Welcome to the lesson on Boost.Range! Modern C++ emphasizes cleaner, more readable, and maintainable code. A powerful tool to achieve this is range-based operations, particularly using Boost.Range. This lesson will introduce you to Boost.Range, showing how to perform range-based operations efficiently.

By the end, you’ll understand the basic concepts of Boost.Range, the benefits of using it, and how to apply it to tasks like filtering, transforming, and combining ranges.

In C++, a range is a sequence of elements you can iterate over using iterators. Unlike traditional containers (like std::vector), ranges are more flexible, allowing operations like filtering and transformation directly on the range. Using ranges brings clear advantages: improved readability, maintainability, and less boilerplate code. You don’t need to know how ranges are implemented, only how to use them, making your code more reusable.

Boost.Range, a part of the Boost libraries, provides range-based algorithms and adaptors. It lets you perform actions on sequences with more readable syntax and less boilerplate code. An essential namespace in Boost.Range is boost::adaptors. This namespace, a declarative space that holds a set of related classes and functions, contains various adaptors to perform complex operations concisely.

As a reminder, in C++, a namespace is a declarative region that provides a scope to the identifiers (names of types, functions, variables, etc.) inside it. Namespaces are used to organize code into logical groups and to avoid name conflicts that can occur especially when your code base includes multiple libraries.

Let's look at a simple example using Boost.Range:

C++
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1#include <boost/range/adaptor/filtered.hpp>
2#include <vector>
3#include <iostream>
4
5bool is_even(int x) {
6    return x % 2 == 0;
7}
8
9int main() {
10    std::vector<int> nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
11    auto evens = nums | boost::adaptors::filtered(is_even);
12
13    for (int n : evens) {
14        std::cout << n << " ";
15    }
16    return 0;
17// Output: 2 4 6 8
18}

Let's break down what the code snippet does:

• Include the required Boost Range Headers. Here, we use filtered to filter our data.
• Define the Filter Function is_even.
• Create a sample vector for testing.
• Apply the filter. Let's focus on this part more:

C++
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1auto evens = nums | boost::adaptors::filtered(is_even);

This line creates a new range, evens, which includes only the even elements from nums. Here, boost::adaptors::filtered(is_even) takes the predicate is_even and filters the elements of nums. The | operator applies this filter to nums. It is called the pipe operator, and we will talk more about it later this course. By now, just remember how to use it!

Instead of including each adaptor individually like <boost/range/adaptor/filtered.hpp>, you can include all adaptors using <boost/range/adaptors.hpp>. It depends on your code style. Usually, including all adaptors is beneficial in terms of the code style if you use multiple adaptors within one code.

Boost.Range offers several other adaptors:

• boost::adaptors::transformed: For transforming elements
• boost::adaptors::reversed: For reversing the range

Using boost::adaptors::transformed:

C++
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1#include <boost/range/adaptor/transformed.hpp>
2#include <vector>
3#include <iostream>
4
5int square(int x) {
6    return x * x;
7}
8
9int main() {
10    std::vector<int> nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
11    auto squares = nums | boost::adaptors::transformed(square);
12    
13    for (int n : squares) {
14        std::cout << n << " ";
15    }
16    return 0;
17// Output: 1 4 9 16 25 36 49 64 81
18}

This code squares each element in nums. As you can see, the way we use transformed is very similar to the way we use filtered. This uniformity makes it easy to work with this library. In this case, transformed(square) squares each number in the vector nums.

Using boost::adaptors::reversed:

C++
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1#include <boost/range/adaptor/reversed.hpp>
2#include <vector>
3#include <iostream>
4
5int main() {
6    std::vector<int> nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
7    auto reversed_nums = nums | boost::adaptors::reversed;
8    
9    for (int n : reversed_nums) {
10        std::cout << n << " ";
11    }
12    return 0;
13// Output: 9 8 7 6 5 4 3 2 1
14}

In this example, the elements of nums are iterated in reverse order using boost::adaptors::reversed. The usage of reversed is straightforward, but note that it doesn’t take any arguments. It follows a similar pattern to filtered and transformed but simpler. This consistency simplifies learning and applying different adaptors.

Another useful feature provided by Boost.Range is the boost::distance function, which helps determine the number of elements in a range. This can be particularly useful when dealing with custom ranges or iterators.

Here's a simple example to demonstrate its usage:

C++
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1#include <boost/range/distance.hpp>
2#include <boost/range/adaptor/filtered.hpp>
3#include <vector>
4#include <iostream>
5
6bool is_even(int x) {
7    return x % 2 == 0;
8}
9
10int main() {
11    std::vector<int> nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
12    auto evens = nums | boost::adaptors::filtered(is_even);
13    
14    std::cout << "Number of even numbers: " << boost::distance(evens) << std::endl;
15    return 0;
16// Output: Number of even numbers: 4
17}

In this example, boost::distance(evens) calculates the number of elements in the filtered range evens. This demonstrates how you can use boost::distance to easily determine the size of a range, enhancing the utility and flexibility of range-based operations.

Pros:

• Readable Code: Operations on ranges are expressed declaratively, making the code more readable.
• Less Boilerplate: There's less boilerplate for common operations.
• Composable: Operations can be easily chained for cleaner code, which we will explore more in the next units.

Cons:

• Performance Overhead: Using adaptors can add a slight performance overhead.

To recap:

• Discussed the importance and advantages of range-based operations in C++.
• Introduced the Boost.Range library and its key components, particularly boost::adaptors.
• Walked through a code example showing how to filter a range using boost::adaptors::filtered.
• Explored the boost::adaptors::transformed adaptor and combining multiple adaptors for complex operations.
• Discussed the pros and cons of using Boost.Range.
• Demonstrated the utility of boost::distance in determining the size of a range.

Now it’s time to put what you've learned into practice. In the upcoming practice session, you will get hands-on experience with filtering, transforming, and combining ranges using the Boost.Range library. Apply the concepts discussed in this lesson and explore additional adaptors to deepen your understanding. Good luck!