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From: Bo Persson on 3 Aug 2010 07:23 Mathias Gaunard wrote: > On Aug 2, 1:10 am, "Bo Persson" <b...(a)gmb.dk> wrote: > >> No, they will not have to check for any other states than they >> already have. > > My point is that you will need to introduce an extra "empty" state > in order to make use of move semantics, and that state may require > additional branches in each of the object primitives. Many objects already have an empty state, like the one they get from a default constructor. If it doesn't, the move operations can leave the object in an non-empty state. The only requrement is that the object remains in a valid state. You don't have to invent a special moved-from state if that is expensive. Bo Persson -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Seungbeom Kim on 4 Aug 2010 11:14
On 2010-08-03 15:23, Bo Persson wrote: > Mathias Gaunard wrote: >> On Aug 2, 1:10 am, "Bo Persson" <b...(a)gmb.dk> wrote: >> >>> No, they will not have to check for any other states than they >>> already have. >> >> My point is that you will need to introduce an extra "empty" state >> in order to make use of move semantics, and that state may require >> additional branches in each of the object primitives. > > Many objects already have an empty state, like the one they get from a > default constructor. If it doesn't, the move operations can leave the > object in an non-empty state. The only requrement is that the object > remains in a valid state. You don't have to invent a special > moved-from state if that is expensive. Right. You don't even have to change the object, in which case a move is identical to a copy. Most likely it is when the object doesn't own a resource which you allocate and get a handle for, e.g. dynamically allocated memory, files, sockets, etc. For example: struct point { double coordinates[3]; double mass; // ... }; you just copy the values, and don't even have to reset the original. However, if you don't have to change the source object, you probably don't want to bother to define a move constructor, because the copy constructor does the same thing and will be used in place of the move constructor if it is not defined. And you often don't even have to define the copy constructor operator, because the default copy constructor will just work fine. So, when you want to define a move constructor, it is when the object owns a resource, and the copy constructor has to allocate another resource so that both objects maintain ownership to their respective resources, but the point of the move constructor is that you don't have to allocate another resource but can just "steal" the existing one. And you have to "reset" the source object somehow because it has lost the ownership. # For every occurrence of "constructor" above, you can substitute "assignment operator" and everything still holds. It is just too tedious to write "constructor/assignment operator" each time. :( -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |