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I would like to define int array in which first dimension is choosen dynamically but second is static -- something like int array[][8]. Unfortunatelly I can not allocate such array dynamically. int ary[][8] = new int[11][8]; Produces error:

error: initializer fails to determine size of ‘array’
    6 |     int array[][8] = new int[11][8];
      |                      ^~~~~~~~~~~~~~
2d.cpp:6:20: error: array must be initialized with a brace-enclosed initializer

or when I try following code:

int array[][8] = new int*[11];
array[0] = new int[8];

I get

2d2.cpp:6:22: error: initializer fails to determine size of ‘array’
    6 |     int array[][8] = new int*[11];
      |                      ^~~~~~~~~~~~
2d2.cpp:6:22: error: array must be initialized with a brace-enclosed initializer
2d2.cpp:7:25: error: incompatible types in assignment of ‘int*’ to ‘int [8]’
    7 |     array[0] = new int[8];

Is that even possible in c++?

Trismegistos
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2 Answers2

1

Just use std::vector and std::array:

#include <vector>
#include <array>
#include <iostream>

int main()
{
    using MyArray = std::vector<std::array<int, 8>>;
    MyArray arr {11};
    for (int i {0}; i < 8; ++i)
        arr[i][i] = i;
    for (const auto& v : arr) {
        for (auto x : v) {
            std::cout << x << " ";
        }
        std::cout << std::endl;
    }
}

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Daniel
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  • This is not what I want. I'm confused about c++ syntax in regard of 2d arrays and I want to clarify it. – Trismegistos Apr 27 '20 at 15:42
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    @Trismegistos It is how to achieve what you want to achieve. Arrays in C++ are dumb, and only a higher level of abstraction can achieve what you are looking for. – PaulMcKenzie Apr 27 '20 at 15:44
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    @Trismegistos Why not? It's exactly what you asked for: "int array in which first dimension is choosen dynamically but second is static". – Daniel Apr 27 '20 at 15:44
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I will answer my own question:

int (*array)[8] = new int[11][8];

I forgot how bad C++ rules for creating type definitions -- especially including pointers, arrays and function pointers -- are. I added missing parentheses around array and now it is fine.

Trismegistos
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  • So how is this any advantage over the `std::vector/std::array` version? You have to now concern yourself with deallocating that memory. Modern C++ programs should refrain from using `new[]`. – PaulMcKenzie Apr 27 '20 at 15:57
  • @PaulMcKenzie There's no need to be aggressive. Yes, if this were a "real problem," I'd suggest using that too, and if you hadn't mentioned it I would have commented that as an alternative. But expressions like `new int[11][8]` are part of the language, like it or not, and there is value in knowing how to use them. Also, if the only concern is remembering to `delete` the memory, `auto arr = std::make_unique(11);` should work fine. `std::vector` may be conceptually overkill. – HTNW Apr 27 '20 at 16:13