Q3ValueVector Class Reference

Detailed Description

The Q3ValueVector class is a value-based template class that provides a dynamic array.

Q3ValueVector is a Qt implementation of an STL-like vector container. It can be used in your application if the standard vector is not available for your target platforms.

Q3ValueVector<T> defines a template instance to create a vector of values that all have the class T. Q3ValueVector does not store pointers to the members of the vector; it holds a copy of every member. Q3ValueVector is said to be value based; in contrast, Q3PtrList and Q3Dict are pointer based.

Q3ValueVector contains and manages a collection of objects of type T and provides random access iterators that allow the contained objects to be addressed. Q3ValueVector owns the contained elements. For more relaxed ownership semantics, see Q3PtrCollection and friends, which are pointer-based containers.

Q3ValueVector provides good performance if you append or remove elements from the end of the vector. If you insert or remove elements from anywhere but the end, performance is very bad. The reason for this is that elements must to be copied into new positions.

Some classes cannot be used within a Q3ValueVector: for example, all classes derived from QObject and thus all classes that implement widgets. Only values can be used in a Q3ValueVector. To qualify as a value the class must provide:

• a copy constructor;
• an assignment operator;
• a default constructor, i.e., a constructor that does not take any arguments.

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.

Q3ValueVector uses an STL-like syntax to manipulate and address the objects it contains.

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 Q3ValueVector<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( 70000 );

     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 most recent change 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 move forward or backward from this position by incrementing/decrementing the iterator. The iterator returned by end() points to the element which is one past the last element 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 element of a vector is by using operator[]. This function provides random access and will return a reference to the element located at the specified index. Thus, you can access every element directly, in constant time, providing you know the location of the element. It is undefined to access an element that does not exist (your application will probably crash). For example:

 Q3ValueVector<int> vec1;  // an empty vector
 vec1[10] = 4;  // WARNING: undefined, probably a crash

 Q3ValueVector<QString> vec2(25); // initialize with 25 elements
 vec2[10] = "Dave";  // OK

Whenever inserting, removing or referencing elements in a vector, always make sure you are referring to valid positions. For example:

 void func( Q3ValueVector<int>& vec )
 {
     if ( vec.size() > 10 ) {
         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.

It is safe to have multiple iterators on the vector at the same time. Since Q3ValueVector 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 element and to all following elements become invalidated. Inserting into the middle of the vector will invalidate all iterators. For convenience, the function back() returns a reference to the last element in the vector, and front() returns a reference to the first element. 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:

 Q3ValueVector<int> vec( 3 );
 vec.push_back( 1 );
 vec.push_back( 2 );
 vec.push_back( 3 );
 ...
 if ( !vec.empty() ) {
     // OK: modify the first element
     int& i = vec.front();
     i = 18;
 }
 ...
 Q3ValueVector<double> dvec;
 double d = dvec.back(); // undefined behavior

Because Q3ValueVector manages memory dynamically, it is recommended that you contruct a vector with an initial size. Inserting and removing elements happens fastest when:

• Inserting or removing elements happens at the end() of the vector;
• The vector does not need to allocate additional memory.

By creating a Q3ValueVector 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 will be wasted if it is never used.

Because Q3ValueVector is value-based there is no need to be careful about deleting elements 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().

Q3ValueVector is shared implicitly, which means it can be copied in constant time. If multiple Q3ValueVector 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 Q3ValueVector is being used in a multi-threaded program, you must protect all access to the vector. See QMutex.

There are several ways to insert elements into the vector. The push_back() function insert elements into the end of the vector, and is usually fastest. The insert() function can be used to add elements at specific positions within the vector.

Items can be also be removed from the vector in several ways. There are several variants of the erase() function which removes a specific element, or range of elements, from the vector.

Q3ValueVector stores its elements in contiguous memory. This means that you can use a Q3ValueVector in any situation that requires an array.