class Result // Interface - all methods are abstract
{
public:
   // The same as previous

   virtual bool IsAsync() = 0;

   // Mt methods are presented without #ifdef guard
};

class RegularResult : public Result // The name could be different
{
public:
    bool IsAsync() override { return false; }

    // Mt methods returning falses, -1 etc.
};

class AsyncResult : public Result
{
public:
    bool IsAsync() override { return true; }
    
    // Return correct values for MT
};

One<Result> result(new AsyncResult());