Provided by: libbobcat-dev_6.03.02-2_amd64 bug

NAME

       FBB::fswap - generic template fast swap function

SYNOPSIS

       #include <bobcat/fswap>

DESCRIPTION

       The  information stored in objects frequently needs to be swapped. A well-known example is
       the swapping operation required when implementing an overloaded assignment  operator.  For
       example, the generic form of the operator assignment operator is:

           Class &operator=(Class const &other)
           {
               Class tmp(other);
               swap(tmp);
               return *this;
           }

       The  swap functionality merely swaps the content of the current object and another object.
       The standard std::swap function calls the class’s  operator=  function  to  swap  objects.
       Newer  implementations  might  use  move-operations  to increase the speed of the swapping
       operation, but in both cases some form of  the  assignment  operator  must  be  available.
       Swapping,  however, might be possible when assignment isn’t. Classes having reference data
       members usually don’t offer assignment operators but  swapping  might  be  a  well-defined
       operation.

       It  is  well known that objects can be installed in a block of memory using placement new,
       using a block of memory the size of the object to construct the object  it.  This  is  the
       foundation of the template function FBB::fswap (fast swap). This swap function merely uses
       the memory occupied by objects to implement the swapping operation and it may therefore be
       used  with classes having const data members, reference data members, ponters to allocated
       memory etc, etc. The function simply uses a spare block of memory the size of  the  object
       to  be  swapped.  It  then  uses  memcpy(3)  to  swap the information contained in the two
       objects, using the spare block of memory as a placeholder.

       Classes may define data members that do not support fast swapping. If  such  data  members
       can  be  swapped using either std::swap or their own swap member, then overloaded versions
       of fswap can be used to fast-swap objects of such classes. Also, classes may inherit  from
       base  classes  that  do  not  support  fast-swapping, but that either offer their own swap
       members or can be swapped using std::swap. For these cases overloaded  versions  of  fswap
       are also available. The classes std::string and std::unordered_map are examples of classes
       whose objects might not be swappable using a fast swap  method.  Therefore,  in  practice,
       classes  defining members of such classes should use one of the overloaded fswap functions
       for swapping their objects.

NAMESPACE

       FBB
       All constructors, members, operators and manipulators, mentioned  in  this  man-page,  are
       defined in the namespace FBB.

INHERITS FROM

       -

ENUMERATION

       The enumereation SwapMode is used to specify a specific swapping-method:

       o      SWAPMEMBER  is  selected  by  default,  but it can also explicitly be specified. It
              indicates that selected members are swapped using their own  swap  member  function
              having prototpype void Type::swap(Type &rhs).

       o      STDSWAP can be specified to indicates that selected members should be swapped using
              std::swap.  Specific members not supporting fast swapping  can  always  be  swapped
              using  a  specific swap-method. E.g., when SWAPMEMBER is selected, but a particular
              data member does not offer a swap-member, then std::swap can be specified for  just
              that member.

SWAP FUNCTIONS

       o      fswap(Type &lhs, Type &rhs):
              This  template  function  swaps  the  content  of  two objects. It can be used with
              classes having const data members, reference members, pointer members  or  standard
              value-typed data members;

       o      fswap(Type &lhs, Type &rhs, member, ...):
              This  function  is provided with a list of member names (i.e., members of the class
              Type) that do not support fast swapping. Those members are swapped using their swap
              member,  while all other members are fast-swapped. When using lists of members, the
              selected members must be listed in their declaration order or a  std::runtime_error
              exception is generated when the function is used.

       o      fswap<SwapMode::SWAPMEMBER>(Type &lhs, Type &rhs, member, ...):
              This  function  acts identically as the previous function, but explicitly specifies
              its default swapping method.

              Each of the members specified in the list of members  can  be  specified  by  their
              names,  in  which  case  the  specified  swapping  method  is  used.  Alternatively
              stdswap(member) can be used, in which case std::swap is used for  swapping  member;
              or swapmember(member) can be used, in which case that member’s swap member function
              is used for swapping member;

       o      fswap<SwapMode::STDSWAP>(Type &lhs, Type &rhs, member, ...):
              This function is also provided with a list of member names that do not support fast
              swapping. Those members are swapped using std::swap. As with the previous function,
              members can be specified by their names, or stdswap(member)  or  swapmember(member)
              can be used;

       o      fswap[swapMode](&firstMember, Type &lhs, Type &rhs [, member, ...]):
              This  function’s first argument is the address of Type’s first data member (usually
              specified as &d_first inside Type’s own swap member).  It  is  used  when  Type  is
              derived from a base class that itself does not support fast swapping. This function
              may optionally be provided with a SwapMode template non-type argument  (by  default
              SWAPMEMBER  is  used),  and may also be provided with a list of members (optionally
              using stdswap and swapmember).

EXAMPLE

       #include <iostream>

       #include "../fswap"

       using namespace FBB;

           // Demo class, members d_v3 and d_v4 cannot be memcpy-fast swapped
           //
       class Demo
       {
           size_t d_v1;
           size_t d_v2;
           std::string d_v3;
           std::string d_v4;
           size_t d_v5;

           public:
               Demo(size_t value = 0);
               void show(char const *msg);
               void swap(Demo &rhs);
       };

       Demo::Demo(size_t value)
       :
           d_v1(value),
           d_v2(value + 1),
           d_v3(std::to_string(value + 2)),
           d_v4(std::to_string(value + 3)),
           d_v5(value + 4)
       {}

           // fast-swap 2 objects, except for d_v3 and d_v4, which are
           // swapped by either std::swap or their own .swap() members
           //
       void Demo::swap(Demo &rhs)
       {
                               // This is OK, after commenting out the
       //  fswap(*this, rhs);  // string members

                               // specifying members that should be swapped
                               // using std::swap. These members MUST be
                               // specified in their class declaration order
           fswap(*this, rhs, d_v3, d_v4);
       //  fswap(*this, rhs, d_v4, d_v3);  // this won’t work...

                                   // same, explicitly requesting the
                                   // swap-mode
       //  fswap<SwapMode::SWAPMEMBER>(*this, rhs, d_v3, d_v4);

                                   // explicitly requesting another
                                   // swap-mode
       //  fswap<SwapMode::STDSWAP>(*this, rhs, d_v3, d_v4);

                                   // default, starting at a begin-member
                                   // NOTE: the example does NOT swap d_v1
       //  fswap(&d_v2, *this, rhs, d_v3, d_v4);

                                   // use fastswap, but start at the
                                   // member d_v1 (use this for derived
                                   // classes whose base class do not
                                   // support fast swapping.
                                   // Before using this example comment
                                   // out the class’s std::string members
       //  fswap(&d_v1, *this, rhs, d_v3);

                                   // same, explicitly requesting the
                                   // swap method, swapping all
       //  fswap<SwapMode::SWAPMEMBER>(&d_v1, *this, rhs, d_v3, d_v4);

                                   // explicitly requesting another
                                   // swap-mode
       //  fswap<SwapMode::STDSWAP>(&d_v1, *this, rhs, d_v3, d_v4);

                                   // use stdswap by default, but not
                                   // for d_v4, for which .swap() is
                                   // used
       //  fswap(&d_v1, *this, rhs, d_v3, swapmember(d_v4));

                                    // same
       //  fswap<SwapMode::STDSWAP>(&d_v1, *this, rhs, d_v3,
       //                                              swapmember(d_v4));

                                    // explicitly requesting the already
                                    // default swap method is OK
       //  fswap(&d_v1, *this, rhs, swapmember(d_v3), stdswap(d_v4));

       }

       void Demo::show(char const *msg)
       {
           std::cout << msg << ". " << d_v1 <<
                           ", " << d_v2 <<
                           ", " << d_v3 <<
                           ", " << d_v4 <<
                           ", " << d_v5 <<
                           ’\n’;
       }

       using namespace std;

       int main()
       {
           Demo d1(10);
           Demo d2(20);

           d1.show("This is d1:");
           d2.show("This is d2:");

           cout << "swapping...\n";
           d1.swap(d2);

           d1.show("This is d1:");
           d2.show("This is d2:");

       }

FILES

       bobcat/fswap - defines the class interface

SEE ALSO

       bobcat(7), memcpy(3)

BUGS

       The fswap functions should not be applied mechanically to swap objects of  classes  having
       pointer data members defining, e.g., a linked list. Consider a list of four objects like:

           A -> B -> C -> D

       fast-swapping B and C would result in the following corrupted list:

                      +------+
                      |      |
           A -> C -+  +-> B -+   +-> D
                   |             |
                   +-------------+

       However, classes implementing a data structure like a linked-list might still benefit from
       fast swapping operations: by implementing their own swap member they could first use  fast
       swapping  to  swap  the  objects,  followed  by  another  fast swap to unswap their `next’
       pointers.

       The fswap function should also not be used for objects defining (back-)pointers  to  their
       own data. Consider the following objects using pointers to data and (back-)pointers to the
       original objects:

           Before fswapping:
               A                                  B
              +--------+   +-----------+         +--------+   +-----------+
              |        |   |           |         |        |   |           |
            +--> *Aimp------> *A (back)--+     +--> *Bimp------> *B (back)--+
            | |        |   |           | |     | |        |   |           | |
            +--**Aimp  |   +-----------+ |     +--**Bimp  |   +-----------+ |
              +--------+ <---------------+       +--------+ <---------------+

           After fswapping:
                            +-------------------------------+
                         +--|-------------------------------|-+
           +-------------|--|-----------------+             | |
           |   A         |  v                 |   B         | v
           |  +--------+ | +-----------+      |  +--------+ | +-----------+
           |  |        | | |           |      |  |        | | |           |
         +-----> *Bimp---+ |  *A (back)--+    +---> *Aimp---+ |  *B (back)--+
         | |  |        |   |           | |       |        |   |           | |
         | +---**Bimp  |   +-----------+ |    +---**Aimp  |   +-----------+ |
         |    +--------+ <---------------+    |  +--------+ <---------------+
         +------------------------------------+

       After the swap **Bimp should point to Bimp’s address (now at A), but in fact it points  to
       Aimp’s  address  (now  at  B).  Likewise,  the back pointers still point at their original
       objects rather than at their swapped objects.

       All stream classes define such pointers and can therefore not be swapped using fswap.

       The bottom line being that fswap should only be used for self-defined classes for which it
       can be proven that fast-swapping does not corrupt the values of its pointer data.

BOBCAT PROJECT FILES

       o      https://fbb-git.gitlab.io/bobcat/: gitlab project page;

       o      bobcat_6.03.02-x.dsc: detached signature;

       o      bobcat_6.03.02-x.tar.gz: source archive;

       o      bobcat_6.03.02-x_i386.changes: change log;

       o      libbobcat1_6.03.02-x_*.deb: debian package containing the libraries;

       o      libbobcat1-dev_6.03.02-x_*.deb:  debian  package  containing the libraries, headers
              and manual pages;

BOBCAT

       Bobcat is an acronym of `Brokken’s Own Base Classes And Templates’.

COPYRIGHT

       This is free software, distributed under the terms  of  the  GNU  General  Public  License
       (GPL).

AUTHOR

       Frank B. Brokken (f.b.brokken@rug.nl).