QMemArray

NAME

SYNOPSIS

#include <qmemarray.h>

Inherited by QByteArray and QPointArray.

Public Members

Protected Members

RELATED FUNCTION DOCUMENTATION

DESCRIPTION

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

See also begin() and end().  

QMemArray::Iterator

See also begin() and end().  

MEMBER FUNCTION DOCUMENTATION

QMemArray::QMemArray ( int, int ) [protected]

QMemArray::QMemArray ()

See also isNull().  

QMemArray::QMemArray ( int size )

The elements are left uninitialized.

See also resize() and isNull().  

QMemArray::QMemArray ( const QMemArray<type> & a )

See also assign().  

QMemArray::~QMemArray ()

QMemArray<type> & QMemArray::assign ( const QMemArray<type> & a )

See also operator=().  

QMemArray<type> & QMemArray::assign ( const type * data, uint size )

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

This can be used to both read and set an element.

See also operator[]().  

Iterator QMemArray::begin ()

ConstIterator QMemArray::begin () const

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

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

See also find().  

QMemArray<type> QMemArray::copy () const

See also detach() and duplicate().  

uint QMemArray::count () const

See also size().

Example: scribble/scribble.cpp.  

type * QMemArray::data () const

The array is a null array if data() == 0 (null pointer).

See also isNull().

Examples:  

void QMemArray::detach () [virtual]

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 )

See also copy().  

QMemArray<type> & QMemArray::duplicate ( const type * data, uint size )

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 ()

ConstIterator QMemArray::end () const

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 )

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

Returns the position of v, or -1 if v could not be found.

See also contains().  

bool QMemArray::isEmpty () const

isEmpty() is equivalent to isNull() for QMemArray (unlike QString).  

bool QMemArray::isNull () const

A null array has size() == 0 and data() == 0.  

uint QMemArray::nrefs () const

QMemArray::operator const type * () const

See also data().  

bool QMemArray::operator!= ( const QMemArray<type> & a ) const

The two arrays are compared bitwise.

See also operator==().  

QMemArray<type> & QMemArray::operator= ( const QMemArray<type> & a )

Equivalent to assign( a ).  

bool QMemArray::operator== ( const QMemArray<type> & a ) const

The two arrays are compared bitwise.

See also operator!=().  

type & QMemArray::operator[] ( int index ) const

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 )

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 )

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.

Note: By default, SpeedOptim is not available for general use since it is only available if Qt is built in a particular configuration.

See also size().

Example: fileiconview/qfileiconview.cpp.  

bool QMemArray::resize ( uint size )

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 )

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

The array is a null array if size() == 0.

See also isNull() and resize().  

void QMemArray::sort ()

See also bsearch().  

bool QMemArray::truncate ( uint pos )

Returns TRUE if successful, i.e. if the memory can be allocated; otherwise returns FALSE.

Equivalent to resize(pos).

See also resize().  

RELATED FUNCTION DOCUMENTATION

QDataStream & operator<< ( QDataStream & s, const QByteArray & a )

See also Format of the QDataStream operators.  

QDataStream & operator>> ( QDataStream & s, QByteArray & a )

See also Format of the QDataStream operators.  

Q_UINT16 qChecksum ( const char * data, uint len )

The checksum is independent of the byte order (endianness).

SEE ALSO

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