override operator[] and call method

Question

I implemented SpareArray class. It's a big one, so there is no sense to show it all, but what is important is that is has insert method, which makes it possible to insert values at any index (from zero to "infinity"). So, my client code may look like so:

auto arr = new SpareArray<int>{};
arr.insert(100, 1);

The above code inserts value 1 at index 100. Now, I want to be able to use square brakets notation to get the same result:

auto arr = new SpareArray<int>{};
arr[100] = 1; //I want this line to internally call arr.insert(100, 1);

So, how can operator[] be difined to internally call insert method? I need this call, because insert method has some business logic and I want this business logic to take place also in this case (in case of []).

Possible duplicate of [What are the basic rules and idioms for operator overloading?](https://stackoverflow.com/questions/4421706/what-are-the-basic-rules-and-idioms-for-operator-overloading) — , Feb 04 '18 at 11:21
@TheDude. I address a concrete issue. You could say, that my question is a possible duplicate of what can be found in C++ reference. IMHO. I may be wrong. — Jacobian, Feb 04 '18 at 11:22
And what do you want to happen when the client doesn't assign a value, when it reads one that isn't there? — StoryTeller - Unslander Monica, Feb 04 '18 at 11:23
And why can't you simply use your `insert()` function encapsulated in that operator overload? Mind to elaborate about that please? — , Feb 04 '18 at 11:24
Obviously, but `arr[100]` has to have *some* value. That's according to the language itself. — StoryTeller - Unslander Monica, Feb 04 '18 at 11:24
@Jacobian You can do as done with some standard containers, have a `const` and a non `const`version. — , Feb 04 '18 at 11:25
[How much research effort is expected of Stack Overflow users? - Meta StackOverflow](https://meta.stackoverflow.com/a/261593/5267751) — user202729, Feb 04 '18 at 11:27
When it did not insert one, it will print a default value (business logic of this SpareArray, where default values are not stored). — Jacobian, Feb 04 '18 at 11:27
So why can't you just insert the default value, and return a reference to it for modification? — StoryTeller - Unslander Monica, Feb 04 '18 at 11:28

Rakete1111 · Accepted Answer · 2018-02-04T11:31:49.173

1

You can return a proxy that does the final call to insert and acts as a wrapper around an element of your array.

template<typename T>
struct Proxy {
  Proxy(SpareArray<T> &This, std::size_t Index) : This(This), Index(Index) {}

  T &operator=(const T &Value) {
    This.insert(Index, Value);
    return This.get(Index); // or however you get an element
  }

  operator T&() { return This.get(Index); }
  operator const T&() const { return This.get(Index); }

private:
  SpareArray<T> &This;
  std::size_t Index;
};

For simplicity, I didn't add more but I would expect such a class to have proper noexcept semantics and a move assignment operator, for example. You also might want to make sure that you delete the copy constructors and such.

edited Feb 04 '18 at 11:31

answered Feb 04 '18 at 11:28

Rakete1111

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Thank you, sir! I got the idea and will check it. – Jacobian Feb 04 '18 at 11:31
Would you be so kind to show how this proxy can be used inside some arbitrary class `SpareArray`? Just an idea would be enough. Now when I try to make it a friend class inside SpareArray and return it from operator[] override, I get a ton of error messages. – Jacobian Feb 04 '18 at 12:18
@Jacobian Why does the proxy have to be a friend? What are the error messages? I can only guess because there are lots of things that could be wrong. – Rakete1111 Feb 04 '18 at 13:04
I will address it in a new question, that I'm just about to ask at SO. – Jacobian Feb 04 '18 at 13:06
If you have some time, please, have a look - https://stackoverflow.com/questions/48608414/templated-proxy-design-pattern – Jacobian Feb 04 '18 at 13:13
Sorry for bothering you once again. I have a tiny question. Now, this construct started to work: `auto arr = new SpareArray{}; arr[100] = 1;`. But this one stopped working - `std::cout << arr[100] << std::endl;`. Even though, my `SpareArray` class has such an overload: `T const& operator[] (int index) const {...}`. It seems like compiler still calls here proxied version of `operator[]`. Probably, you know what may be wrong with that. – Jacobian Feb 04 '18 at 14:37
@Jacobian Do what I did, because it seems like you didn't: Always return the proxy object, and provide a conversion operator inside the proxy object so that it implicitly converts itself to the element. – Rakete1111 Feb 04 '18 at 14:40

override operator[] and call method

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