How can I pass the pointer of a static 2D array to a structure/class?

Question

I have a problem when I try to pass the pointer of an array (which contains parameters needed by some functions in my program) to a structure, which then should be passed to those functions. GSL for example wants me to pass parameters in this way.

A little example program looks like this:

#include <iostream>

using namespace std;

struct myparams
{
    double  * a;
    double ** b;
};

int main()
{
    double c[10]   = {0,1,2,3,4,5,6,7,8,9};
    double d[4][3] = {{1,2,3},{4,5,6},{7,8,9},{10,11,12}};

    double** e = new double*[4];
    for (int i = 0; i < 4; i++) {
       e[i] = new double[3];
    }

    myparams params;

    // THIS WORKS:
    params.a = c;
    for (int i = 0; i < 10; i++) {
        cout << params.a[i] << endl;
    }

    // THIS DOESN'T WORK
    params.b = d;

    // THIS WORKS:
    params.b = e;

    delete[] e;
}

What is the problem with

 params.b = d

The Compiler complains with "cannot convert 'double[4][3]' to 'double**' in assignment" or something like that (translated from german).

double[n][m] cannot decay into double **, while double[n] can decay into double *. That's why the first assignment is valid, and the latter isn't. — Zeta
– Zeta, Commented Feb 15, 2013 at 14:08
Also see stackoverflow.com/a/13273979/560648 and stackoverflow.com/a/9557855/560648 and stackoverflow.com/a/7139014/560648 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Feb 15, 2013 at 14:15

Community · Accepted Answer · 2017-05-23 12:03:02Z

11

double d[4][3]; is an array of arrays. double b**; is a pointer to pointer. These types are not the same (the common "arrays are just pointers" you might have read on the internet is wrong).

Elements of d are of type double[3]. Arrays, when passed around, decay to pointers to their first element (see section 4.2. of C++ standard). d will decay to double(*)[3] (a pointer to array of 3 doubles).

Long story short, double(*)[3] is not convertible to double** and this is what compiler is telling you.

If you need to keep d as it is, you need to declare b as double (*b)[3];

For more in-depth explanation, refer to this SO question.

edited May 23, 2017 at 12:03

CommunityBot

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answered Feb 15, 2013 at 14:16

jrok

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5 Comments

GriffinPeterson Over a year ago

Thank you! This was almost the solution. It's not double (*b)[4], but double (*b)[3].

Lightness Races in Orbit Over a year ago

I like how you accepted the incorrect answer over the correct one. That makes sense.

jrok Over a year ago

@LightnessRacesinOrbit If you point out what's incorrect, I'll fix it. Thank you.

Lightness Races in Orbit Over a year ago

@jrok: GriffinPeterson (the OP!) already did. Read his comment.

jrok Over a year ago

@LightnessRacesinOrbit Aye, typo.

Some programmer dude · Accepted Answer · 2024-08-26 13:12:25Z

8

It's because a pointer to a pointer (or an array of pointers as it can be used as) is not the same as an array of arrays. The layout in memory is incompatible.

For example, lets say we have these two declarations:

char aa[2][2] = { { 'a', 'b' }, { 'c', 'd' } };
char **pp;

pp = new char*[2];
pp[0] = new char[2];
pp[1] = new char[2];

The array aa looks like this in memory:

+----------+----------+----------+----------+
| aa[0][0] | aa[0][1] | aa[1][0] | aa[1][1] |
+----------+----------+----------+----------+

The "array" pp meanwhile looks like this:

+------+------+
| a[0] | a[1] |
+------+------+
   |      |
   |      v
   |      +---------+---------+
   |      | a[1][0] | a[1][1] |
   |      +---------+---------+
   V
   +---------+---------+
   | a[0][0] | a[0][1] |
   +---------+---------+

The "array" pp contains pointers to another "array"

edited Aug 26, 2024 at 13:12

answered Feb 15, 2013 at 14:08

Some programmer dude

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11 Comments

GriffinPeterson Over a year ago

Thank you for the quick response! But which member do I need in my structure myparams to store the pointer to the array d?

Lightness Races in Orbit Over a year ago

Okay I think that's better than my diagram. Dammit.

Some programmer dude Over a year ago

@LightnessRacesinOrbit Well, I think yours look more "professional" than my crude ASCII art :)

Lightness Races in Orbit Over a year ago

@JoachimPileborg: I did it in pbrush!

Lightness Races in Orbit Over a year ago

@GriffinPeterson: It's unwieldy, though. You're much better off with a 1D array representation.

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