""空字符串，会创建一个对象，分配内存空间

NULL，不会创建对象，不会分配内存空间，意思是不存在的


#include<iostream>usingnamespacestd;intmain(){std::stringstrTemp;if(strTemp.empty()){cout<<"isempty"<<endl;}else{cout<<"isnotempty"<<endl;}strTemp="";if(strTemp.empty()){cout<<"isempty"<<endl;}else{cout<<"isnotempty"<<endl;}if(strTemp==""){cout<<"isempty"<<endl;}else{cout<<"isnotempty"<<endl;}/*if(strTemp==NULL){cout<<"isempty"<<endl;}else{cout<<"isnotempty"<<endl;}*/return0;}

在以下代码最后，用NULL与字符类型进行比较，会直接导致编译错误！

test.cpp:37:17: error: no match for ‘operator==’ in ‘strTemp == 0’

从编译的错误提示看：NULL为数值0


#include<iostream>usingnamespacestd;intmain(){if(NULL==0){cout<<"NULLisnumber0"<<endl;}else{cout<<"NULLisnotnumber0"<<endl;}intnNum;cout<<"nNUm="<<nNum<<endl;nNum=NULL;cout<<"nNUm="<<nNum<<endl;cout<<static_cast<void*>(NULL)<<endl;return0;}

从这段代码，进一步验证了，NULL对就的值为数值0，地址为空

注意：将NULL赋值为int变量时会报警告

NULL.cpp:18:9: warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null]