我需要一个在 C++ 中具有一个动态维度的二维数组

Question

I need to create a 2-dimensions array, where there are 8 "rows" but "columns" are not predetermined

someting along this example:

[0], ['mum', 'dad', 'uncle']
[1], ['brother', 'sister']
[2], ['friend', 'colleague', 'boss', 'employee']
.... and so on

than I have to scroll the first index (1 to 8) and read every possible values of all columns (for each "index" I have the count of how many elements are in it.

and I need to read using something like

ary[2][3]    --->that would return "boss"

Following other exampes I am using

unsigned char (*ary)[n] = malloc(sizeof(unsigned char[8][n]));

but it doesn't compile and gives:

error: cannot initialize a variable of type 'unsigned char **' with an rvalue of type 'void *'

HOw do I declare and read this kind of array correctly in C++ ?

Answer 1

If the number of rows will be known at compile time you can use std::array

std::array<std::vector<std::string>, number_of_rows>

You can also use

std::vector<std::string>[number_of_rows]

But working with raw arrays is not as convenient as std::array .

If the number of rows will not be known until runtime you can have a vector of vectors like

std::vector<std::vector<std::string>>

Answer 2

When using C++, avoid using malloc . Prefer to use new instead.

Assuming n is a compile time constant, you can use:

unsigned char (*ary)[n] = new unsigned char[8][n];

If n is a run time variable, you most likely need to use:

unsigned char (*ary)[8] = new unsigned char[n][8];

You can avoid the problems of dealing with dynamically allocated memory if:

1. You use a std::array for the array of unsigned char .
2. You use a std::vector to capture the dynamic nature of the data.

---

std::struct<std::array<unsigned char, 8>> ary(n);

Answer 3

First, this syntax is more C than C++ (familiarize yourself with new for example, or arrays/vectors/STL etc...). In your case, if you really want C syntax (malloc) then only malloc when you know how much you want to malloc during run time. You said you don't want the same n for all rows, so your line doesn't make logical sense (in addition to being illegal):

char *a[8];
a[0] = malloc(sizeof(char)*the_amount_of_chars_plus_null_termination_in_all_strings);
/* You have to test malloc succeeded! */
/*For each row a different size, which must be known at run time*/

You don't say how you obtain each row, so I don't know if you plan on per-calculating these values, or getting strings from input and then you can use strlen+1 for each one.