Assign const 1D array from const 2D array row inside a header file c++

Question

I have searched in a few places already for a solution to this problem and have come up short. Essentially what I am looking to do is to assign a const 1D array from one of the rows of a const 2d array and specifically to do it in a header file.

Say I have

const int Arr1[2][3] = {{1,2,3},{4,5,6}};

and I want to initialize an array (within a header file) from this array. I know normally I could do:

const int *Arr2 = Arr1[1];

or

const int *Arr2 = Arr1[0];

but doing so in a header file gives multiple definition errors. So my question is if there is a nice way to do this.

πάντα ῥεῖ · Accepted Answer · 2016-07-07 00:28:13Z

2

The multiple definition errors don't have anything to do with your assignment.

Just say in your header

 extern const int *Arr2;

and initialize it in a separate translation unit:

 const int Arr1[2][3] = {{1,2,3},{4,5,6}};
 const int *Arr2 = Arr1[1];

Maybe selectively from preprocessor directives:

 const int Arr1[2][3] = {{1,2,3},{4,5,6}};
 #ifdef ZERO_PATH
 const int *Arr2 = Arr1[0];
                     // ^ ZERO_PATH
 #else
 const int *Arr2 = Arr1[1];
 #endif

answered Jul 7, 2016 at 0:28

πάντα ῥεῖ

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Comments

M.M · Accepted Answer · 2016-07-07 01:18:31Z

Your code:

const int *Arr2 = Arr1[1];

initializes a pointer, not an array. Arrays and pointers are different. It's not clear from your post whether you're content with using a pointer, but if so then you either need to setup as in πάντα ῥεῖ's answer; or give Arr2 internal linkage:

const int *const Arr2 = Arr1[1];    // if you do not plan to change Arr2
static const int *Arr2 = Arr1[1];   // if you do

To actually use an array you can write:

const int Arr2[3] = { Arr1[1][0], Arr1[1][1], Arr1[1][2] };

There's no way to use a whole C-style array as initializer for another array of the same type. But if you use C++-style arrays you can do that:

const array<array<int, 3>, 2> Arr1 = {{ {1,2,3},{4,5,6} }};

const array<int, 3> Arr2 = Arr1[0];

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