Casting a void pointer to a 2D String array pointer (C/CPP)

I'm using a library which requires a function with a void* pointer as a parameter. I have a 2D string array and I want to pass that array through that parameter and extract it inside the function. I successfully passed the array as a pointer but I don't know how to convert that pointer back to my array.

This is my current code:

String str_array[100][10];

int callback(void* data) {

  String* str_array_ptr[100][10] = (String* [100][10])data;

  (*str_array_ptr)[0][0] = "text";

  return 0;

}

void test() {
  callback(&str_array);
}

However, when compiling, I obtain the following error message:

> error: ISO C++ forbids casting to an array type 'String* [100][10]' [-fpermissive]

PS: I'm trying to use the SQLite library's sqlite3_exec() function and store the result of a "SELECT SQL query" into a 2D string array.

SQLite C Interface - One-Step Query Execution Interface

You cannot cast a pointer to an array. Instead you access your array through another pointer. That pointer has type String (*)[10]. Like this

String str_array[100][10];

int callback(void* data) { 

    String (*str_array_ptr)[10] = (String (*)[10])data;

    str_array_ptr[0][0] = "text"; // Note no '*'

    return 0;

}

void test() {
    callback(str_array); // Note no '&'
}

Both the way you create the pointer, you don't need to use &, and the way you access the pointer, you don't need to use *, are also wrong in your code. See the code above for details.

The fundamental issue here (and maybe the issue you are misunderstanding) is the difference between String *x[10]; and String (*x)[10];. In the first case x is an array of 10 pointers to String, in the second case x is a pointer to an array of ten String. It's the second option that you want.

The line, String* str_array_ptr[100][10]; does not declare a pointer to a 2D array of String objects; rather, it declares a 2D array of pointers to String objects.

The syntax for declaring a pointer to a 2D array is tricky; in your case, it is the following (to declare str_array_ptr as a pointer to a 100 x 10 array of String objects):

String (*str_array_ptr)[100][10];

Casting to such a pointer is (arguably) even trickier; using a C++ static_cast (which you can when the source operand is a void*), you get the following code:

#include <iostream>
#include <string>
using String = std::string;

String str_array[100][10];

int callback(void* data) {

    String (*str_array_ptr)[100][10] = static_cast<String (*)[100][10]>(data);
    (*str_array_ptr)[0][0] = "text";
    return 0;
}

void test() {
    callback(&str_array);
}

One could argue that, if you really wish to use such a pointer to a 2D array (there are likely better design solutions in C++, such as the answer posted by john), then you could define a type for that pointer with a typedef or using... statement (e.g. using StrArrPtrType = decltype(&str_array);); however, the C++ and C programming communities generally frown (severely and rightly) on hiding pointers behind typedef or using aliases.