std::cout << str_manip("string to manipulate");

std::string str;
str_manip(str);
std::cout << str;

#include <string>

// copying
std::string str_manip(std::string str)
{
        // manipulate str
        return str;
}

// in-place
void str_manip(std::string& str)
{
        // manipulate str
}

error: call of overloaded 'str_manip(std::__cxx11::string&)' is ambiguous

std::string str;
str_manip(str);
std::cout << str;

#include <string>

// copying
std::string str_manip(const std::string& str)
{
        std::string dup = str;
        // manipulate dup
        return dup;
}

// in-place
void str_manip(std::string& str)
{
        // manipulate str
}