Edmund's blog

Writing code.

Reversed range-based for loop

With a range-based for loop you can easily iterate over all elements in a container.

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std::list<int> items{1, 2, 3};

for (auto&& item : items)
    std::cout << item << ' ';

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1 2 3

But what if you want to reverse iterate over the items?

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#include "ReverseAdapter.h"

for (auto&& item : MakeReverse(items))
    std::cout << item << ' ';

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The C++ standard library does not provide a reverse adapter, so we have to write our own. This one works for both const and non-const containers.

ReverseAdapter.h
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#ifndef REVERSE_ADAPTER_H
#define REVERSE_ADAPTER_H


template<class T>
class ReverseAdapter
{
public:
    ReverseAdapter(T& container) : m_container(container) {}
    typename T::reverse_iterator begin() { return m_container.rbegin(); }
    typename T::reverse_iterator end() { return m_container.rend(); }

private:
    T& m_container;
};

template<class T>
class ConstReverseAdapter
{
public:
    ConstReverseAdapter(const T& container) : m_container(container) {}
    typename T::const_reverse_iterator begin() { return m_container.rbegin(); }
    typename T::const_reverse_iterator end() { return m_container.rend(); }

private:
    const T& m_container;
};


template<class T>
ReverseAdapter<T> MakeReverse(T& container)
{
    return ReverseAdapter<T>(container);
}

template<class T>
ConstReverseAdapter<T> MakeReverse(const T& container)
{
    return ConstReverseAdapter<T>(container);
}


#endif  // REVERSE_ADAPTER_H