一个简单的C++问题... 这个为什么不能把小写变成大写

Question

vector text;
string s = "hello world" ;
text.push_back(s);
for (auto temp = text.begin(); temp != text.end() && temp->empty(); temp++)
{
for (auto temp2=(*temp).begin();temp2!=(*temp).end();temp2++)
*temp2 = toupper(*temp2);
}
for (auto it = text.cbegin(); it != text.cend() && !it->empty(); ++it)
cout << *it << endl;