054关于const

注：IDE vs2013

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const定义常量

#include <iostream>
#include <stdlib.h>
using namespace std;
int main()
{
	const int x = 3; 
	system("pause");
	return 0;
}

此时的x为一个常量，不能再给x赋值，比如x=5;就是错误的；
此时const相当于#define。

const与指针

1

#include <iostream>
#include <stdlib.h>
using namespace std;

int main()
{
	int x = 3;
	int y = 4;
	const int *p = &y;
	cout << *p << "," << x << "," << y << endl;
	p = &x;
	cout << *p << "," << x << "," << y << endl;
	x = 5;
	cout << *p << "," << x << "," << y << endl;
	system("pause");
	return 0;
}

const int *p = &y;等价于int const *p = &y;，此时定义*p为一个常量，不能为其赋值，比如*p=10;就是错误的，但是可以对p赋值，上述程序相当于将p的指向由y改向x，并且可以通过改变x来改变*p。
如图：

2

#include <iostream>
#include <stdlib.h>
using namespace std;

int main()
{
	int x = 4;
	int z = 5;
	int *const q = &z;
	cout << *q << "," << z << endl;
	//q = &x; // 错误
	*q = 10;
	cout << *q << "," << z << endl;
	z = 6;
	cout << *q << "," << z << endl;
	system("pause");
	return 0;
}

将q定义为常量，不能对q赋值，即不能改变q的指向，但是可以对*q赋值，同时也会改变z的值，z的改变也会改变*q。

const与引用

#include <iostream>
#include <stdlib.h>
using namespace std;

int main()
{
	int x = 3;
	int y = 4;
	int const &z = x;
	cout << z << ',' << x << endl;
	//z = y; //错误
	x = 5;
	cout << z << "," << x << endl;


	system("pause");
	return 0;
}

不能再对z赋值，但是可以改变x从而对z进行改变。

指针与引用

#include <iostream>
#include <stdlib.h>
using namespace std;
int main()
{
	int x = 10;
	int *p = &x;
	cout << x << "," << *p << endl;
	cout << &x << "," << p << endl;

	system("pause");
	return 0;
}

x *p表示数字
&x p表示地址