不能返回局部对象的引用

#include<iostream>
#include<string>
using namespace std;
string make_plural(size_t, const string&, const string&);
const string &shorterString(const string &, const string &);
const string &mainip(const string&);
char &get_val(string &, string::size_type);
int main(void)
{
cout << make_plural(1, "dog", "s") << endl;
cout << make_plural(2, "dog", "s") << endl;

string string1 = "1234";
string string2 = "abc";
cout << shorterString(string1, string2) << endl;

cout << mainip("jiajia") << endl;

string s("123456");
cout << s << endl;
get_val(s, 0) = 'a';

cout << s << endl;

getchar();
return 0;
}
//返回非引用
string make_plural(size_t i, const string &word, const string &ending)
{
return (i == 1) ? word : word + ending;
}
//返回引用
const string &shorterString(const string &s1, const string &s2)
{
return s1.size()<s2.size() ? s1 : s2;
}
const string &mainip(const string &s)
{
string ret = s;
return ret;
}
//引用返回左值
char &get_val(string &str, string::size_type ix)
{
return str[ix];
}