How Can I Reverse Iterate Through a Container Using C 11's Range-Based for-Loops?

Reverse Iteration in Range-Based for-Loops with C 11

In C , the range-based for-loop provides a convenient way to iterate through the elements of a container. However, there is no built-in container adapter that can reverse the direction of iterators, making it challenging to iterate over a container in reverse order.

Concept: Container Adapters

Before exploring a solution, it's important to understand the concept of container adapters. These are objects that wrap a container and provide a modified view of its elements. Adapters can filter, transform, or otherwise manipulate the elements returned by the underlying container.

Solution: Boost's reverse Adapter

To reverse the direction of iterators and enable reverse iteration in range-based for-loops, we can leverage the boost::adaptors::reverse adapter from the Boost C Libraries. This adapter wraps a container and returns a reversed sequence of its elements.

Example

Consider the following example:

#include <list>
#include <iostream>
#include <boost/range/adaptor/reversed.hpp>

int main() {
    std::list<int> x { 2, 3, 5, 7, 11, 13, 17, 19 };

    // Iterate in reverse order
    for (auto i : boost::adaptors::reverse(x))
        std::cout << i << '\n';

    // Iterate in forward order
    for (auto i : x)
        std::cout << i << '\n';
}

In this example, the reversed adapter is used to iterate over the list x in reverse order. The output will be:

19
17
13
11
7
5
3
2

Note: The boost::adaptors::reverse adapter is part of the Boost C Libraries, which is a collection of open-source libraries that can be downloaded and integrated into existing C projects.

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