NULL與""空字符串的區別

""空字符串，會創建一個對象，分配內存空間

NULL，不會創建對象，不會分配內存空間，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;
}

在以下代碼最后，用NULL與字符類型進行比較，會直接導致編譯錯誤！

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

從編譯的錯誤提示看：NULL為數值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;
}

從這段代碼，進一步驗證了，NULL對就的值為數值0，地址為空

注意：將NULL賦值為int變量時會報警告

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