返回未知大小的std :: array

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!

Answer 1

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

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()); .

Answer 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.

Answer 3

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).