std::map element in heap or stack?
in [C++]
|
Prev: Need to analyze C source code and determine loops (program analysis)
Next: Fast Assignment of POD Struct Whose Members Have Copy Constructors
From: shuisheng on 8 Dec 2009 08:49 Dear All, I am wondering the inserted element in std::map is located in heap or stack? For example std::map<int, double> a; a[0] = 3.14; The element of the map: pair<int, double>(0, 3.14) is in heap or stack? I think to know it is important for memory management. Thank you a lot ahead, Shuisheng -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Anders Dalvander on 9 Dec 2009 05:59 On Dec 9, 2:49 am, shuisheng <shuishen...(a)yahoo.com> wrote: > I am wondering the inserted element in std::map is located in heap or > stack? Hi Shuisheng, It all depends on the allocator for each specific map: It could be allocated on the stack, but most often using the default allocator it is allocated in the free-store. Regards, Anders Dalvander -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Ulrich Eckhardt on 9 Dec 2009 05:55 shuisheng wrote: > I am wondering the inserted element in std::map is located in heap or > stack? For example > > std::map<int, double> a; > a[0] = 3.14; Since you can insert elements into the container at runtime, either it stores them internally (which could store them on the "stack") or externally (i.e. on the heap). If it was internally, you would have to resize the object dynamically, but C++ objects always have a fixed size. Alternatively, you could allocate "enough" spare memory for internal storage (a fixed-capacity container), but that isn't really useful. The only other possibility is to use the heap, which is what is used. > The element of the map: pair<int, double>(0, 3.14) [...] Danger: The element type for a map<int, double> is pair<int const, double>! Uli -- Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Taras Shevchuk on 9 Dec 2009 06:05 On 09.12.2009 3:49, shuisheng wrote: > Dear All, > > I am wondering the inserted element in std::map is located in heap or > stack? For example > > std::map<int, double> a; > a[0] = 3.14; > > The element of the map: pair<int, double>(0, 3.14) is in heap or > stack? I think to know it is important for memory management. > > Thank you a lot ahead, > > Shuisheng > Inserted elements in all std containers are located in heap. Taras Shevchuk -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Goran Pusic on 9 Dec 2009 06:05
On Dec 9, 2:49 am, shuisheng <shuishen...(a)yahoo.com> wrote: > Dear All, > > I am wondering the inserted element in std::map is located in heap or > stack? For example > > std::map<int, double> a; > a[0] = 3.14; > > The element of the map: pair<int, double>(0, 3.14) is in heap or > stack? I think to know it is important for memory management. That's probably implementation-specific, and you can certainly change the allocator of any STL container. So you would have to consult doc for standard library you use. That said, map is usually implemented as a red-black tree and tree nodes are on heap. (Tree nodes, if I understand correctly, contain instances of value_type, which are key/value pairs of your map). Note that stack is a bad storage idea for any container as stack should be considered a scarce resource. E.g. on 32-bit Linux with pthreads, you default should be 8MB. It should be 1MB on a 32-bit Windows XP. That's peanuts compared to a couple of GB of virtual memory. Goran. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |