CODEWITHSUNDEEP
c++arraysmultidimensional-arraydata-structuresstdmap
xWourFo
xWourFo

Reputation: 47

Creating a std::map containing 2d arrays?

I was trying to create a map to associate 2d arrays to characters. I tried to do it in this way:

map<char, bool[6][4]> m;
bool charA[6][4] = { 0,1,1,0,
                     1,0,0,1,
                     1,1,1,1,
                     1,0,0,1,
                     1,0,0,1,
                     1,0,0,1 };

m.insert(make_pair('a', charA));

But it doesn't work; maybe because the array is not dynamically allocated?

Upvotes: 2

Views: 717

Answers (1)

JeJo
JeJo

Reputation: 32722

Since you are using and have access to compiler or newer versions, use std::array instead of the c-style array.

In your case, you can replace it with std::array<std::array<bool, 4>, 6>. Now you can do as follows: See a demo

#include <array> // std::array

std::map<char, std::array<std::array<bool, 4>, 6>> m;
std::array<std::array<bool, 4>, 6> arr = { 0,1,1,0,
                       1,0,0,1,
                       1,1,1,1,
                       1,0,0,1,
                       1,0,0,1,
                       1,0,0,1 };

// now you can
m.emplace('a', arr); // construct the entry in-place (std::map::emplace)

Maybe because the array is not dynamically allocated?

This has nothing to do with the allocations, rather the problem is with the c-style arrays. You simply can not copy it to the map like that. In other words, not copyable!

More read:

Future reads:

As @TedLyngmo pointed out, in the given example, even the std::map::emplace is used, a copy of the array (i.e. arr) will take place. This could be much expensive for larger arrays or arrays with data types, which are expensive to copy. In that case, you need to use the std::map::emplace, in the way shown in the linked website.

You can refer to the following examples for further examples:

Upvotes: 2

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