一起来练习C++的指针

	int a = 10, b = 20;
	int* p = &a;
	p = &b;		// 改变指向的权限  ?
	*p = 30;	// 修改内存的权限  ?
	const int* cp = &a;	// 限制修改权限
	//*cp = 100;		// error：表达式必须是可修改的左值	修改 ?
	cp = &b;			// ok.								指向 ?
	int* const pa = &a;	// 限制指向权限
	*pa = 100;			// ok.								修改 ?
	//pa = &b;			// error：表达式必须是可修改的左值  指向 ?

	int a = 10;
	int* p = &a;	// int*  <==  int *
	int* q = p;				// int*  <== int*
	const int* cp = p;		// const int*  <==  int*	权限缩小，?
	int* const pa = p;		// int*	 <== int*			注意：int* const pa;是“int*”类型
	//int* p1 = cp;	// error：int*  <== const int *     权限放大，?
	int* p2 = pa;	// ok.    int*  <== int*

int *       <==  int *		// int* 包含 int* 和 int* const类型
int const * <==  int *		// int const * <=等同=> const int *
// 以上公式反过来赋值就是错误的..

	int a = 10;
	const int* p = &a;
	int* q = p;		
	int a = 10;
	int* const p = &a;
	int* q = p;
	int a = 10;
	int* const p = &a;
	int* const q = p;
	int a = 10;
	int* const p = &a;
	const int* q = p;

	int a = 10;
	const int* p1 = &a;
	int* q1 = p1;				// error：无法从const int * 转为 int *
	/* 分析：
		int* <= cosnt int*
	*/
	int* const p2 = &a;
	int* q2 = p2;
	/* 分析：
		int* <= int*
	*/
	int* const p3 = &a;
	int* const q3 = p3;
	/* 分析：
		int* <= int*
	*/
	int* const p4 = &a;
	const int* q4 = p4;
	/* 分析：
		cosnt int* <= int*
	*/

// 选项A
int a = 10;
int* p1 = &a;
const int** q1 = &p1;	
// 选项B
int a = 10;
int* p2 = &a;
int* const* q2 = &p2;
// 选项C
int a = 10;
int* p3 = &a;
int** const q3 = &p3;
// 选项D
int a = 10;
int* const p4 = &a;
int** q4 = &p4;	
// 选项E
int a = 10;
const int* p5 = &a;
int* const* q5 = &p5;	

	int* p1 = &a;
	const int** q1 = &p1;	//error  无法从“int * *”转换为“const int** ”

	// 方案一
	const int* p12 = &a;
	const int** q12 = &p12;

	// 方案二
	int* p11 = &a;
	const int* const* q11 = &p1;

	int* p2 = &a;			
	int* const* q2 = &p2;
	/* 分析：
		int* const*  <==  int* *
		const修饰 *q2，即cosnt修饰一级指针
		cosnt*  <== *				// 去掉前面的 int* <= int*
		int const *  <== int *		// 添加一个任意类型，如int
			如?所示，这是一个权限缩小的一级指针赋值，?
	*/

	int* p3 = &a;
	int** const q3 = &p3;
	cout << typeid(q3).name() << endl;	//输出 q3 类型   int * *
	/* 分析：
		int**const  <== int* *  即  int**  <== int* *
		两边类型相同，无需进行特殊判断，?
	*/

	int n = 10;
	// 使用typeid(valtypr).name() 输出变量类型
	int const* p1 = &n;		// int const *
	int* const p2 = &n;		// int *			// 忽略const
	int* p = &n;
	int** q = &p;
	//int const** q1 = &p;		
	int const* const* q1 = &p;	   // int const* const*
	int* const* q2 = &p;		   // int* const*
	int** const q3 = &p;		   // int**		// 忽略cosnt

	int* const p4 = &a;
	int** q4 = &p4;			//error  "int *const *"类型的值不能用于初始化"int **类型的实体

	const int* p5 = &a;
	int* const* q5 = &p5;	//error  无法从"const int **"转换为"int *const *"

	//1 int* <= int*
	int* p51 = &a;					// const int* p5  ?   int* p51
	int* const* q51 = &p51;
	//2 const int*  <= const int*
	const int* p52 = &a;
	const int* const* q52 = &p52;	// int* const* q5 ?   const int* const* q52