Return std::array of unknown size

Question

I would like to return a multidimensional std::array from my function. The size of the returned array shall be determined by the size of an input argument. Something like:

std::array<std::array<unsigned int, n>, n> foo(std::vector<int> vec){
    unsigned int n = vec.size;
    std::array<std::array<unsigned int, n>, n> result;
    return result;
}

It would be nice to solve this without an annoying additional template argument. std::vector instead of std::array seems not to be as straightforward to initialize with n (undetermined) items as std::array (without explicit initialization). How can this be made possible? Thank you!

The size of an std::array must be known at compile time. That's why the size is templated — Francisco Gallego Salido
– Francisco Gallego Salido, Commented Dec 12, 2017 at 23:24
Why do you need to use std::array as a return type rather than std::vector? — user2100815
– user2100815, Commented Dec 12, 2017 at 23:26
@ F. Salido: For C-style like arrays, you can do someting like: int[] foo() { // ... @N. Butterworth: If I used std::vector, I would have to initialize the complete object with n^2 elements with a nested for-loop -- I strive to omit this. @George; If there is no better solution than your best-so-far one, I would like to see them as an answer to be able to mark it as the best one :-) — user7427029
– user7427029, Commented Dec 12, 2017 at 23:35
@user7427029 int[] foo() doesn't compile, as well as int foo()[]. — HolyBlackCat
– HolyBlackCat, Commented Dec 13, 2017 at 0:20
@HolyBlackCat that's an extension I believe.. only way to organize and return multidimensional array of size that wasn't known at compile time is a naked pointer, or a class that is a wrapper class around naked pointer. vectors are ok to act as replacement, a redefined iterator ma allow a trick of emulating multidimensional. — Swift - Friday Pie
– Swift - Friday Pie, Commented Dec 13, 2017 at 0:25

Francisco Gallego Salido · Accepted Answer · 2017-12-12 23:45:30Z

2

First thing you need to know is that std::array has its size fixed at compile time, as the documentation from cppreference sais:

std::array is a container that encapsulates fixed size arrays.

This container is an aggregate type with the same semantics as a struct holding a C-style array T[N] as its only non-static data member.

If n comes from std::cin or from the command line arguments (or whatever kind of input out of compile time), then the compiler can't deduce the type, and therefore it will throw an error.

The most sensible way to do this is with std::vector, and you can do it like this:

std::vector<std::vector<unsigned int>> foo(std::vector<int> vec){
    unsigned int n = vec.size();
    std::vector<std::vector<unsigned int>> result(n, std::vector<unsigned int>(n));
    return result;
}

Just initialize every vector using the size constructor of the vector.

answered Dec 12, 2017 at 23:45

Francisco Gallego Salido

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2 Comments

Swift - Friday Pie Over a year ago

I looks like that RVO in this case is impossible

Francisco Gallego Salido Over a year ago

@Swift Well, you can always force a move if you don't want to create a new copy, it's not a big deal.

George · Accepted Answer · 2017-12-13 00:03:46Z

2

You can create a n sized std::vector of n sized std::vector s using its c'tor, i.e:

std::vector<std::vector<unsigned int>> result(n, std::vector<unsigned int>(n, 0));

NOTE: According to cppreference.com, the second parameter used in the above example of std::vector s c'tor is for the value of each item to be created: c'tor signature:

vector( size_type count, const T& value, const Allocator& alloc = Allocator());.

edited Dec 13, 2017 at 0:03

answered Dec 12, 2017 at 23:42

George

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3 Comments

Swift - Friday Pie Over a year ago

it's possible, but it is allocation cost nightmare.. or nightmare for anyone who would try to multithread, using such structure of storage

George Over a year ago

@Swift "it's possible, but it is allocation cost nightmare" Why? And I doubt multithreading is relevant but why would it be an issue to use std::vector<std::vector<>>(especially in a small program)?

Swift - Friday Pie Over a year ago

C++ is a language that can hides complexity of code behind simple structures That type is essentially an array with uneven lines, because every "line" represented by vector and can change its size independently. But it's not an analog of python's numty. There is no way to represent it as 2D array without copying all data to another storage. If OP ever needs to do the latter (very common problem with working with numerous APIs (e.g. OpenGL) or interfacing with python or other languages), only effective way he can do it is to make a wrapper class around single dimensional vector.

Innokentiy Alaytsev · Accepted Answer · 2017-12-12 23:45:53Z

0

Try using Boost.MultiArray. It allows you to create multidimensional arrays with contents of arbitrary content type. I have used it (the boost::multi_array_ref part to be more specific) and it works pretty nice. An important feature is the ability to create array views (and slices based on views).

answered Dec 12, 2017 at 23:45

Innokentiy Alaytsev

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