Detailed Description
The QMemArray class is a template class that provides arrays of simple types.
QMemArray is implemented as a template class. Define a template
instance QMemArray<X> to create an array that contains X items.
QMemArray stores the array elements directly in the array. It can
only deal with simple types (i.e. C++ types, structs, and classes
that have no constructors, destructors, or virtual functions).
QMemArray uses bitwise operations to copy and compare array
elements.
The QPtrVector collection class is also a kind of array. Like most
collection classes, it uses pointers
to the contained items.
QMemArray uses explicit sharing with a
reference count. If more than one array shares common data and one
of the arrays is modified, all the arrays are modified.
The benefit of sharing is that a program does not need to duplicate
data when it is not required, which results in lower memory use
and less copying of data.
An alternative to QMemArray is QValueVector. The QValueVector class
also provides an array of objects, but can deal with objects that
have constructors (specifically a copy constructor and a default
constructor). QValueVector provides an STL-compatible syntax and is
implicitly shared.
Example:
#include <qmemarray.h>
#include <stdio.h>
QMemArray<int> fib( int num ) // returns fibonacci array
{
Q_ASSERT( num > 2 );
QMemArray<int> f( num ); // array of ints
f[0] = f[1] = 1;
for ( int i = 2; i < num; i++ )
f[i] = f[i-1] + f[i-2];
return f;
}
int main()
{
QMemArray<int> a = fib( 6 ); // get first 6 fibonaccis
for ( int i = 0; i < a.size(); i++ )
qDebug( "%d: %d", i, a[i] );
qDebug( "1 is found %d times", a.contains(1) );
qDebug( "5 is found at index %d", a.find(5) );
return 0;
}
Program output:
0: 1
1: 1
2: 2
3: 3
4: 5
5: 8
1 is found 2 times
5 is found at index 4
Note concerning the use of QMemArray for manipulating structs or
classes: Compilers will often pad the size of structs of odd sizes
up to the nearest word boundary. This will then be the size
QMemArray will use for its bitwise element comparisons. Because
the remaining bytes will typically be uninitialized, this can
cause find() etc. to fail to find the element. Example:
// MyStruct may be padded to 4 or 8 bytes
struct MyStruct
{
short i; // 2 bytes
char c; // 1 byte
};
QMemArray<MyStruct> a(1);
a[0].i = 5;
a[0].c = 't';
MyStruct x;
x.i = '5';
x.c = 't';
int i = a.find( x ); // may return -1 if the pad bytes differ
To work around this, make sure that you use a struct where
sizeof() returns the same as the sum of the sizes of the members
either by changing the types of the struct members or by adding
dummy members.
QMemArray data can be traversed by iterators (see begin() and
end()). The number of items is returned by count(). The array can
be resized with resize() and filled using fill().
You can make a shallow copy of the array with assign() (or
operator=()) and a deep copy with duplicate().
Search for values in the array with find() and contains(). For
sorted arrays (see sort()) you can search using bsearch().
You can set the data directly using setRawData() and
resetRawData(), although this requires care.
See also Shared Classes and Non-GUI Classes.
Member Type Documentation
QMemArray::ConstIterator
A const QMemArray iterator.
See also begin() and end().
QMemArray::Iterator
A QMemArray iterator.
See also begin() and end().
Member Function Documentation
QMemArray::QMemArray ( int, int ) [protected]
Constructs an array without allocating array space. The
arguments should be (0, 0). Use at your own risk.
QMemArray::QMemArray ()
Constructs a null array.
See also isNull().
QMemArray::QMemArray ( int size )
Constructs an array with room for size elements. Makes a null
array if size == 0.
The elements are left uninitialized.
See also resize() and isNull().
QMemArray::QMemArray ( const QMemArray<type> & a )
Constructs a shallow copy of a.
See also assign().
QMemArray::~QMemArray ()
Dereferences the array data and deletes it if this was the last
reference.
QMemArray<type> & QMemArray::assign ( const QMemArray<type> & a )
Shallow copy. Dereferences the current array and references the
data contained in a instead. Returns a reference to this array.
See also operator=().
QMemArray<type> & QMemArray::assign ( const type * data, uint size )
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Shallow copy. Dereferences the current array and references the
array data data, which contains size elements. Returns a
reference to this array.
Do not delete data later; QMemArray will call free() on it
at the right time.
type & QMemArray::at ( uint index ) const
Returns a reference to the element at position index in the array.
This can be used to both read and set an element.
See also operator[]().
Iterator QMemArray::begin ()
Returns an iterator pointing at the beginning of this array. This
iterator can be used in the same way as the iterators of
QValueList and QMap, for example.
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns a const iterator pointing at the beginning of this array.
This iterator can be used in the same way as the iterators of
QValueList and QMap, for example.
int QMemArray::bsearch ( const type & v ) const
In a sorted array (as sorted by sort()), finds the first
occurrence of v by using a binary search. For a sorted
array this is generally much faster than find(), which does
a linear search.
Returns the position of v, or -1 if v could not be found.
See also sort() and find().
int QMemArray::contains ( const type & v ) const
Returns the number of times v occurs in the array.
See also find().
QMemArray<type> QMemArray::copy () const
Returns a deep copy of this array.
See also detach() and duplicate().
uint QMemArray::count () const
Returns the same as size().
See also size().
Example: scribble/scribble.cpp.
type * QMemArray::data () const
Returns a pointer to the actual array data.
The array is a null array if data() == 0 (null pointer).
See also isNull().
Examples: fileiconview/qfileiconview.cpp and network/networkprotocol/nntp.cpp.
void QMemArray::detach () [virtual]
Detaches this array from shared array data; i.e. it makes a
private, deep copy of the data.
Copying will be performed only if the reference
count is greater than one.
See also copy().
Reimplemented in QBitArray.
QMemArray<type> & QMemArray::duplicate ( const QMemArray<type> & a )
Deep copy. Dereferences the current array and obtains a copy of
the data contained in a instead. Returns a reference to this
array.
See also copy().
QMemArray<type> & QMemArray::duplicate ( const type * data, uint size )
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Deep copy. Dereferences the current array and obtains a copy of
the array data data instead. Returns a reference to this array.
The size of the array is given by size.
See also copy().
Iterator QMemArray::end ()
Returns an iterator pointing behind the last element of this
array. This iterator can be used in the same way as the iterators
of QValueList and QMap, for example.
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Returns a const iterator pointing behind the last element of this
array. This iterator can be used in the same way as the iterators
of QValueList and QMap, for example.
bool QMemArray::fill ( const type & v, int size = -1 )
Fills the array with the value v. If size is specified as
different from -1, then the array will be resized before being
filled.
Returns TRUE if successful, i.e. if size is -1, or size is
!= -1 and the memory can be allocated; otherwise returns FALSE.
See also resize().
int QMemArray::find ( const type & v, uint index = 0 ) const
Finds the first occurrence of v, starting at position index.
Returns the position of v, or -1 if v could not be found.
See also contains().
bool QMemArray::isEmpty () const
Returns TRUE if the array is empty; otherwise returns FALSE.
isEmpty() is equivalent to isNull() for QMemArray (unlike
QString).
bool QMemArray::isNull () const
Returns TRUE if the array is null; otherwise returns FALSE.
A null array has size() == 0 and data() == 0.
uint QMemArray::nrefs () const
Returns the reference count for the shared array data. This
reference count is always greater than zero.
QMemArray::operator const type * () const
Cast operator. Returns a pointer to the array.
See also data().
bool QMemArray::operator!= ( const QMemArray<type> & a ) const
Returns TRUE if this array is different from a; otherwise
returns FALSE.
The two arrays are compared bitwise.
See also operator==().
QMemArray<type> & QMemArray::operator= ( const QMemArray<type> & a )
Assigns a shallow copy of a to this array and returns a
reference to this array.
Equivalent to assign( a ).
bool QMemArray::operator== ( const QMemArray<type> & a ) const
Returns TRUE if this array is equal to a; otherwise returns
FALSE.
The two arrays are compared bitwise.
See also operator!=().
type & QMemArray::operator[] ( int index ) const
Returns a reference to the element at position index in the
array.
This can be used to both read and set an element. Equivalent to
at().
See also at().
void QMemArray::resetRawData ( const type * data, uint size )
Removes internal references to the raw data that was set using
setRawData(). This means that QMemArray no longer has access to
the data, so you are free to manipulate data as you wish.
You can now use the QMemArray without affecting the original data, for example by calling setRawData() with a pointer to some
other data.
The arguments must be the data and length, size, that were
passed to setRawData(). This is for consistency checking.
See also setRawData().
bool QMemArray::resize ( uint size, Optimization optim )
Resizes (expands or shrinks) the array to size elements. The
array becomes a null array if size == 0.
Returns TRUE if successful, or FALSE if the memory cannot be
allocated.
New elements are not initialized.
optim is either QGArray::MemOptim (the default) or
QGArray::SpeedOptim. When optimizing for speed rather than memory
consumption, the array uses a smart grow and shrink algorithm that
might allocate more memory than is actually needed for size
elements. This speeds up subsequent resize operations, for example
when appending many elements to an array, since the space has
already been allocated.
See also size().
Example: fileiconview/qfileiconview.cpp.
bool QMemArray::resize ( uint size )
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
Resizes (expands or shrinks) the array to size elements. The
array becomes a null array if size == 0.
Returns TRUE if successful, i.e. if the memory can be allocated;
otherwise returns FALSE.
New elements are not initialized.
See also size().
QMemArray<type> & QMemArray::setRawData ( const type * data, uint size )
Sets raw data and returns a reference to the array.
Dereferences the current array and sets the new array data to data and the new array size to size. Do not attempt to resize
or re-assign the array data when raw data has been set. Call
resetRawData(data, size) to reset the array.
Setting raw data is useful because it sets QMemArray data without
allocating memory or copying data.
Example I (intended use):
static char bindata[] = { 231, 1, 44, ... };
QByteArray a;
a.setRawData( bindata, sizeof(bindata) ); // a points to bindata
QDataStream s( a, IO_ReadOnly ); // open on a's data
s >> <something>; // read raw bindata
a.resetRawData( bindata, sizeof(bindata) ); // finished
Example II (you don't want to do this):
static char bindata[] = { 231, 1, 44, ... };
QByteArray a, b;
a.setRawData( bindata, sizeof(bindata) ); // a points to bindata
a.resize( 8 ); // will crash
b = a; // will crash
a[2] = 123; // might crash
// forget to resetRawData: will crash
Warning: If you do not call resetRawData(), QMemArray will attempt
to deallocate or reallocate the raw data, which might not be too
good. Be careful.
See also resetRawData().
uint QMemArray::size () const
Returns the size of the array (maximum number of elements).
The array is a null array if size() == 0.
See also isNull() and resize().
void QMemArray::sort ()
Sorts the array elements in ascending order, using bitwise
comparison (memcmp()).
See also bsearch().
bool QMemArray::truncate ( uint pos )
Truncates the array at position pos.
Returns TRUE if successful, i.e. if the memory can be allocated;
otherwise returns FALSE.
Equivalent to resize(pos).
See also resize().
Related Functions
Writes byte array a to the stream s and returns a reference
to the stream.
See also Format of the QDataStream operators.
Reads a byte array into a from the stream s and returns a
reference to the stream.
See also Format of the QDataStream operators.
Q_UINT16 qChecksum ( const char * data, uint len )
Returns the CRC-16 checksum of len bytes starting at data.
The checksum is independent of the byte order (endianness).
This file is part of the Qt toolkit.
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