How to overload an in-place and a copying string manipulation functions?

Question

I would like to be able to do the following:

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

as well as

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

For this, I have two functions

#include <string>

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

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

but they produce the following error:

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

How can I overcome this?

Answer 1

The problem is with this call:

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

The compiler doesn't know which version of str_manip to call.

You can change your functions to look like this:

#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
}

Now, the compiler knows that the ambiguous call has to be the function that takes the non-const parameter. You can also be sure that your call that returns a std::string to the << operator isn't modifying your string.