Cant make a vector of fixed size arrays?

Question

I have this odd problem

    vector<unsigned int[3]> tris;
    for (unsigned int i = 0; i < idx.size() - 2; i++) {
        unsigned int push[] = {idx[i], idx[i+1], idx[i+2]};
        tris.push_back(push); //<- this is where it goes belly up
    }

The code piece is supposed to unravel a triangle strip index list into triangle indices but wont compile under vs10. Thoughts?

AnT stands with Russia · Accepted Answer · 2013-06-06T19:51:03.857

No, unless you wrap your arrays into a struct or use something like std::array.

The naked C-style array type is not copyable or assignable, which makes it ineligible to serve as a standard container element.

P.S. It should be noted though that C++11 switched to a more elaborate (per-method) approach to specifying requirements to container element type, instead of the more broad approach used by C++03. The above ineligibility claim is based on C++03. I'm not ready to say that it is so unconditionally true for C++11 as well... But it is certainly true even in C++11 if you insist on using push_back in your code.

P.P.S. In C++11 one can probably get away with std::list of naked C-style arrays and using emplace to construct new elements. But not with std::vector.

This is the better answer, as it concisely points out *why* the example is invalid, not just what to do. — Richard J. Ross III, Jun 06 '13 at 19:35

score 12 · Answer 2 · answered Jun 06 '13 at 19:33

12

Use std::vector<std::array<unsigned, 3>> instead.

answered Jun 06 '13 at 19:33

Juraj Blaho

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This answer looks like it works, but it doesn't. I mean, it works because it lays out everything in a flat contiguous sequence of bytes. But you cannot do `push_back(arr)` on it and hoping that it will append the new `arr`. But you can use `std::memcpy`. – daparic Jun 21 '20 at 22:20
In C++11 you can actually write tris.push_back(std::array{idx[i], idx[i+1], idx[i+2]}); Combined with the fact that tris.data() is the equivalent C-style array, makes this choice the closest to the original intention, in my opinion. – MikeLima May 08 '22 at 10:55

score 0 · Answer 3 · edited May 23 '17 at 11:53

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If you are using C++11 you can use tuples.

std::vector < std::tuple< unsigned int, unsigned int, unsigned int > > tris;

https://stackoverflow.com/a/15734156/846686

A less 'elegant' solution can be a pair of a pair

    std::vector < std::pair< unsigned int, std::pair<unsigned int, unsigned int> > tris;

but it may result in a very confused code to read...

edited May 23 '17 at 11:53

Community

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answered Dec 30 '16 at 14:21

campisano

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Cant make a vector of fixed size arrays?

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