double pointer undefined c++ code example

Example 1: double pointer malloc in c

char *x;  // Memory locations pointed to by x contain 'char'
char **y; // Memory locations pointed to by y contain 'char*'

x = (char*)malloc(sizeof(char) * 100);   // 100 'char'
y = (char**)malloc(sizeof(char*) * 100); // 100 'char*'

// below is incorrect:
y = (char**)malloc(sizeof(char) * 50 * 50);
// 2500 'char' not 50 'char*' pointing to 50 'char'

Example 2: double pointers C++

#include <stdio.h>

int main(void)
{
    int value = 100;
    int *value_ptr = &value;
    int **value_double_ptr = &value_ptr;

    printf("Value: %d\n", value);
    printf("Pointer to value: %d\n", *value_ptr);
    printf("Double pointer to value: %d\n", **value_double_ptr);
}