Range-based for loop on a dynamic array?

Question

There is a range-based for loop with the syntax:

for(auto& i : array)

It works with constant arrays but not with pointer based dynamic ones, like

int *array = new int[size];
for(auto& i : array)
   cout<< i << endl;

It gives errors and warnings about failure of substitution, for instance:

Error] C:\Users\Siegfred\Documents\C-Free\Temp\Untitled2.cpp:16:16: error: no matching function for call to 'begin(int*&)'

How do I use this new syntax with dynamic arrays?

Answer 1

To make use of the range-based for-loop you have to provide either begin() and end() member functions or overload the non-member begin() and end() functions. In the latter case, you can wrap your range in a std::pair and overload begin() and end() for those:

    namespace std {
        template <typename T> T* begin(std::pair<T*, T*> const& p)
        { return p.first; }
        template <typename T> T* end(std::pair<T*, T*> const& p)
        { return p.second; }
    }

Now you can use the for-loop like this:

    for (auto&& i : std::make_pair(array, array + size))
        cout << i << endl;

Note, that the non-member begin() and end() functions have to be overloaded in the std namespace here, because pair also resides in namespace std. If you don't feel like tampering with the standard namespace, you can simply create your own tiny pair class and overload begin() and end() in your namespace.

Or, create a thin wrapper around your dynamically allocated array and provide begin() and end() member functions:

    template <typename T>
    struct wrapped_array {
        wrapped_array(T* first, T* last) : begin_ {first}, end_ {last} {}
        wrapped_array(T* first, std::ptrdiff_t size)
            : wrapped_array {first, first + size} {}

        T*  begin() const noexcept { return begin_; }
        T*  end() const noexcept { return end_; }

        T* begin_;
        T* end_;
    };

    template <typename T>
    wrapped_array<T> wrap_array(T* first, std::ptrdiff_t size) noexcept
    { return {first, size}; }

And your call site looks like this:

    for (auto&& i : wrap_array(array, size))
         std::cout << i << std::endl;

Example