""(空字符串) 、" "（空格字符串）、empty() 判断字符串是否为空

“”表示空字符串  “ ”表示含有一个空格的字符串不为空字符串  empty()) 判断字符串是否为空 

#include<iostream>  //代码1 表示
#include<string>
using namespace std;
int main()
{
//string max = NULL;//错误
string max1 = "";
string max2 = " ";
string max3 = "dd";


printf("%d\n", max1.size());//0
printf("%d\n", max2.size());//1
printf("%d\n", sizeof(max1));//28  一般是4  vs2013是28
printf("%d\n", sizeof(max2));//28  一般是4  vs2013是28
printf("%d\n", max2.empty());//0  empty()  //判断字符串是否为空 
printf("%d\n", sizeof(max2.empty()));//1
printf("%d\n", max3.empty());//0
printf("%d\n", sizeof(max3.empty()));//1
printf("%d\n", max1.empty());//1 是空
printf("%d\n", sizeof(max1.empty()));//1
//printf("%d\n", max2.empty().size());//错 非法
}


//0
//1
//28
//28
//0
//1
//0
//1
//1
//1
//请按任意键继续. . .



#include<iostream>  //代码1 表示
#include<string>
using namespace std;
int main()
{
//string max = NULL;//错误
string max1 = "";
string max2 = " ";
string max3 = "dd";


if (max1 == "")
cout << 1<<endl;
if (max1.empty())
cout << 1 << endl;
//if (max1 == NULL)  //错误  max1是字符串不可与NULL比较
// cout << 1 << endl;


if (max2 == "")
cout << 2 << endl;
if (max2.empty())
cout << 2 << endl;


if (max2 == " ")
cout << 3 << endl;
}


//1
//1
//3
//请按任意键继续. . .