初始化和导航char [] []

Question

Let's say I have a char[2][3] named charPP, 2 rows, 3 columns.

1. What is the best way to initialize the whole thing with dummy data?
2. How would I change the value of a certain cell? For example, make it so the pointer to the last column in each row is a nullptr?
3. What would happen if I did a reinterpret_cast<char**>(charPP[0]) ?

Sorry if this is generic or vague, I'm just looking for some basic understanding.

Answer 1

As for case number 3, I guess you really means reinterpret_cast<char**>(charPP) ? Well you simply can't access an array of arrays as a pointer to pointer, because the memory layout is not compatible.

Lets say you have an array char a[2][2] , it's memory layout would be

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

However, if you use pointer to pointer (ie char ** ), which you used as the above array, it would look like this:

+------+------+-----+
| a[0] | a[1] | ... |
+------+------+-----+
  |      |
  |      V
  |      +---------+---------+---------+-----+
  |      | a[1][0] | a[1][1] | a[1][2] | ... |
  |      ----------+---------+---------+-----+
  V
  +---------+---------+---------+-----+
  | a[0][0] | a[0][1] | a[0][2] | ... |
  +---------+---------+---------+-----+

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For case number 2, while it's true that you can use an array as a pointer (arrays decays to pointers), it's still an array so you can't make parts of it "point" to null. Lets take the example above, with char a[2][2] , you can't do a[0] = nullptr .

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In case number 1, you do know how to access a single cell for a simple (one-dimensional) array don't you? It's just the same for multi-dimensional arrays, as long as you remember that you have an array of arrays .

Answer 2

1. char charPP[2][3] = { 0 };

2. charPP[i][j] = 0; char[0][2] = 0; char[1][2] = 0;

3. Possible mayhem since you're casting a char value(-128 to 127) to a char** .