Declaration and initialize pointer-to-pointer char-array

Question

Concerning a pointer to pointer I wonder if its ok with the following:

    char** _charPp = new char*[10];

     for (int i = 0; i < 10; i++)
    ´   _charPp[i] = new char[100];

That is - if I want a pointer to pointer that points to 10 char-arrays.

The question - is this ok now or do I have to make some kind of initializing of every char-arrays? Then - how do I do that?

I will later on in the program fill these arrays with certain chars but i suspect that the arrays should be initialized with values before such as "0"

why dont you use `std::vector >` and reserve the wanted size for each inner vector? — Zaiborg, May 12 '14 at 10:11
Is there a reason you are trying to use pointers when you don't seem to understand them? Why not use a `vector`? This is C++ after all — Engineer2021, May 12 '14 at 10:19

Netherwire · Accepted Answer · 2014-05-12T10:27:26.963

Yes, it looks pretty much suitable,

int arraySizeX = 100;
int arraySizeY = 100;
char** array2d = new char*[arraySizeX] // you've just allocated memory that
                                       // holds 100 * 4 (average) bytes
for(int i = 0; i < arraySizeX; ++i)
{
    array2d[i] = new char[arraySizeY] // you've just allocated a chunk that 
                                       //holds 100 char values. It is 100 * 1 (average) bytes
    memset(array2d[i], 0, arraySizeY) // to make every element NULL
}

I will later on in the program fill these arrays with certain chars but i suspect that the arrays should be initialized with values before such as "0"

No, there's no "default value" in C++. You need to assign 0 to the pointer to make it null, or use memset() to do it with arrays.

I didn't downvote but it's probably because the `memset` is more effort than going `new char[arraysizeY]();` (and could also be slower) — M.M, May 12 '14 at 10:49

score 1 · Answer 2 · answered May 12 '14 at 10:43

Netherwire's answer shows correctly how to initialize the chars to 0. I'll address the first part of your question.

There is no requirement to initialize them yet. As long as you initialize the chars before they're read, it's correct.

Depending on the structure of your code and how you use the arrays, it may be safer to do anyway since it can be hard to find the bug if you eventually do read some of them before initialization.

Also, remember to later delete[] the arrays that you've allocated. Or better yet, consider using std::vector instead (and possibly std::string depending on what you use those arrays for.)

Declaration and initialize pointer-to-pointer char-array

2 Answers2