使用指向 C 中的数组的指针查找数组中的元素数

Question

please help me to implement function in C that take the pointer of array and return the number of elements in that array. I have an array of type My_Type like this:

typedef struct My_Type My_Type ;

struct My_Type {
    char *array[100];  //100 is the maximum length the array could have
}

My_Type *my_array = malloc(sizeof(My_Type));

After creating a heap memory for my_array, the array was added n number of element (n<=100). The function I'm trying to write look like this:

int Count(My_Type *array)

Thank you so much!

Answer 1

It is not possible in C. C arrays do not have any metadata about their length or type. The only way is to store the size in the array or use more complex data type.

typedef struct
{
    size_t size;
    int arr[];
}int_arr_t;


int_arr_t *allocate(int_arr_t *array, size_t newsize)
{
    int_arr_t *arr = realloc(array, sizeof(*arr) + newsize * sizeof(arr -> arr[0]));

    if(arr)
    {
        arr -> size = newsize;
    }
    return arr;
}

size_t getCount(int_arr_t *arr)
{
    return arr ? arr -> size : 0;
}

Some remarks: malloc(sizeof(My_Type)); do not use types in the sizeof only objects. If you change the type of the object you will not have to change other code. For example to follow type form your question only typedef has to be changed:

typedef struct
{
    size_t size;
    char *arr[];
}int_arr_t;

The rest of the code will calculate the correct sizes without any changes. It is much safer and less prone to errors.