avoid cost of std::mutex when not multi-threading?

Question

Suppose I have an application that may or may not have spawned multiple threads. Is it worth it to protect operations that need synchronization conditionally with a std::mutex a

Answer 1

You are on the right track - write the functional part withot synchronization and add it externally, if and when needed.

Instead of the explicit if-block I would still instantiate the lock, and hide the complexity in there.

template 
struct faster_lock{
  faster_lock(Mutex& mutex) lock here, possibly with nested RAII {}
  ~faster_lock()noexcept { unlock here, or nested RAII }
};

{
  faster_lock lock(mutex);
  operation_requiring_synchronization();
}

And the last note - if you have atomic flag anyway you can just turn it into a spinlock and keep your logic simpler.