C++中cout/cin对于空指针、字符指针的处理，及输入>>、 输出

本文链接：http://blog.csdn.net/ncepuwanghui/article/details/23712735

#include <iostream>
using namespace std;


void func(char* p)
{
	p = new char[128];
	//*p = "Hello World";//错误写法：invalid conversion from ‘const char*’ to ‘char’
	p = "Hello World";//有警告：将一个字符串常量转换为char*时存在风险,应使用常量指针
	cout << "&p = " << &p << endl;
	cout << "p = " << (void*)p << endl;
	cout << "*p = " << *p << endl;
	cout << "p = " << p  << endl;
}


int main(int argc, char const *argv[])
{
	char *ptr = NULL;
	//char *ptr = 0;
	cout << "-------------------------------------------------" << endl;
	cout << "&ptr = " << &ptr << endl;//存放指针ptr的单元地址
	cout << "ptr = " << (void*)ptr << endl;//指针ptr指向的单元地址
	//cout << "*ptr = " << *ptr << endl;//段错误,无法访问0x0地址单元
	cout << "ptr = " << ptr << endl;//指针ptr所指单元内容,而不是指针ptr指向的单元地址
	//此处由于ptr是空指针，ptr所指单元内容为0，与字符串结束符'\0'值相同
	//因此当输出ptr内容后输出终止,不会输出后面的“<<endl”
	//cout在输出前要调用strlen来取得字符串的长度，不可避免得要有读字符串的操作
	//也就是读了内存地址为0,导致输出流异常，可用cout.clear();重置输出流
	
	//if (!cout)
	//if (cout.bad())
	if (cout.fail())
	{
		cout.clear();//设置内部错误状态，如果用缺省参量调用，则清除所有错误位
		cout << "\nOutput stream aready normal" << endl;
	}
	//执行结果如下：
	/*
	 * [wanghui@wanghui 0409]$ g++ -o exe func.cpp && ./exe 
	 * &ptr = 0xbfd78f6c
	 * ptr = 0
	 * ptr = [wanghui@wanghui 0409]$ 
	 */


	/*
	 *C++的IO流是通过操作符"重载"来实现的.
	 *	OS& operator <<(OS&, char*) // 输出字符串的函数
	 *	OS& operator <<(OS&, void*) // 输出指针地址的函数
	 *	当cout << p时; 如果p是个char*, 那么就会调用前一个函数, 用字符串方式输出.
	 *	如果p是其他的指针, 包括void*,int*等等; 都会匹配第二个函数, 用指针方式输出.
	 *	cout为流对象，自动识别输出数据的类型.
	 *	对于字符地址，即指针类型，就是输出其所在地址区间的所有字符，直到字符串结束符'\0'为止.
	 */
	cout << "-------------------------------------------------" << endl;
	func(ptr);
	cout << "-------------------------------------------------" << endl;
	cout << "&ptr = " << &ptr << endl;//存放指针ptr的单元地址
	cout << "ptr = " << (void*)ptr << endl;//指针ptr指向的单元地址
	//cout << "*ptr = " << *ptr << endl;//段错误,无法访问0x0地址单元
	cout << "ptr = " << ptr << endl;//指针ptr所指单元内容,而不是指针ptr指向的单元地址
	//if (!cout)
	//if (cout.bad())
	if (cout.fail())
	{
		cout.clear();//设置内部错误状态，如果用缺省参量调用，则清除所有错误位
		cout << "\nOutput stream aready normal" << endl;
	}
	
	//delete ptr;
	delete[] ptr;
	//测试其他数据类型空指针	
	cout << "-------------------------------------------------" << endl;
	int *p = NULL;
	cout << "&p = " << &p << endl;//存放指针p的单元地址
	//cout << "p = " << (void*)p << endl;
	cout << "p = " << p << endl;//指针p指向的单元地址
	//cout << "*p = " << *p << endl;//段错误,无法访问0x0地址单元


	return 0;
}

运行结果如下：

[wanghui@wanghui 0409]$ g++ -o exe func.cpp 
func.cpp: In function ‘void func(char*)’:
func.cpp:19: warning: deprecated conversion from string constant to ‘char*’
[wanghui@wanghui 0409]$ ./exe 
-------------------------------------------------
&ptr = 0xbfb2da2c
ptr = 0
ptr = 
Output stream aready normal
-------------------------------------------------
&p = 0xbfb2da10
p = 0x8048d44
*p = H
p = Hello World
-------------------------------------------------
&ptr = 0xbfb2da2c
ptr = 0
ptr = 
Output stream aready normal
-------------------------------------------------
&p = 0xbfb2da28
p = 0

参考文章：
http://www.cplusplus.com/forum/beginner/74372/
http://bytes.com/topic/c/answers/494457-null-pointer-cout
http://blog.sina.com.cn/s/blog_70a5a49d01014qmr.html