Access pointer in range based for loop

Question

I have a dynamic array wrapper template like this:

class Wrapper {
  public:
    T* const start;
    T* const end;
    T* begin() const { return start; }
    T* end() const { return end; }
    /*
       more code
    */
};

which gives value and reference access via the loops:

Wrapper<T> wrapper;
for(auto val : wrapper) {
  //do smth to value
}
for(auto& ref : wrapper) {
  //do smth to reference
}

. I now want to create a range based for loop which is equivalent to this:

for(auto ptr = wrapper.start; ptr != wrapper.end; ptr++) {
  //do smth to pointer
}

, i.e. I want a range based loop over wrapper to give access to a pointer. Is there a way to do this without creating an array of pointers to pointers inside my wrapper?

Edit:

Dani's solution in the comment works as has already been pointed out here. I was actually wondering if there was a way to make the following syntax:

for(auto ptr : wrapper) {
  //do smth
}

work as the C-style for loop above.

You mean something like: for(auto& ref : wrapper) { auto ptr = &ref; } ? Can I somehow alter the begin() and end() methods to get this working implicitly insite this: for(auto ptr : wrapper) { /* do smth */ } ? — Yuri Lifanov
– Yuri Lifanov, Commented Oct 13, 2016 at 19:48

Chris Drew · Accepted Answer · 2016-10-14 08:16:46Z

1

You could write your own custom iterator that returns a T* when dereferenced

class iterator {
    T* ptr;
public:
    explicit iterator(T* ptr) : ptr(ptr) {}
    iterator& operator++() {ptr++; return *this;}
    bool operator!=(iterator other) const {return ptr != other.ptr;}
    T* operator*() const {return ptr;}
};

And then return this iterator from your begin and end functions.

I think it is slightly surprising behavior for an array wrapper to have though.

Live demo.

edited Oct 14, 2016 at 8:16

answered Oct 13, 2016 at 20:48

Chris Drew

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1 Comment

Yuri Lifanov Over a year ago

That looks great. Thanks. The array wrapper is just a minimal example.

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