Constructor calling another constructor?

**cpjust** · 02-10-2008

I thought this was possible (and I don't see why it wouldn't be), but VC++ gives me a

error C2614: 'TicketComparator' : illegal member initialization: 'TicketComparator' is not a base or member

with the following code:

Code:

class TicketComparator
{
public:
	TicketComparator( const std::set<int>*  winningNumbers,
				std::set<int>*  matchingNums )
		:	m_WinningNumbers( winningNumbers ),
			m_MatchingNums( matchingNums ) {}

	TicketComparator( TicketComparator&  rhs )
		:	TicketComparator( rhs.m_WinningNumbers, rhs.m_MatchingNums ) {}
...
};

Am I doing something wrong, or is calling a constructor from another constructor not allowed?

**tabstop** · 02-10-2008

You can call a constructor from a constructor, but not from an initializer, I don't think.

**whiteflags** · 02-10-2008

Originally Posted by tabstop

You can call a constructor from a constructor, but not from an initializer, I don't think.

Base class constructors can be called in the initializer list of a derived class' constructor. Because copy constructors are also constructors, they can have an initializer list.

But copy constructors should call their members' copy constructors if possible. If that's not possible, that means that you have to do a deep copy of some pointer, probably. It would be better to use a smart pointer based on whatever your needs may be, due to the fact that smart pointers are easy to reassign and handle their own object.

There's one ugly way to do it if you have to use a raw pointer. From memory:

Code:

class TicketComparator
{
public:
    TicketComparator ( const TicketComparator & rhs )
    {
        delete m_matchingNumbers;
        delete m_winningNumbers;

        m_matchingNumbers = new std::set<int>();
        m_winningNumbers = new std::set<int>();

        if( m_matchingNumbers && m_winningNumbers )
        {
            *m_matchingNumbers = rhs->m_matchingNumbers;
            *m_winningNumbers = rhs->m_winningNumbers;
        }
    }
    // more implementation...
private:
    std::set<int> * m_matchingNumbers, 
                  * m_winningNumbers;
};

**CornedBee** · 02-10-2008

Am I doing something wrong, or is calling a constructor from another constructor not allowed?

Delegating constructors are not possible in C++98. They will be in C++09, small consolation as that may be.

**cpjust** · 02-10-2008

Thanks.
Does anyone know if there was a logical reason why they decided that calling non-base constructors in initialization lists shouldn't be allowed?

**grumpy** · 02-10-2008

Because, if common code was required in two or more constructors, a member function could be defined and called by both constructors. Delegating constructors are therefore technically unnecessary, and were viewed as an unnecessary feature by the standard committee. The original proposal for delegating constructors also did not define what was required particularly well (the syntax was essentially what is put forward now, but the actual semantics were poorly defined). The combination of a technically unnecessary feature being the subject of a poorly defined technical proposal led to rejection.

**Daved** · 02-11-2008

But a delegating constructor can be called from the initializer list, whereas a member function could not. So technically there is a difference, right?

**CornedBee** · 02-11-2008

More importantly, a delegated-to constructor can have an initializer list of its own, whereas a member function cannot.
Which is why the feature will be there in 09.
I think the restriction is something like: the initializer list may contain a delegation to another constructor of the same class or a list of base and member initializers, but not both. This is necessary to ensure sane semantics.

**laserlight** · 02-11-2008

I think the restriction is something like: the initializer list may contain a delegation to another constructor of the same class or a list of base and member initializers, but not both. This is necessary to ensure sane semantics.

Sounds good. I kind of liked the constructor chaining thing in Java, though part of its added usefulness was due to the lack of default arguments as a language feature.

**brewbuck** · 02-11-2008

Originally Posted by cpjust

I

Code:

	TicketComparator( const std::set<int>*  winningNumbers,
				std::set<int>*  matchingNums )
		:	m_WinningNumbers( winningNumbers ),
			m_MatchingNums( matchingNums ) {}

	TicketComparator( TicketComparator&  rhs )
		:	TicketComparator( rhs.m_WinningNumbers, rhs.m_MatchingNums ) {}

This copy constructor is unnecessary. The other constructor just copies values into member variables. The default copy constructor would do exactly the same thing.

**cpjust** · 02-11-2008

Originally Posted by brewbuck

This copy constructor is unnecessary. The other constructor just copies values into member variables. The default copy constructor would do exactly the same thing.

Yeah, I guess you're right. I made a lot of changes to the class, so I'm not sure if I needed it before or not, but I sure don't now.

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