std::vector<int> array{ 5 }; // Calls std::vector::vector(std::initializer_list<int>), 1 element: 5

Class assignment using std::initializer_list

You can also use std::initializer_list to assign new values to a class by overloading the assignment operator to take a std::initializer_list parameter. This works analogously to the above. We’ll show an example of how to do this in the quiz solution below.

Note that if you implement a constructor that takes a std::initializer_list, you should ensure you do at least one of the following:

1. Provide an overloaded list assignment operator
2. Provide a proper deep-copying copy assignment operator

Here’s why: consider the above class (which doesn’t have an overloaded list assignment or a copy assignment), along with following statement:

1	array = { 1, 3, 5, 7, 9, 11 }; // overwrite the elements of array with the elements from the list

First, the compiler will note that an assignment function taking a std::initializer_list doesn’t exist. Next it will look for other assignment functions it could use, and discover the implicitly provided copy assignment operator. However, this function can only be used if it can convert the initializer list into an IntArray. Because { 1, 3, 5, 7, 9, 11 } is a std::initializer_list, the compiler will use the list constructor to convert the initializer list into a temporary IntArray. Then it will call the implicit assignment operator, which will shallow copy the temporary IntArray into our array object.

At this point, both the temporary IntArray’s m_data and array->m_data point to the same address (due to the shallow copy). You can already see where this is going.

At the end of the assignment statement, the temporary IntArray is destroyed. That calls the destructor, which deletes the temporary IntArray’s m_data. This leaves our array variable with a hanging m_data pointer. When you try to use array->m_data for any purpose (including when array goes out of scope and the destructor goes to delete m_data), you’ll get undefined results (and probably a crash).

Rule

If you provide list construction, it’s a good idea to provide list assignment as well.

Summary

Implementing a constructor that takes a std::initializer_list parameter allows us to use list initialization with our custom classes. We can also use std::initializer_list to implement other functions that need to use an initializer list, such as an assignment operator.

Quiz time

Question #1

Using the IntArray class above, implement an overloaded assignment operator that takes an initializer list.

The following code should run:

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int main() { IntArray array { 5, 4, 3, 2, 1 }; // initializer list for (int count{ 0 }; count < array.getLength(); ++count) std::cout << array[count] << ' ';   std::cout << '\n';   array = { 1, 3, 5, 7, 9, 11 };   for (int count{ 0 }; count < array.getLength(); ++count) std::cout << array[count] << ' ';   std::cout << '\n';   return 0; }

This should print:

5 4 3 2 1 
1 3 5 7 9 11

Show Solution

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#include <cassert> // for assert() #include <initializer_list> // for std::initializer_list #include <iostream>   class IntArray { private: int m_length {}; int *m_data {};   public: IntArray() = default;   IntArray(int length) : m_length{ length }, m_data{ new int[length]{} } {   }   IntArray(std::initializer_list<int> list) : // allow IntArray to be initialized via list initialization IntArray(static_cast<int>(list.size())) // use delegating constructor to set up initial array { // Now initialize our array from the list int count{ 0 }; for (auto element : list) { m_data[count] = element; ++count; } }   ~IntArray() { delete[] m_data; // we don't need to set m_data to null or m_length to 0 here, since the object will be destroyed immediately after this function anyway }   IntArray(const IntArray&) = delete; // to avoid shallow copies IntArray& operator=(const IntArray& list) = delete; // to avoid shallow copies   IntArray& operator=(std::initializer_list<int> list) { // If the new list is a different size, reallocate it int length{ static_cast<int>(list.size()) }; if (length != m_length) { delete[] m_data; m_length = length; m_data = new int[length]{}; }   // Now initialize our array from the list int count{ 0 }; for (auto element : list) { m_data[count] = element; ++count; }   return *this; }   int& operator[](int index) { assert(index >= 0 && index < m_length); return m_data[index]; }   int getLength() const { return m_length; } };   int main() { IntArray array { 5, 4, 3, 2, 1 }; // initializer list for (int count{ 0 }; count < array.getLength(); ++count) std::cout << array[count] << ' ';   std::cout << '\n';   array = { 1, 3, 5, 7, 9, 11 };   for (int count{ 0 }; count < array.getLength(); ++count) std::cout << array[count] << ' ';   std::cout << '\n';   return 0; }

16.x -- Chapter 16 comprehensive quiz

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16.6 -- Container classes

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179 comments to 16.7 — std::initializer_list

• chai

  February 25, 2021 at 4:33 am · Reply

  hi there,
  In the IntArray assignment operator overload any idea why I get a buffer overrun warning?

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  auto length{ new_list.size() };             if (length != m_size)         {             //If not then  de-allocate and resize.             delete[] m_intArray;               m_size = length;             //m_intArray = new int[length] {};    // No warning if this line is active instead.             m_intArray = new int[new_list.size()] {};    //buffer overrun warning         }

• chai

  February 23, 2021 at 4:32 pm · Reply

  hi, just wondering if:

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  IntArray(std::initializer_list<int> list): // allow IntArray to be initialized via list initialization         IntArray(static_cast<int>(list.size())) // use delegating constructor to set up initial array     {         // Now initialize our array from the list         int count{ 0 };         for (int element : list)         {             m_data[count] = element;// is this better than m_data [ count++ ] = element; ?             ++count;         }     }

  thanks.

• chai

  February 23, 2021 at 4:28 pm · Reply

  Hi, I am just wondering if

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  IntArray(std::initializer_list<int> list): // allow IntArray to be initialized via list initialization         IntArray(static_cast<int>(list.size())) // use delegating constructor to set up initial array     {         // Now initialize our array from the list         int count{ 0 };         for (int element : list)         {             m_data[count] = element;// is this better than writing m_data[ count++ ] = element; ?             ++count;         }     }

  thanks.

• yeokaiwei

  December 22, 2020 at 3:21 am · Reply

  Dear Alex,
  Your sudden changeon the ordering of the syllabus is extremely confusing (22 December 2020).

  What are the the reserved expansions for?

  How do we do continue on the syllabus?

  • Alex

    December 22, 2020 at 10:00 am · Reply

    The main page will always show the recommended reading order.

    Yesterday 2 main changes were made:
    * Chapter 7 was moved ahead of Chapter 8 (previously chapter S)
    * The remaining chapters were renumbered to remove some of the temporary lettering and make room for future expansion. You can ignore the reserved expansions for now -- some of the existing chapters are too long and will be split/reorganized in the future.

    • yeokaiwei

      December 23, 2020 at 10:51 am · Reply

      Hi Alex,
      Thank you for the lessons.

      I'll follow your advice then.

      The numbering has changed quite a bit so my folders have a big gap in between Chapter 10 to Chapter 17.

      Could the learncpp website upload images in the WordPress plugin?

      It's hard to show error messages.

• Lucky Abby

  November 17, 2020 at 11:00 pm · Reply

  I tried to initialize array object using std::initializer_list object but the compiler complains unable to convert the list into int (in my case), is it the only way to use initializer_list is to wrap the array inside class?

  • nascardriver

    November 19, 2020 at 8:11 am · Reply

    Can you post your code?

    • Lucky Abby

      November 20, 2020 at 4:22 am · Reply

      Hi Nascardriver,

      here is my code:

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      #include <iostream> #include <initializer_list> #include <array>   int main() {     std::initializer_list<int> iList{2,4,6,8,10};     std::array<int,5> array{iList};     for(auto e:array) std::cout << e << ' '; }

      • nascardriver

        November 20, 2020 at 9:02 am · Reply

        Built-in arrays and `std::array` (Which is a built-in array in disguise) cannot be initialized from an `std::initialized_list`. You can copy the elements from the `std::initializer_list` into the `std::array`:

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        #include <algorithm> #include <array> #include <initializer_list> #include <iostream>   int main() {   std::initializer_list<int> iList{2, 4, 6, 8, 10};   std::array<int, 5> array{};     std::copy_n(iList.begin(), std::min(array.size(), iList.size()), array.begin());     for (auto e : array) std::cout << e << ' '; }

        If you have otherwise made sure that `array.size() >= iList.size()`, you can use this instead

        1	std::ranges::copy(iList, array.begin());

        • Lucky Abby

          November 20, 2020 at 10:46 am · Reply

          Thank you nascardriver.

          I found out that new c++20 features like std::ranges::copy and few ranges copy is quite useful for other cases, cmiimw.

• Mince

  September 26, 2020 at 2:18 pm · Reply

  Visual Studio 2019 gives me a C6386 warning (Buffer overrun while writing to 'm_arr': the writable size is 'size*4' bytes, but '8' bytes might be written.)
  for the following implementation of the quiz question:

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  // List Assignment IntArray& IntArray::operator=(const std::initializer_list<int> source) {     int size = static_cast<int>(source.size());     assert(size > 0 && "Array size must be greater than 0");       delete[] m_arr;     m_arr = new int[size] {};     m_len = size;       int i{ 0 };     for (int num : source)     {         if (i < size) // Fix false C6386 warning             m_arr[i++] = num;     }       return *this; }

  The code works fine in the cases I tested and I don't see any cases where var "i" would overflow "m_arr". I'd just like to make sure that this indeed an error on Visual Studio's part and I'm not missing anything.

  • nascardriver

    September 27, 2020 at 1:33 am · Reply

    Yes, this is a common problem in VS. Avoid C-style arrays wherever possible and VS should be quiet.

• gabrielepompa88

  September 25, 2020 at 9:03 am · Reply

  In lesson [P.6.16 - An introduction to std::vector](https://www.learncpp.com/cpp-tutorial/6-16-an-introduction-to-stdvector/) you made this example in which - to initialize a std::vector<int> of five zeros - we had to use direct initialization (instead of brace initialization).

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  #include <iostream> #include <vector> int main() {     // Using direct initialization, we can create a vector with 5 elements,     // each element is a 0. If we use brace initialization, the vector would     // have 1 element, a 5.     std::vector<int> array(5);     std::cout << "The length is: " << array.size() << '\n';     for (int i : array)         std::cout << i << ' ';     std::cout << '\n';     return 0; }

  If I understand correctly std::initializer_list, the reason should be that, internally, std::vector uses a std::initializer list kind of constructor, which thus favor a matching initializer_list constructor over other potentially matching constructors. In other words,

  1	std::vector<int> array{ 5 };

  would resolve into a vector of one element (a five) because we would be (unintentionally) calling its initializer_list constructor and 5 would be interpreted as size parameter? If my understanding is correct, it can be useful to mention at this point. Thanks

  • nascardriver

    September 26, 2020 at 2:08 am · Reply

    I added an example with `std::vector` to the lesson. Thanks for the suggestion!

    • gabrielepompa88

      September 26, 2020 at 8:59 am · Reply

      Thank you nascardriver, I'm really enjoying this! Now, with your example, the connection is clear. Maybe you can also refer to this section on std::initializer_list from the P.6.16 on std::vector from where that example comes from.

      For other readers: I messed up the final part of my previous comment:  

      1	5 would be interpreted as size parameter?

      should be

      1	5 would be interpreted as the only element of array and not as the length (size parameter) of a zero-initialized array?

• Gamer_to_be

  September 18, 2020 at 5:36 pm · Reply

  If I want to iterate trough elements of IntArray through for-each loop, does the following make sense?

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  int* end()     {         if (!m_data)             return nullptr;         return &m_data[m_length];     }     int* begin()     {         if (!m_data)             return nullptr;         return &m_data[0];     }

  //main

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  #include <iostream> #include "IntArray2.h" #include <vector> int main() {          IntArray array{ 1,2,3,8,10 };          std::cout << *array.begin();              for(const auto& item:array)        std::cout << item << ' ';          return 0; }

  • nascardriver

    September 19, 2020 at 11:21 pm · Reply

    You cannot access

    1	m_data[m_length]

    This element doesn't exist, trying to access it causes undefined behavior. The same goes for `m_data[0]` if the array is empty. Use pointer arithmetic instead.

    Binding references to fundamental types is slower than copying them. Your range-based for-loop shouldn't use references, because the elements are `int`.