Little surprises #4: string concatenation edition

Another one that came up at work today. A method was behaving as if an argument was taking on it's default value even though there was an argument right there in the code.

Abstracted to it's bare essentials it starts with a method

class Thing {
   // c'tors and so on
   void method(const std::string & s1, 
               const std::string & s2 = "");
};

which was called in a section of code that had significant history

Thing thing;
thing.method("string"// oldWayToDeduceString(),
             "second string");

Do you see it?

Only one argument is being passed to Thing::method due to string concatenation and the comma that used to separate the first argument from the second being commented out. The compiler doesn't complain because the second argument takes on it's default value.

The real failure here (and it was mine, if you really need to know) was not removing the commented code after testing and before committing this code the last time 'round.