int const &foo(int const &a)
{
    return a;
}

int const &f = foo(3); 

struct Foo
{
    Foo &operator=(int);
};

Foo &convert(Foo &a)
{
    return a;
}

Foo f;
convert(f) = 6;

std::string getStr();
std::string &myStr = getStr();

foo(3);

#include <string>

std::string getStr(std::string const &a_str)
{
    return a_str;
}

int main(int argc, char *argv[])
{
    std::string &s = getStr("123");
}

#include <string>

std::string const &getStr(std::string const &a_str)
{
    return a_str;
}

int main(int argc, char *argv[])
{
    std::string const &s = getStr("123");
}