23、关于字符串常量存储的一个易犯错误的地方

// Win32_test.cpp : 定义控制台应用程序的入口点。
#include "stdafx.h"
#include "iostream"
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
	char *a = "hello";
	char *b = "hello";
	char s[] = "hello";
	//*b = 'b'; error
	s[0] = 'b';
	//a,b指针指向的是常量区的数据"hello”，而s[]在栈中分配内存并复制了
	//常量数据"abcdef”的拷贝。所以，我们修改字符数组s，实际上只是修改的在
	//栈中的拷贝，而没有修改文本常量区的数据。
	if (a == b)
		cout << "a == b" << endl; //输出a == b
	if ("test" == "test")
		cout << "test == test" << endl; //输出test == test
	return 0;
}

#include <stdio.h> 
int main() 
{ 
	char *p="abcdef"; //should be : const char * p = "abcdef";
	//...
} 
/*
apparently, var p is allocated from current stack frame, 
the string constant literal: "abcdef" lies in the .rdata segment,
if u use the expresstion: p[2]='W', which will change the rdata's data,
os refuse this action, but the compiler and linker pass.
so a running-error will popup.
*/

//another ex;
#include <stdio.h> 
int main() 
{ 
	char s[]="abcdef";//s在栈，“abcdef”在数据区
	//...
} 
/*
this behavior is okay, 
this memory from s is alloated from the current stack frame,
r-vale "abcdef" will be copies to that place that is already allocated 
through above step. notice! u change only the copy NOT origin, so this program 
may support 'reload'.

//main.cpp
 int a=0;  //全局初始化区
 char *p1;  //全局未初始化区
 main()
 {
  intb;栈
  char s[]="abc";  //栈
  char *p2;     //栈
  char *p3="123456";  //123456\0在常量区，p3在栈上。
  static int c=0；  //全局（静态）初始化区
  p1 = (char*)malloc(10);
  p2 = (char*)malloc(20);  //分配得来得10和20字节的区域就在堆区。
  strcpy(p1,"123456");  //123456\0放在常量区，编译器可能会将它与p3所向"123456"优化成一个地方。
}