There is an argument between the Rambam [Shabat chapter 14] and Tosphot about what constitutes a private domain.
Tosphot and the Rashba both hold three walls constitutes a private domain on Shabat. But the Rambam makes a difference. If the alley is open to a public domain then it is just a Carmalit. [A place that is not a private domain nor a public domain.] But if open to a carmalit then it is a private domain. That is the three walls reduce it one level down.
There is a lot to go into here because of a few gemaras in Eruvin which seem to be clearly like Tosphot. [And I wanted to add that the Karban Eda in the Yerushalmi holds that the Rambam holds a Lehi is considered a wall from the Torah but an overhead board is just derabanan.]
But the thing I wanted to point out here is something I mentioned a few years ago--that you really do not see the Gemara making the distinction about a public domain having 600,000 people walking through it. So on Shabat my approach is to carry only in a pocket. This you can see in Ketubot chapter 3 and also in Bava Batra that the thief taking out a purse on Shabat is obligated for Shabat when the purse has changed domain, not when the object in the purse has changed domains.[You can see this more clearly in Bava Batra but I have forgotten the sugia over there.]
But unless it is really absolutely necessary I think it is best to stay home on Shabat and avoid all the problems. Besides that usually people need to recover from Shabat ion Sunday. So it really is not much of a day of rest for most people.
Tosphot and the Rashba both hold three walls constitutes a private domain on Shabat. But the Rambam makes a difference. If the alley is open to a public domain then it is just a Carmalit. [A place that is not a private domain nor a public domain.] But if open to a carmalit then it is a private domain. That is the three walls reduce it one level down.
There is a lot to go into here because of a few gemaras in Eruvin which seem to be clearly like Tosphot. [And I wanted to add that the Karban Eda in the Yerushalmi holds that the Rambam holds a Lehi is considered a wall from the Torah but an overhead board is just derabanan.]
But the thing I wanted to point out here is something I mentioned a few years ago--that you really do not see the Gemara making the distinction about a public domain having 600,000 people walking through it. So on Shabat my approach is to carry only in a pocket. This you can see in Ketubot chapter 3 and also in Bava Batra that the thief taking out a purse on Shabat is obligated for Shabat when the purse has changed domain, not when the object in the purse has changed domains.[You can see this more clearly in Bava Batra but I have forgotten the sugia over there.]
But unless it is really absolutely necessary I think it is best to stay home on Shabat and avoid all the problems. Besides that usually people need to recover from Shabat ion Sunday. So it really is not much of a day of rest for most people.
