invalid types for array subscript while addressing function

Question

My code:

.h file:

extern const int* g_position;

inline DWORD pos_x(BYTE v) {
    if (1 <= v && v <= 3)
        return g_position[v][0];

    return 0;
}

inline DWORD pos_y(BYTE v) {
    if (1 <= v && v <= 3)
        return g_position[v][1];

    return 0;
}

.cpp file:

const int* g_position = (int*) 0x86b2fdc;

What I am trying to do is address the function which is basically a two-dimensional array:

(DWORD (*)[4][2]) 0x86b2fdc<g_position>

For some reason I am getting the following error(s) while compiling:

In function 'gev::DWORD pos_y(BYTE)':
error: invalid types 'const int[int]' for array subscript
    return g_position[v][0];

What is wrong in my code? I am declaring the g_position as an array type so it should work.

g_position is only int *, so you can't dereference twice. Also, what exactly are you trying to do? In general, hard-coding a pointer is a bad idea. In any case the extern const ... declaration of g_position needs to match whatever declares that object at 0x86b2fdc — Drew McGowen
– Drew McGowen, Commented Aug 6, 2014 at 2:01

Remy Lebeau · Accepted Answer · 2014-08-06 20:02:48Z

1

g_position is declared as int*, which allows only one level of dereferencing, but you are trying to perform two dereferences instead. You need to change the declaration to int** instead:

extern const int** g_position;

const int** g_position = (int**) 0x86b2fdc;

Update:

An int** is not quite the same thing as an int[][]. However, if you take into account that an int[4][2] has the same memory layout as an int[8], and you can access the content of an int[] using an int*, then try this instead:

extern const int* g_position;

inline DWORD pos_x(BYTE v)
{
    if (1 <= v && v <= 3)
        return g_position[(v*4)+0];

    return 0;
}

inline DWORD pos_y(BYTE v)
{
    if (1 <= v && v <= 3)
        return g_position[(v*4)+1];

    return 0;
}

const int* g_position = (int*) 0x86b2fdc;

edited Aug 6, 2014 at 20:02

answered Aug 6, 2014 at 2:34

Remy Lebeau

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5 Comments

Lucas Over a year ago

Getting this error: invalid conversion from 'int**' to 'const int**' [-fpermissive]. When I change the (int**) to the (const int**) my application is crashing.

Lucas Over a year ago

Here is a backtrace snippet: #0 0x286b54c6 in gev::pos_y (e=3 '\003') at src/pos.h:14

Remy Lebeau Over a year ago

If it is crashing, then the data at the memory address likely does not represent a true 2-dimensional array to begin with.

Lucas Over a year ago

it does, this is the data at the memory address: int g_position[4][2] = { { 0, 0}, { 3461, 8480}, { 4115, 2340}, { 7875, 7575} };

Lightness Races in Orbit Over a year ago

"An int** is not quite the same thing as an int[][]" It's not in any way even close to being the same thing.

Lightness Races in Orbit · Accepted Answer · 2014-08-09 15:01:06Z

Is 0x86b2fdc the address of an array of arrays? Or the address of an array of pointers to arrays? Or is it the address of a pointer to something?

Assuming it is the address of an array of arrays of 4 ints, you can just declare g_position as a pointer to such:

extern const int (*g_position)[4];  /* pointer to arrays of 4 ints */

in the .cpp file:

const int (*g_position)[4] = (int (*)[4])0x86b2fdc;

Your basic confusion seems to be confusing pointers and arrays, which are not the same thing:

an array is a sequence of objects laid out one after the other in memory. You can't do anything with an array other than initialize it, get its size, and get a pointer to it or its 0th element
a pointer is an address that refers to memory. The memory pointed at may contain a single object of the appropriate type, or it may contain an array (in which case the pointer points at the 0th element of the array.

Whenever you refer to an array (except when using sizeof or unary &), the compiler silently gives you a pointer to the 0th element of the array, so the name of such a pointer is mostly interchangeable with the name of the array. Your pos_ functions don't use sizeof or & on g_position, so for your purposes, they are interchangeable. you just need to get the type correct.

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