QList<A*> indexOf

[Moppel]

Hi,

can someone explain this compiler error below?

#include <QList>
 
class A
{
 
public:
	A(){ _list = new QList<A*>(); }
	~A(){ delete _list;}
	int indexOfChild(const A* x) const { return _list->indexOf( x, 0); }
 
private:
	QList<A*> *_list;
};

#include <QList>

class A
{

public:
	A(){ _list = new QList<A*>(); }
	~A(){ delete _list;}
	int indexOfChild(const A* x) const { return _list->indexOf( x, 0); }

private:
	QList<A*> *_list;
};

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a.cpp: In member function `int A::indexOfChild(const A*) const':
a.cpp:9: error: invalid conversion from `const A*' to `A*'
a.cpp:9: error:   initializing argument 1 of `int QList<T>::indexOf(const T&,
   int) const [with T = A*]'

a.cpp: In member function `int A::indexOfChild(const A*) const':
a.cpp:9: error: invalid conversion from `const A*' to `A*'
a.cpp:9: error:   initializing argument 1 of `int QList<T>::indexOf(const T&,
   int) const [with T = A*]'

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Why takes the conversion from 'const A*' to 'A*' place?
if T = A*
this is
int QList<A*>::indexOf(const A*&, int) const
But A*& ??

References and pointers....

Can someone enligthen me, please?

Thanks,

Moppel

[wysota]

You are trying to pass a pointer to a non-const object to a method which requires a const object. The error (with pointer and references) is just a result of trying to find a match for aguments. Compiler tries different combinations and maybe a reference is the last thing it tries, so it fails on it and returns such a combination. Either remove the "const" modifier from the method parameter or make sure you pass a const object.

[jpn]

The list contains non-constant pointers and you are passing a constant pointer for QList::indexOf() method.

Try:

int indexOfChild(A* x) const { return _list->indexOf( x, 0); }

int indexOfChild(A* x) const { return _list->indexOf( x, 0); }

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or

QList<const A*>

QList<const A*>

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which way fits better for your need..

[Moppel]

That would not work since the calling function passes a const pointer:

QModelIndex AModel::itemToIndex(const A *x, int column) const{
	const A *parent = x->parent();
	int row = parent->indexOfChild(x);
...

QModelIndex AModel::itemToIndex(const A *x, int column) const{
	const A *parent = x->parent();
	int row = parent->indexOfChild(x);
...

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Of course I could get rid of all the const. But I don't want to do that.

But it does work if I use the QList like this.

#include <QList>
 
class A
{
 
public:
	A(A* parent = 0){ _list = new QList<const A*>(); _parent = parent;}
	~A(){ delete _list;}
	int indexOfChild(const A* x) const { return _list->indexOf( x, 0); }
 
private:
	int i;
	A *_parent;
	QList<const A*> *_list;
};

#include <QList>

class A
{

public:
	A(A* parent = 0){ _list = new QList<const A*>(); _parent = parent;}
	~A(){ delete _list;}
	int indexOfChild(const A* x) const { return _list->indexOf( x, 0); }

private:
	int i;
	A *_parent;
	QList<const A*> *_list;
};

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I still don't really understand the previous error.
QList's method is declared like this.

int QList::indexOf ( const T & value, int from = 0 ) const

int QList::indexOf ( const T & value, int from = 0 ) const

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The method will not modify T, or how do I have to read the const T& here?

I guess I have to get my Bruce Eckel's out again.

[dublet]

Simple, quick solution:

int indexOfChild(const A* x) const { return _list->indexOf( (A *)x, 0); }

int indexOfChild(const A* x) const { return _list->indexOf( (A *)x, 0); }

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[wysota]

Simple, quick solution:

Simple, quick and dirty It breaks the "const" modifier.

[mhennings]

This is a very valid question.
Why not search a non-const QList for a const value? Searching won't change anything in the list, the constness of operator==() ensures that.
Even more, if t would not be passed by const reference but by const value, e.g.:
int indexOf(T const t, int from = 0) const;
the non-const -> const cast would happen automatically in the comparison operator.

In fact, I'm wondering why QList / QVector doesn't use T const* n and T const *e as the iterators.