Small Steps V — Making Your Own Types

Reading without copying

Read without copying.

What you will learn

A const reference lets you read the original without copying it, while promising not to change it through that reference.

Try it

int sum(const Array<int>& numbers)
{
    int total = 0;

    for (int i = 0; i < numbers.length(); i = i + 1)
        total = total + numbers[i];

    return total;
}

void small_main()
{
    Array<int> numbers = {3, 7, 2, 9, 4};

    print("Sum: ", sum(numbers));
}

In const Array<int>& numbers, & marks a reference to the original and const makes it read-only. The output is Sum: 25.

The sum needs no copy of the whole array. Attempting to change it causes a compilation error. The earlier Array<int> parameter is also valid, but copying can be costly for large arrays.

Exercise

Change the earlier find_largest(Array<int> numbers) to use const Array<int>&. The function does not modify the Array.

int find_largest(Array<int> numbers)
{
    int largest = numbers[0];

    for (int i = 1; i < numbers.length(); i = i + 1)
        if (numbers[i] > largest)
            largest = numbers[i];

    return largest;
}

void small_main()
{
    Array<int> numbers = {4, 12, 3, 9};
    print(find_largest(numbers));
}

Hint

Change only the parameter to const Array<int>& numbers; the algorithm can stay the same.

int find_largest(const Array<int>& numbers)
{
    int largest = numbers[0];

    for (int i = 1; i < numbers.length(); i = i + 1)
        if (numbers[i] > largest)
            largest = numbers[i];

    return largest;
}

void small_main()
{
    Array<int> numbers = {4, 12, 3, 9};
    print(find_largest(numbers));
}