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 contents 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 swaping
operation, but in both cases some form of the assignment operator must be
available. Swapping, however, might be possible when assignemnt 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.
The function uses partial specializations to optimize the swapping operation for
objects of sizes 1, 2, 4 or 8 bytes. It uses
memcpy(3) for objects of
other sizes.
NAMESPACE¶
FBB
All constructors, members, operators and manipulators, mentioned in this
man-page, are defined in the namespace
FBB.
INHERITS FROM¶
-
SWAP FUNCTION¶
- o
- fswap(Type &lhs, Type &rhs):
This template function swaps the contents of the two objects. It can be used
with classes having const data members, reference members, pointer members
or standard value-typed data members.
EXAMPLE¶
#include <iostream>
#include "../fswap"
class Demo
{
std::ostream &d_out;
size_t d_value;
public:
Demo(std::ostream &out = std::cerr, size_t value = 0)
:
d_out(out),
d_value(value)
{}
void show(char const *msg)
{
d_out << msg << ". Value: " << d_value << ’\n’;
}
};
using namespace std;
int main()
{
Demo d1;
Demo d2(cout, 12);
FBB::fswap(d1, d2);
d1.show("This is d1"); // to cerr: 12
d2.show("This is d2"); // to cout: 0
}
FILES¶
bobcat/fswap - defines the class interface
SEE ALSO¶
bobcat(7),
memcpy(3)
BUGS¶
The
fswap function 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.
DISTRIBUTION FILES¶
- o
- bobcat_3.23.01-x.dsc: detached signature;
- o
- bobcat_3.23.01-x.tar.gz: source archive;
- o
- bobcat_3.23.01-x_i386.changes: change log;
- o
- libbobcat1_3.23.01-x_*.deb: debian package holding the
libraries;
- o
- libbobcat1-dev_3.23.01-x_*.deb: debian package holding the
libraries, headers and manual pages;
- o
- http://sourceforge.net/projects/bobcat: public archive location;
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).