16.7 — std::initializer_list

Consider a fixed array of integers in C++:

int array[5];

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

#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:

#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:

#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:

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.

#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>.

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 ranged-based for loop 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:

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

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:

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 array->m_data as a dangling 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).

Best practice

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:

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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