Changing the construction of std::array

Question

I'd like to change the construction of std::array<unsigned int, size> the following way:

myarray a {}; should construct [0,1,2,3,4...] instead of [0,0,0,0,...]
myarray a {5,2}; should construct [5,2,2,3,4...] instead of [5,2,0,0,0...]

One obvious solution is to have a class:

template <size_t size>
struct myarray {
    std::array<int, size> ar;
    myarray() { for (size_t i=0; i<size; i++) ar[i]=i; }
    myarray(std::initializer_list<unsigned int> il) :ar(il) {
        for (size_t i=il.size(); i<size; i++) ar[i]=i; 
    } 
    ...
};

However it forces me to wrap every single member of ar. As far as I understand, inheriting from std::array is not a very good idea. Is there a third reasonable way ?

@Jarod42 : Are you suggesting some kind of factory function ? Then how do you ensure that the array is always constructed trough this function ? I don't want a user to call the usual constructor inadvertently. — hivert, Mar 28 '14 at 08:08
Yes I suggested a factory function. But it seems that it doesn't fit your needs. — Jarod42, Mar 28 '14 at 08:24

score 0 · Answer 1 · answered Mar 28 '14 at 08:43

It would be a very bad idea to change it globally. For all you know, std::cout internally uses a std::array. Even in your own code, the principle of least surprise dictates that std::array<int, 5> creates [0,0,0,0,0]. That's the whole point of std:: - "STandarD".

So, the proper solution is to use a function which returns an array constructed like you want.

BTW, you can replace the for-loop with a single call to std::iota(begin(ar)+il.size(), end(ar), il.size())

score 0 · Accepted Answer · edited May 23 '17 at 10:26

0

I believe my earlier post on the similar topic would be useful.

https://stackoverflow.com/a/21651027/2724703

edited May 23 '17 at 10:26

Community

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answered Mar 28 '14 at 08:45

Mantosh Kumar

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So you are saying that there is no critical problem inheriting form `std::array`. – hivert Mar 29 '14 at 08:15

Changing the construction of std::array

2 Answers2