19.2 — Dynamically allocating arrays

In addition to dynamically allocating single values, we can also dynamically allocate arrays of variables. Unlike a fixed array, where the array size must be fixed at compile time, dynamically allocating an array allows us to choose an array length at runtime (meaning our length does not need to be constexpr).

Author’s note

In these lessons, we’ll be dynamically allocating C-style arrays, which is the most common type of dynamically allocated array.

While you can dynamically allocate a std::array, you’re usually better off using a non-dynamically allocated std::vector in this case.

To allocate an array dynamically, we use the array form of new and delete (often called new[] and delete[]):

#include <cstddef>
#include <iostream>

int main()
{
    std::cout << "Enter a positive integer: ";
    std::size_t length{};
    std::cin >> length;

    int* array{ new int[length]{} }; // use array new.  Note that length does not need to be constant!

    std::cout << "I just allocated an array of integers of length " << length << '\n';

    array[0] = 5; // set element 0 to value 5

    delete[] array; // use array delete to deallocate array

    // we don't need to set array to nullptr/0 here because it's going out of scope immediately after this anyway

    return 0;
}

Because we are allocating an array, C++ knows that it should use the array version of new instead of the scalar version of new. Essentially, the new[] operator is called, even though the [] isn’t placed next to the new keyword.

The length of dynamically allocated arrays has type std::size_t. If you are using a non-constexpr int, you’ll need to static_cast to std::size_t since that is considered a narrowing conversion and your compiler will warn otherwise.

Note that because this memory is allocated from a different place than the memory used for fixed arrays, the size of the array can be quite large. You can run the program above and allocate an array of length 1,000,000 (or probably even 100,000,000) without issue. Try it! Because of this, programs that need to allocate a lot of memory in C++ typically do so dynamically.

Dynamically deleting arrays

When deleting a dynamically allocated array, we have to use the array version of delete, which is delete[].

This tells the CPU that it needs to clean up multiple variables instead of a single variable. One of the most common mistakes that new programmers make when dealing with dynamic memory allocation is to use delete instead of delete[] when deleting a dynamically allocated array. Using the scalar version of delete on an array will result in undefined behavior, such as data corruption, memory leaks, crashes, or other problems.

One often asked question of array delete[] is, “How does array delete know how much memory to delete?” The answer is that array new[] keeps track of how much memory was allocated to a variable, so that array delete[] can delete the proper amount. Unfortunately, this size/length isn’t accessible to the programmer.

Dynamic arrays are almost identical to fixed arrays

In lesson 17.8 -- C-style array decay, you learned that a fixed array holds the memory address of the first array element. You also learned that a fixed array can decay into a pointer that points to the first element of the array. In this decayed form, the length of the fixed array is not available (and therefore neither is the size of the array via sizeof()), but otherwise there is little difference.

A dynamic array starts its life as a pointer that points to the first element of the array. Consequently, it has the same limitations in that it doesn’t know its length or size. A dynamic array functions identically to a decayed fixed array, with the exception that the programmer is responsible for deallocating the dynamic array via the delete[] keyword.

Initializing dynamically allocated arrays

If you want to initialize a dynamically allocated array to 0, the syntax is quite simple:

int* array{ new int[length]{} };

Prior to C++11, there was no easy way to initialize a dynamic array to a non-zero value (initializer lists only worked for fixed arrays). This means you had to loop through the array and assign element values explicitly.

int* array = new int[5];
array[0] = 9;
array[1] = 7;
array[2] = 5;
array[3] = 3;
array[4] = 1;

Super annoying!

However, starting with C++11, it’s now possible to initialize dynamic arrays using initializer lists!

int fixedArray[5] = { 9, 7, 5, 3, 1 }; // initialize a fixed array before C++11
int* array{ new int[5]{ 9, 7, 5, 3, 1 } }; // initialize a dynamic array since C++11
// To prevent writing the type twice, we can use auto. This is often done for types with long names.
auto* array{ new int[5]{ 9, 7, 5, 3, 1 } };

Note that this syntax has no operator= between the array length and the initializer list.

For consistency, fixed arrays can also be initialized using uniform initialization:

int fixedArray[]{ 9, 7, 5, 3, 1 }; // initialize a fixed array in C++11
char fixedArray[]{ "Hello, world!" }; // initialize a fixed array in C++11

Explicitly stating the size of the array is optional.

Resizing arrays

Dynamically allocating an array allows you to set the array length at the time of allocation. However, C++ does not provide a built-in way to resize an array that has already been allocated. It is possible to work around this limitation by dynamically allocating a new array, copying the elements over, and deleting the old array. However, this is error prone, especially when the element type is a class (which have special rules governing how they are created).

Consequently, we recommend avoiding doing this yourself. Use std::vector instead.

Quiz time

Question #1

Write a program that:

std::string supports comparing strings via the comparison operators < and >. You don’t need to implement string comparison by hand.

Your output should match this:

How many names would you like to enter? 5
Enter name #1: Jason
Enter name #2: Mark
Enter name #3: Alex
Enter name #4: Chris
Enter name #5: John

Here is your sorted list:
Name #1: Alex
Name #2: Chris
Name #3: Jason
Name #4: John
Name #5: Mark

A reminder

You can use std::getline() to read in names that contain spaces (see lesson 5.9 -- Introduction to std::string).

A reminder

To use std::sort() with a pointer to an array, calculate begin and end manually

std::sort(array, array + arrayLength);

Show Solution

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