C language-data type and pointer summary

Common data type bytes

Types of	16th	32nd	64th
char	1	1	1
short int	2	2	2
int	2	4	4
unsigned int	2	4	4
float	4	4	4
double	8	8	8
long	4	4	8
long long	8	8	8
unsigned long	4	4	8

16-bit compiler: char *: 2 bytes

32-bit editor: char *: 4 bytes

64-bit editor: char *: 8 bytes

The difference between const int * p and int * const p is analyzed using code:

#include <iostream>

using namespace std;

int main() {
	int  i1 = 40;
	int i2 = 20;
	const int *p;
	p = &i1;
	p = &i2; //不报错
	*p = 80; //报错
	//表明如果const在*前面表示*p是常量。
	i2 = 80;
	cout << *p << endl;
	return 0;
}

#include <iostream>

using namespace std;

int main() {
	int  i1 = 40;
	int i2 = 20;
	//p = &i1;  //报错
	int * const p = &i1;
	//p = &i2; //报错
	*p = 80; //不报错
	//表明如果const在*后面表示p是常量。
	i2 = 80;
	cout << *p << endl;
	return 0;
}

Add three situations

Case 1: int * p pointer to const int i1 constant

#include <iostream>

using namespace std;

int main() {
	const int i1 = 20;
	int *p;
	//p = &i1; //报错，const int 类型的i1的地址是不能赋值给指向int类型地址的指针pi的，否
	//则pi岂不是能修改i1的值了吗？
	p = (int *) &i1;
	cout << *p << endl;
	*p = 80; 
	cout << *p << endl;
	cout << i1 << endl;
	return 0;
}

Output answer:

The value of i1 has not changed.

#include <iostream>

using namespace std;

int main() {
	const int i1 = 20;
	
	int *p;
	p = (int *) &i1;
	cout << p << endl;
	cout << *p << endl;
	*p = 80;
	cout << p << endl; 
	cout << *p << endl;
	cout << i1 << endl;
	cout << &i1 << endl;
	return 0;
}

Output:

The reason is currently unknown.

Case 2: const int * pi pointer points to const int i1

The two types are the same and can be assigned as shown in the following program. i1 cannot be modified by pi and i1.

#include <iostream>

using namespace std;

int main() {
	const int i1 = 20;
	const int *p;
	p = &i1;
	cout << *p << endl;
	return 0;
}

Case 3: Use const int * const p1 to declare a pointer

#include <iostream>

using namespace std;

int main() {
	int i1 = 20;
	int i2 = 10;
	const int *const p = &i1;//必须在定义是初始化。 
	//p = &i2; //报错 
	//*p = 80; //报错 
	cout << *p << endl;
	return 0;
}

The pointer variable itself is in a memory address like other variables. We can also make a pointer point to this address.

short int *p;

short int ** pp; declares a pointer variable pp, which is used to store (or point to) the address of a pointer variable of type short int *.

pp = & p; is to assign the address of p to pp.

example:

#include <iostream>

using namespace std;

int main() {
	short int a;
	short int *p;
	short int **pp;
	a = 10;
	p = &a;
	pp = &p;
	cout << a << endl;
	cout << *p << endl;
	cout << **pp << endl;
	return 0;
}

The output is: