当按参数传递的第一个括号为空时，为什么C++字符数据类型为3d数组

#include <iostream>

void printArray(char arr[][3][4], int rows, int cols, int depth) {
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            for (int k = 0; k < depth; k++) {
                std::cout << arr[i][j][k] << " ";
            }
            std::cout << std::endl;
        }
        std::cout << std::endl;
    }
}

int main() {
    char arr[][3][4] = {
        { {'a', 'b', 'c', '\0'}, {'d', 'e', 'f', '\0'}, {'g', 'h', 'i', '\0'} },
        { {'j', 'k', 'l', '\0'}, {'m', 'n', 'o', '\0'}, {'p', 'q', 'r', '\0'} }
    };

    int rows = sizeof(arr) / sizeof(arr[0]);
    int cols = sizeof(arr[0]) / sizeof(arr[0][0]);
    int depth = sizeof(arr[0][0]) / sizeof(arr[0][0][0]);

    printArray(arr, rows, cols, depth);

    return 0;
}