| Home | All Classes | Main Classes | Annotated | Grouped Classes | Functions | |
The QValueVector class is a value-based template class that provides a dynamic array. More...
#include <qvaluevector.h>
QValueVector is a Qt implementation of an STL-like vector container. It can be used in your application if the standard vector is not available. QValueVector is part of the Qt Template Library.
QValueVector<T> defines a template instance to create a vector of values that all have the class T. Please note that QValueVector does not store pointers to the members of the vector; it holds a copy of every member. QValueVector is said to be value based; in contrast, QPtrList and QDict are pointer based.
QValueVector contains and manages a collection of objects of type T and provides random access iterators that allow the contained objects to be addressed. QValueVector owns the contained items. For more relaxed ownership semantics, see QPtrCollection and friends which are pointer-based containers.
QValueVector provides good performance if you append or remove items from the end of the vector. If you insert or remove items from anywhere but the end, performance is very poor, since many items will need to be copied into new positions.
Some classes cannot be used within a QValueVector - for example, all classes derived from QObject and thus all classes that implement widgets. Only values can be used in a QValueVector. To qualify as a value the class must provide:
Note that C++ defaults to field-by-field assignment operators and copy constructors if no explicit version is supplied. In many cases this is sufficient.
QValueList's function naming is consistent with the other Qt classes (e.g., count(), isEmpty()). QValueList also provides extra functions for compatibility with STL algorithms, such as size() and empty(). Programmers already familiar with the STL list can use these functions instead.
Example:
#include <qvaluevector.h>
#include <qstring.h>
#include <stdio.h>
class Employee
{
public:
Employee(): s(0) {}
Employee( const QString& name, int salary )
: n( name ), s( salary )
{ }
QString name() const { return n; }
int salary() const { return s; }
void setSalary( int salary ) { s = salary; }
private:
QString n;
int s;
};
int main()
{
typedef QValueVector<Employee> EmployeeVector;
EmployeeVector vec( 3 ); // vector of 3 Employees
vec[0] = Employee( "Bill", 50000 );
vec[1] = Employee( "Steve", 80000 );
vec[2] = Employee( "Ron", 60000 );
Employee joe( "Joe", 50000 );
vec.push_back( joe ); // vector expands to accommodate 4 Employees
joe.setSalary( 4000 );
EmployeeVector::iterator it;
for( it = vec.begin(); it != vec.end(); ++it )
printf( "%s earns %d\n", (*it).name().latin1(), (*it).salary() );
return 0;
}
Program output:
Bill earns 50000
Steve earns 80000
Ron earns 60000
Joe earns 50000
As you can see, the latest changes to Joe's salary did not affect the value in the vector because the vector created a copy of Joe's entry.
Many Qt functions return const value vectors; to iterate over these you should make a copy and iterate over the copy.
There are several ways to find items in the vector. The begin() and end() functions return iterators to the beginning and end of the vector. The advantage of getting an iterator is that you can now move forward or backward from this position by incrementing/decrementing the iterator. The iterator returned by end() points to the items which is one past the last item in the container. The past-the-end iterator is still associated with the vector it belongs to, however it is not dereferenceable; operator*() will not return a well-defined value. If the vector is empty(), the iterator returned by begin() will equal the iterator returned by end().
The fastest way to access an item of a vector is by using operator[]. This function provides random access and will return a reference to the item located at the specified index. Thus, you can access every item directly, in constant time, providing you know the location of the item. It is undefined to access an item that does not exist (your application will probably crash). For example:
QValueVector<int> vec1; // an empty vector vec1[10] = 4; // undefined, probably a crash QValueVector<QString> vec2( 25 ); // initialize with 25 items vec2[10] = "Bill"; // OK
Whenever inserting, removing or referencing items in a vector, always make sure you are referring to valid positions. For example:
void func( QValueVector<int>& vec )
{
if ( vec.size() > 9 ) {
vec[9] = 99; // OK
}
}
The iterators provided by vector are random access iterators, therefore you can use them with many generic algorithms, for example, algorithms provided by the STL or the QTL.
Another way to find an item in the vector is by using the std::find() or qFind() algorithms. For example:
QValueVector<int> vec;
...
QValueVector<int>::const_iterator it = qFind( vec.begin(), vec.end(), 3 );
if ( it != vector.end() )
// 'it' points to the found item
It is safe to have multiple iterators on the vector at the same time. Since QValueVector manages memory dynamically, all iterators can become invalid if a memory reallocation occurs. For example, if some member of the vector is removed, iterators that point to the removed item and to all following items become invalidated. Inserting into the middle of the vector will invalidate all iterators. For convenience, the function back() returns a reference to the last item in the vector, and front() one for the first. If the vector is empty(), both back() and front() have undefined behavior (your application will crash or do unpredictable things). Use back() and front() with caution, for example:
QValueVector<int> vec( 3 );
vec.append( 1 );
vec.append( 2 );
vec.append( 3 );
...
if ( !vec.empty() ) {
// OK: modify the first item
int& i = vec.front();
i = 18;
}
...
QValueVector<double> dvec;
double d = dvec.back(); // undefined behavior
Because QValueVector manages memory dynamically, it is recommended to contruct a vector with an initial size. Inserting and removing items happens fastest when:
By creating a QValueVector with a sufficiently large initial size, there will be less memory allocations. Do not use an initial size that is too big, since it will still take time to construct all the empty entries, and the extra space may be wasted if it is never used.
Because QValueVector is value-based there is no need to be careful about deleting items in the vector. The vector holds its own copies and will free them if the corresponding member or the vector itself is deleted. You can force the vector to free all of its items with clear().
QValueVector is shared implicitly, which means it can be copied in constant time. If multiple QValueVector instances share the same data and one needs to modify its contents, this modifying instance makes a copy and modifies its private copy; it thus does not affect the other instances. This is often called "copy on write". If a QValueVector is being used in a multi-threaded program, you must protect all access to the vector. See QMutex.
There are several ways to insert items into the vector. The append() function insert items into the end of the vector. The insert() can be used to add items at specific positions within the vector (normally, inserting items at the end() of the vector is fastest).
Items can be also be removed from the vector in several ways. There are several variants of the erase() function which removes a specific item, or range of items, from the vector.
Vectors can be also sorted with various STL algorithms , or it can be sorted using the Qt Template Library. For example with qBubbleSort():
Example:
QValueVector<int> v( 4 );
v.append( 5 );
v.append( 8 );
v.append( 3 );
v.append( 4 );
qBubbleSort( v );
QValueVector stores its items in contiguous memory. This means that you can use a QValueVector in any situation that requires an array.
See also Qt Template Library Classes, Implicitly and Explicitly Shared Classes, and Non-GUI Classes.
Constructs an empty vector without any elements. To create a vector which reserves an initial amount of space for elements, use QValueVector(size_type n).
Constructs a copy of v.
This operation costs O(1) time because QValueVector is shared implicitly.
The first modification to the vector does however take O(n) time.
Constructs a vector with an initial size of n elements. Each element is initialized with the value of val.
Constructs a copy of v.
This operation costs O(n) time because v is copied.
Destroys the vector, destroying all elements and freeing the memory.
References to the values in the vector and all iterators of this vector become invalidated. Note that it is impossible for an iterator to check whether or not it is valid - QValueVector is tuned for performance, not error checking.
See also insert().
Returns a const reference to the item with index i. If ok is non-null, and the index i is out of range, *ok is set to FALSE and the returned reference is undefined. If the index i is within the range of the vector, and ok is non-null, *ok is set to TRUE and the returned reference is well defined.
This function is provided for STL compatibility. It is equivalent to last().
See also front().
Returns a const iterator pointing to the beginning of the vector. If the vector is empty(), the returned iterator will equal end().
See also isEmpty().
This function is provided for STL compatibility. It is equivalent to isEmpty().
See also size().
Returns a const iterator pointing behind the last item of the vector.
Removes all items from first up to but not including last and returns the position of the next item.
See also last().
This function is provided for STL compatibility. It is equivalent to first().
See also back().
Inserts n copies of x immediately before position x.
See also count().
See also first().
Assigns v to this vector and returns a reference to this vector.
All iterators of the current vector become invalidated by this operation. The cost of such an assignment is O(1) since QValueVector is implicitly shared.
Assigns v to this vector and returns a reference to this vector.
All iterators of the current vector become invalidated by this operation. The cost of this assignment is O(n) since v is copied.
Returns TRUE if each element in this vector equals each corresponding element in x; otherwise returns FALSE.
Returns a const reference to the item at index i. If i is out of range, this function has undefined behavior.
This function is provided for STL compatibility.
This function is provided for STL compatibility. It is equivalent to append().
See also insert().
This function is provided for STL compatibility. It is equivalent to count().
See also empty().
This file is part of the Qt toolkit. Copyright © 1995-2002 Trolltech. All Rights Reserved.
| Copyright © 2002 Trolltech | Trademarks | Qt version 3.1.0-b1
|