C ++ error: a expected initializer before [function name]

The return type for stringThing must be either void or string, not both. You also must include <string>, if you want to use string.

Since you want to output the return value of stringThing() in main, I guess it should be

std::string stringThing (std::string shiftdir, const std::string &teststring)

But then, you must also return a string from your function

if (shiftdir == "right")
    return teststring + " " + "Bit Shifted right";
else
    return teststring + " " + "Bit Shifted left";

for example.

Your parameter std::string &teststring won’t work with your const char* argument. So either declare it as a copy by value string only, or better const string&.