Sorting using qSort(), - if QList contains POINTERS

[joseph]

Hi guys,


How to use qSort() if the QList contains pointers like this...

QList < double* > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>
 
_doubleList .append( new( 2.22) );
_doubleList .append( new( 1.22) );
_doubleList .append( new( 4.22) );
_doubleList .append( new( 3.22) );
 
qSort( _doubleList );           // This code will not sort the data properly

QList < double* > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>

_doubleList .append( new( 2.22) );
_doubleList .append( new( 1.22) );
_doubleList .append( new( 4.22) );
_doubleList .append( new( 3.22) );

qSort( _doubleList );           // This code will not sort the data properly

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while using the above "_doubleList" the data is not sorting .
But if am using the list like this ...

QList < double > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>
 
_doubleList .append(  2.22 );
_doubleList .append(  1.22 );
_doubleList .append( 4.22 );
_doubleList .append( 3.22 );
 
qSort( _doubleList );           // This code WILL  SORT the data properly

QList < double > _doubleList;  
//  this is for simple easyness , you can give QList<employee*>

_doubleList .append(  2.22 );
_doubleList .append(  1.22 );
_doubleList .append( 4.22 );
_doubleList .append( 3.22 );

qSort( _doubleList );           // This code WILL  SORT the data properly

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please tell me how to sort a QList which contains pointers.
thanks in advance

[ChristianEhrlicher]

You have to implement your own lessThan operator like shown here:

http://doc.trolltech.com/4.3/qtalgorithms.html#qSort-2

[joseph]

can anybody help me to sort the following code.
My intention is to sort the sudent object in assending order( based on the _rollNo ).

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	 bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}
	int getRollNo()
		{
		return _rolNo;
		}
private :
	 int _rolNo;
 
};
 
int main()
{
 
QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;
 
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();		
		cout<<"\n" << val->getRollNo();
 
	}
 
        qSort( studentList.begin() , studentList.end(), toAssending );   //  please correct  syntax if am wrong
 
	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();				
		cout<<"\n" << val_1->getRollNo();
	}
 
	qDeleteAll( studentList );
 
 
 
return 0;
}

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	 bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}
	int getRollNo()
		{
		return _rolNo;
		}
private :
	 int _rolNo;

};

int main()
{

QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;

	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();		
		cout<<"\n" << val->getRollNo();
		
	}

        qSort( studentList.begin() , studentList.end(), toAssending );   //  please correct  syntax if am wrong

	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();				
		cout<<"\n" << val_1->getRollNo();
	}
	
	qDeleteAll( studentList );



return 0;
}

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thanks

[jpn]

Make toAssending() a non-member function.

class Student
{
    ...
};
 
bool toAssending( const Student & s1 , const Student & s2 )
{
	cout<<"\n" << __FUNCTION__;
	return s1.getRollNo() < s2.getRollNo();
}
 
int main()
{
	...
        qSort( studentList.begin() , studentList.end(), toAssending );
	...
}

class Student
{
    ...
};

bool toAssending( const Student & s1 , const Student & s2 )
{
	cout<<"\n" << __FUNCTION__;
	return s1.getRollNo() < s2.getRollNo();
}

int main()
{
	...
        qSort( studentList.begin() , studentList.end(), toAssending );
	...
}

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[joseph]

jpn,

I have tried the same code its not working. i am getting compilation error.
here is my code

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	/* 
	bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}*/
	int getRollNo() const
		{
		return _rolNo;
		}
private :
	 int _rolNo;
 
};
 
bool toAssending(  const Student & s1 ,  const Student & s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1.getRollNo() < s2.getRollNo();
}
 
int main()
{
 QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();
		//cout<<"\n" << val;
		cout<<"\n" << val->getRollNo();
 
	}
 
qSort( studentList.begin() , studentList.end(), toAssending );  //here am using the global function. instead of class -member function.
 
	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();		
		cout<<"\n" << val_1;
		cout<<"\n" << val_1->getRollNo();
	}
 
	qDeleteAll( studentList );
 
return 0;
}

class Student
{
public:
	Student() 
		{ cout<<"\n" << __FUNCTION__; }
	Student(int roolNo ):_rolNo( roolNo ) 
		{ cout<<"\n" << __FUNCTION__; }
	~Student() 
		{ cout<<"\n" << __FUNCTION__; }
	/* 
	bool toAssending( Student & s1 , Student & s2 )
		{
			cout<<"\n" << __FUNCTION__;
			return s1.getRollNo() < s2.getRollNo();
		}*/
	int getRollNo() const
		{
		return _rolNo;
		}
private :
	 int _rolNo;

};

bool toAssending(  const Student & s1 ,  const Student & s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1.getRollNo() < s2.getRollNo();
}

int main()
{
 QList<Student*> studentList ;
	studentList.append( new Student(2) );
	studentList.append( new Student(4) );	
	studentList.append( new Student(3) );
	studentList.append( new Student(1) );
	studentList.append( new Student(5) );
	QListIterator<Student*> it( studentList );
	Student* val = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		val = it.next();
		//cout<<"\n" << val;
		cout<<"\n" << val->getRollNo();
		
	}
	
qSort( studentList.begin() , studentList.end(), toAssending );  //here am using the global function. instead of class -member function.

	cout<<"\n   ****** After SORT ***** " ;
	QListIterator<Student*> it_1( studentList );
	Student* val_1 = 0;
	while( it_1.hasNext() )
	{
		val_1 = it_1.next();		
		cout<<"\n" << val_1;
		cout<<"\n" << val_1->getRollNo();
	}
	
	qDeleteAll( studentList );
    
return 0;
}

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here is the compliation error.
please help me to resolve it.

main.cpp(311) : warning C4101: 'assending' : unreferenced local variable
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
        c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(180) : see reference to function template instantiation 'void QAlgorithmsPrivate::qSortHelper<RandomAccessIterator,T,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,const T & ,LessThan)' being compiled
        with
        [
            RandomAccessIterator=QList<Student *>::iterator,
            T=Student *,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
        main.cpp(312) : see reference to function template instantiation 'void qSort<QList<T>::iterator,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,LessThan)' being compiled
        with
        [
            T=Student *,
            RandomAccessIterator=QList<Student *>::iterator,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
 
 
logFile - 12 error(s), 10 warning(s)

main.cpp(311) : warning C4101: 'assending' : unreferenced local variable
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
        c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(180) : see reference to function template instantiation 'void QAlgorithmsPrivate::qSortHelper<RandomAccessIterator,T,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,const T & ,LessThan)' being compiled
        with
        [
            RandomAccessIterator=QList<Student *>::iterator,
            T=Student *,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
        main.cpp(312) : see reference to function template instantiation 'void qSort<QList<T>::iterator,bool(__cdecl *)(const Student &,const Student &)>(RandomAccessIterator,RandomAccessIterator,LessThan)' being compiled
        with
        [
            T=Student *,
            RandomAccessIterator=QList<Student *>::iterator,
            LessThan=bool (__cdecl *)(const Student &,const Student &)
        ]
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(355) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(360) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(362) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(370) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(373) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 2 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous
c:\Qt\4.2.2\include\QtCore\../../src\corelib\tools\qalgorithms.h(385) : error C2664: 'bool (const Student &,const Student &)' : cannot convert parameter 1 from 'Student *' to 'const Student &'
        Reason: cannot convert from 'Student *' to 'const Student'
        No constructor could take the source type, or constructor overload resolution was ambiguous


logFile - 12 error(s), 10 warning(s)

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[jpn]

Right, the comparator function should naturally take pointers since that's what the container contains:

bool toAssending( const Student * s1 , const Student * s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1->getRollNo() < s2->getRollNo();
}

bool toAssending( const Student * s1 , const Student * s2 )
{
    cout<<"\n" << __FUNCTION__;
    return s1->getRollNo() < s2->getRollNo();
}

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[joseph]

could you please me , how this qSort() is working .
Are we passing function pointer to qSort() ...??



thanks

[jpn]

could you please me , how this qSort() is working.

You can always check Qt sources for implementation details. You can find qSort() implementation in src/corelib/tools/qalgorithms.h.

Are we passing function pointer to qSort() ...??

Yes, qSort() calls the passed function to compare items.

[joseph]

First of all thanks jpn,
But please see the below code for compare <double*>

bool putDoubleListInAscendingOrder( const double * d1 , const double * d2 )
{
	cout<<"\n" << __FUNCTION__;
	return *d1 < *d2;
}
 
int main()
{
 
QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );
 
 
QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
        it. toFront();  //  Note tat i am using the same iterator
                            //  According to assistant , iterator will move before 1-st item
                            // if i am calling  it.next(), then it is sapposed to return the first item in
                            // _doubleList .
 
        d= it.next();  
        cout<<"\n d :"  << *d ;   //  Here the expected result is "1.22"
 
// N:B => But am getting the reuslt of cout is =  "5.12"  ...why..????
 
 
return 0;
}

bool putDoubleListInAscendingOrder( const double * d1 , const double * d2 )
{
	cout<<"\n" << __FUNCTION__;
	return *d1 < *d2;
}

int main()
{

QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );


QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
        it. toFront();  //  Note tat i am using the same iterator
                            //  According to assistant , iterator will move before 1-st item
                            // if i am calling  it.next(), then it is sapposed to return the first item in
                            // _doubleList .

        d= it.next();  
        cout<<"\n d :"  << *d ;   //  Here the expected result is "1.22"

// N:B => But am getting the reuslt of cout is =  "5.12"  ...why..????


return 0;
}

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but if am changing the code as below. ( with different iterator or _doubleList.first() will return the expected result ie; "1.22" )

int main()
{
 
QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );
 
 
QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
 
        QListIterator<double*> it_2 (  _doubleList )
        it_2. next()     ; 
        d= it_2. next();
        cout<<"\n d :"  << *d ;   //  Here the result is "1.22"
 
// could you please explain why ...?? or If my understanding is wrong please correct me..
 
return 0;
}

int main()
{

QList<double*> _doubleList ;
	_doubleList.append( new double(5.12) );
	_doubleList.append( new double(2.22) );
	_doubleList.append( new double(3.22) );
	_doubleList.append( new double(1.22) );
	_doubleList.append( new double(4.22) );


QListIterator<double*> it( _doubleList );
	double* d = 0;
	cout<<"\n   ****** Before SORT ***** " ;
	while( it.hasNext() )
	{
		d = it.next();		
		cout<<"\n" << *d;		
	}
        cout<<"\n   ****** After SORT ***** " ;
	qSort( _doubleList.begin(), _doubleList.end() , putDoubleListInAscendingOrder );
     
        QListIterator<double*> it_2 (  _doubleList )
        it_2. next()     ; 
        d= it_2. next();
        cout<<"\n d :"  << *d ;   //  Here the result is "1.22"

// could you please explain why ...?? or If my understanding is wrong please correct me..

return 0;
}

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[ChristianEhrlicher]

'it' is invalid after qSort()

[jpn]

Indeed, as QListIterator docs state:

QListIterator<T> allows you to iterate over a QList<T> (or a QQueue<T>). If you want to modify the list as you iterate over it, use QMutableListIterator<T> instead.

Also, may I ask what's the point of allocating doubles on the heap? Notice that QList is an implicitly shared class.

[joseph]

Indeed, as QListIterator docs state:

Also, may I ask what's the point of allocating doubles on the heap? Notice that QList is an implicitly shared class.

Thanks for you information.
yes you are right. There is no need for <double*>.
But it was just an example.


I have tried with QMutableListIterator . its working perfectly
thanks

[petersvp]

Check this & use functors!

http://stackoverflow.com/questions/1...01277#18301277

[d_stranz]

I am sure that 6 years ago, your advice would have been useful. Meanwhile, the OP's original project was canceled, the company went under, and he's had 4 different jobs since then.

Functors are nice, but when all you need is a simple static function that takes two pointers to double as arguments and returns a bool, adding all that overhead is overkill. And will the person who inherits this code take a look and ask, "Why the heck did they use functors when a static function would have worked just as well? Must be reading too many advanced C++ books".

Been there, done that. Functors have their uses and many C++ generic programming template constructs could not be implemented without them. Sorting a list of pointers by the value of the pointed-to instance isn't one of them.