char **Data[70]={NULL};
What is the correct terminology for this? How else could it be written? What does it look like in memory? I am reading many tutorials on pointers but I don't see it in this syntax. Any help is appreciated. Thanks.
In computer programming, an array of pointers is an indexed set of variables, where the variables are pointers (referencing a location in memory). Pointers are an important tool in computer science for creating, using, and destroying all types of data structures.
Dynamic 1D Array in C++: An array of pointers is a type of array that consists of variables of the pointer type. It means that those variables can point to some other array elements. Example: int *p[3];
Declaration of an Array of Pointers in C In simple words, this array is capable of holding the addresses a total of 55 integer variables. Think of it like this- the ary[0] will hold the address of one integer variable, then the ary[1] will hold the address of the other integer variable, and so on.
We can make separate pointer variables which can point to the different values or we can make one integer array of pointers that can point to all the values.
This structure
char **Data[70]={NULL};
is an array of 70 pointers to pointers to char. The compiler allocates 70 * sizeof(char**)
bytes for this array, which assuming 32-bit pointers is 280 bytes.
If you internally think of a "pointer to char" as a string, which isn't true but it's close enough, then this is an array of 70 pointers to strings. To make some ASCII art and pretend that you have allocated and filled some values....
Array of One or more
char ** char *
+---------+ +---------+
| 0 | --> | ptr | --> "Hello, world"
+---------+ +---------+
| 1 |
+---------+ +---------+
| 2 | ----> | ptr2 | --> "Goodbye, cruel world"
+---------+ +---------+
| 3 |
+---------+ +---------+
| 4 | ------> | ptr3[0] | --> "Message 0"
+---------+ +---------+
... | ptr3[1] | --> "Message 1"
+---------+ +---------+
| 69 | | ptr3[2] | --> "Message 2"
+---------+ +---------+
You could do the above with code like this (error checking malloc return values skipped):
char **Data[70]={NULL};
char **ptr, **ptr2, **ptr3;
ptr = (char **) malloc(sizeof(char *));
*ptr = "Hello, world";
Data[0] = ptr;
ptr2 = (char **) malloc(sizeof(char *));
*ptr2 = "Goodbye, cruel world";
Data[2] = ptr2;
ptr3 = (char **) malloc(10 * sizeof(char *));
Data[4] = ptr3;
ptr3[0] = "Message 0";
ptr3[1] = "Message 1";
...
ptr3[9] = "Message 9";
printf("%s\n", *Data[0]);
printf("%s\n", Data[2][0]);
printf("%s\n", Data[4][0]);
printf("%s\n", Data[4][1]);
...
printf("%s\n", Data[4][9]);
Think of it this way: Each entry in the array is a char **
. Each entry can point to an arbitrary location in memory, said location(s) being char *
and thus being able to point to a null-terminated character array aka "string."
Note carefully the distinction between this and what you get when you allocate a 2D array:
char *Data2[10][70]={NULL};
The allocation of Data2
above gives you a 2-dimensional array of char *
pointers, said 2-d array being allocated in a single chunk of memory (10 * 70 * sizeof(char*)
bytes, or 2800 bytes with 32-bit pointers). You don't have the ability to assign the char **
pointers to arbitrary locations in memory that you have with the single-dimensional array of char **
pointers.
Also note (given above declarations of Data
and Data2
) that the compiler will generate different code for the following array references:
Data[0][0]
Data2[0][0]
Here's another way to think about this: Imagine that you have several arrays of pointers to strings:
char *table0[] = { "Tree", "Bench", "Stream" };
char *table1[] = { "Cow", "Dog", "Cat" };
char *table2[] = { "Banana", "Carrot", "Broccoli" };
char **Data[3];
Data[0] = table0;
Data[1] = table1;
Data[2] = table2;
You have an array of pointers to "array of pointer to char". If you now print the value of data[1][1]
, think of it like this: data[1]
gets you a pointer to the array table1
. Then the value table1[1]
equals "Dog"
.
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