#include<iostream>
#include<string>
using namespace std;
string s = "hello";
for (auto &i : s )
i = toupper(i); //改變成大寫，影響s的值
cout<<s<<endl;   //s的值是 HELLO

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程式碼2：
#include<iostream>
#include<string>
using namespace std;
string s = "hello";
for (auto i : s )
i = toupper(i);   //改變成大寫，不影響s的值
cout<<s<<endl;   //s的值是 hello
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