Unable to use the operator== of QValueList

[yellowmat]

As follow

bool CClassA::operator==(const CClassA& aca)
{
	if( m_dAMember == aca.m_dAMember )
		return true;
	else
		return false;
}

bool CClassA::operator==(const CClassA& aca)
{
	if( m_dAMember == aca.m_dAMember )
		return true;
	else
		return false;
}

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[wysota]

As follow

bool CClassA::operator==(const CClassA& aca)
{
	if( m_dAMember == aca.m_dAMember )
		return true;
	else
		return false;
}

bool CClassA::operator==(const CClassA& aca)
{
	if( m_dAMember == aca.m_dAMember )
		return true;
	else
		return false;
}

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Make it:

bool CClassA::operator==(const CClassA& aca) const
{
	 return ( m_dAMember == aca.m_dAMember );
}

bool CClassA::operator==(const CClassA& aca) const
{
	 return ( m_dAMember == aca.m_dAMember );
}

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Note the "const" at the end of function header.

[yellowmat]

I can also give the definition of the operator== of CClassB

bool CClassB::operator==(const CClassB& acb)
{
	if( m_vlListOfClassA == acb.m_vlListOfClassA) )
		return true;
	else
		return false;
}

bool CClassB::operator==(const CClassB& acb)
{
	if( m_vlListOfClassA == acb.m_vlListOfClassA) )
		return true;
	else
		return false;
}

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[wysota]

I can also give the definition of the operator== of CClassB

bool CClassB::operator==(const CClassB& acb)
{
	if( m_vlListOfClassA == acb.m_vlListOfClassA) )
		return true;
	else
		return false;
}

bool CClassB::operator==(const CClassB& acb)
{
	if( m_vlListOfClassA == acb.m_vlListOfClassA) )
		return true;
	else
		return false;
}

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Same here -- add "const".

[yellowmat]

Thanks a lot Wysota, it compiles.

I have another question about my problem, why must I add the const in the definition ? What does it means ?

It is the first time I must do this with my operator== ... but it is also the first time I use it with a QValueList.

[wysota]

It means that by comparing the "current" (this) object to some other object you don't modify this object.

And this results in making the "current" object "const", which was missing here:

no operator defined which takes a left-hand operand of type 'const class CClassA'

It's strictly a C++ issue and has nothing to do with Qt.

[Michiel]

const is pretty handy, since it will catch some programmer errors. Like, when you accidentally try to change a value you aren't supposed to.

The general rule is: Either don't use const at all, or use it everywhere.

QT wisely does use it, so you have to use it as well if you use QT classes.

The compiler was only warning you. You were basically calling a non-const function from a const function . Or rather, the QValueList was doing this, since the CClassA operator== was not const and the QValueList operator== is.