C++中cout/cin對於空指針、字符指針的處理，及輸入>>、 輸出

本文鏈接：http://blog.csdn.net/ncepuwanghui/article/details/23712735

#include <iostream>
using namespace std;


void func(char* p)
{
	p = new char[128];
	//*p = "Hello World";//錯誤寫法：invalid conversion from ‘const char*’ to ‘char’
	p = "Hello World";//有警告：將一個字符串常量轉換爲char*時存在風險,應使用常量指針
	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;//存放指針ptr的單元地址
	cout << "ptr = " << (void*)ptr << endl;//指針ptr指向的單元地址
	//cout << "*ptr = " << *ptr << endl;//段錯誤,無法訪問0x0地址單元
	cout << "ptr = " << ptr << endl;//指針ptr所指單元內容,而不是指針ptr指向的單元地址
	//此處由於ptr是空指針，ptr所指單元內容爲0，與字符串結束符'\0'值相同
	//因此當輸出ptr內容後輸出終止,不會輸出後面的“<<endl”
	//cout在輸出前要調用strlen來取得字符串的長度，不可避免得要有讀字符串的操作
	//也就是讀了內存地址爲0,導致輸出流異常，可用cout.clear();重置輸出流
	
	//if (!cout)
	//if (cout.bad())
	if (cout.fail())
	{
		cout.clear();//設置內部錯誤狀態，如果用缺省參量調用，則清除所有錯誤位
		cout << "\nOutput stream aready normal" << endl;
	}
	//執行結果如下：
	/*
	 * [wanghui@wanghui 0409]$ g++ -o exe func.cpp && ./exe 
	 * &ptr = 0xbfd78f6c
	 * ptr = 0
	 * ptr = [wanghui@wanghui 0409]$ 
	 */


	/*
	 *C++的IO流是通過操作符"重載"來實現的.
	 *	OS& operator <<(OS&, char*) // 輸出字符串的函數
	 *	OS& operator <<(OS&, void*) // 輸出指針地址的函數
	 *	當cout << p時; 如果p是個char*, 那麼就會調用前一個函數, 用字符串方式輸出.
	 *	如果p是其他的指針, 包括void*,int*等等; 都會匹配第二個函數, 用指針方式輸出.
	 *	cout爲流對象，自動識別輸出數據的類型.
	 *	對於字符地址，即指針類型，就是輸出其所在地址區間的所有字符，直到字符串結束符'\0'爲止.
	 */
	cout << "-------------------------------------------------" << endl;
	func(ptr);
	cout << "-------------------------------------------------" << endl;
	cout << "&ptr = " << &ptr << endl;//存放指針ptr的單元地址
	cout << "ptr = " << (void*)ptr << endl;//指針ptr指向的單元地址
	//cout << "*ptr = " << *ptr << endl;//段錯誤,無法訪問0x0地址單元
	cout << "ptr = " << ptr << endl;//指針ptr所指單元內容,而不是指針ptr指向的單元地址
	//if (!cout)
	//if (cout.bad())
	if (cout.fail())
	{
		cout.clear();//設置內部錯誤狀態，如果用缺省參量調用，則清除所有錯誤位
		cout << "\nOutput stream aready normal" << endl;
	}
	
	//delete ptr;
	delete[] ptr;
	//測試其他數據類型空指針	
	cout << "-------------------------------------------------" << endl;
	int *p = NULL;
	cout << "&p = " << &p << endl;//存放指針p的單元地址
	//cout << "p = " << (void*)p << endl;
	cout << "p = " << p << endl;//指針p指向的單元地址
	//cout << "*p = " << *p << endl;//段錯誤,無法訪問0x0地址單元


	return 0;
}

運行結果如下：

[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

參考文章：
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