Dynamic array of Maps in C++

Question

In a program which I'm coding I need to use an structure as this one:

static std::unordered_map<size_t, int> map[N][M];

The problem is that I need to generate the size of the map dynamically, so I would define:

static std::unordered_map<size_t, int> **map

But I'm having problems allocating it. Any suggestion?

Thanks

As for your current problem with pointers, please read about how to ask good questions and learn how to create a Minimal, Complete, and Verifiable Example. — Some programmer dude
– Some programmer dude, Commented Oct 16, 2017 at 5:58

Galik · Accepted Answer · 2017-10-16 16:25:36Z

2

I recomment you use std::vector to manage your arrays. You can create a vector of vectors like this:

std::vector<std::vector<std::unordered_map<size_t, int>>> map(M, std::vector<std::unordered_map<size_t, int>>(N));

for(int m = 0; m < M; ++m)
{
    for(int n = 0; n < N; ++n)
    {
        map[m][n].emplace(0, 1);
    }
}

If you know the sizes at compile time, you can use std::array which should be faster.

static std::array<std::array<std::unordered_map<size_t, int>, M>, N> map;

edited Oct 16, 2017 at 16:25

answered Oct 16, 2017 at 6:09

Galik

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3 Comments

Chaosit Over a year ago

Just a minor addition to this answer - in case if you know the size of your arrays at compile time - I'd suggest to use std::array instead of vector. This way the compiler can make better optimizations to your code

Jowi Over a year ago

In this case I know the size on runtime. Using the vector approach I found a solution. static std::vector<std::vector<std::unordered_map<size_t, int>>> map; map.resize( num_of_col , std::vector<std::unordered_map<size_t, int>>( num_of_row ) );

Galik Over a year ago

@Chaosit Added a comment.

Jay · Accepted Answer · 2017-10-16 05:58:39Z

0

You only need to allocate the space of N * sizeof(std::unordered_map<size_t, int> *).

answered Oct 16, 2017 at 5:58

Jay

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