NULL与""空字符串的区别

""空字符串，会建立一个对象，分配内存空间

NULL，不会建立对象，不会分配内存空间，意思是不存在的

#include <iostream>
using namespace std;

int main()
{
        std::string strTemp;

        if( strTemp.empty() )
        {
                cout<<"is empty "<<endl;
        }
        else
        {
                cout<<"is not empty"<<endl;
        }

        strTemp = "";

        if( strTemp.empty() )
        {
                cout<<"is empty"<<endl;
        }
        else
        {
                cout<<"is not empty"<<endl;
        }

        if( strTemp == "" )
        {
                cout<<"is empty"<<endl;
        }
        else
        {
                cout<<"is not empty"<<endl;
        }

        /*
        if( strTemp == NULL )
        {
                cout<<"is empty"<<endl;
        }
        else
        {
                cout<<"is not empty"<<endl;
        }
        */

        return 0;
}

在如下代码最后，用NULL与字符类型进行比较，会直接致使编译错误！

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>
using namespace std;

int main()
{
        if( NULL == 0 )
        {
                cout<<"NULL is number 0"<<endl;
        }
        else
        {
                cout<<"NULL is not number 0"<<endl;
        }

        int nNum;
        cout<<"nNUm = "<<nNum<<endl;

        nNum = NULL;
        cout<<"nNUm = "<<nNum<<endl;

        cout<<static_cast<void*>(NULL)<<endl;

        return 0;
}

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]