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Can anyone explain to me the exact use of private constructors in c++ ?

Apr 18 '06 #1
12 8985
Preets ha scritto:
Can anyone explain to me the exact use of private constructors in c++ ?

For example for implement singleton pattern
Apr 18 '06 #2

Preets wrote:
Can anyone explain to me the exact use of private constructors in c++ ?


If you write a singleton class in C++. you will need to play some
tricks make all constructors private, including default, copy and
assignment operators.

Go to http://msdn.microsoft.com/msdnmag/issues/03/02/CQA/
to find an example of singleton class.

Apr 18 '06 #3
can a class derive from singleton class ? And, can any number of
instances of that derived class be created ?

Apr 18 '06 #4
Basic idea is that if you are making constructor private you will not
be able to create object of that class externaly. It is usefull in many
cases. singleton pattern is only one among them...

Apr 18 '06 #5

a class can derive from a singleton class if the singleton's
constructor is protected but if it is private then ? and what about the
number of instances of the derived class ? will that be also one only ?

Apr 18 '06 #6
If the constructor is private then also you can derive with one
technique. You can make the class to be derived as a friend of class
which's constructor is private.

Apr 18 '06 #7
Preets wrote:
can a class derive from singleton class ? And, can any number of
instances of that derived class be created ?


You can check "More effective C++ by Scott Meyers" 'Item 26: Limiting
the number of objects of a class' for a discussion about singletons and
singletons as base classes.

But singleton is only one of the uses of private Constructors (and the
most famous one, I suppose). there are some other uses such as
prohibiting the instantiation of object for a class (if you want to
facilitate only the use of it's static properties) or forcing the use
of what is called the "virtual constructor" pattern.

Apr 18 '06 #8
"dan2online " <da********@gma il.com> writes:

Hi,
Preets wrote:
Can anyone explain to me the exact use of private constructors in c++ ?


If you write a singleton class in C++. you will need to play some
tricks make all constructors private, including default, copy and
assignment operators.


Could you explain the reasons why would somebody write a singleton class in
C++ instead of using the good old C-style singleton pattern?

The advantages of the C-style singleton (file-local data with public
interface) are better information hiding, and built in compiler firewall.
But what are the advantages of the singleton class?
Apr 20 '06 #9
Preets wrote:
Can anyone explain to me the exact use of private constructors in c++ ?


Besides singletons and the virtual constructor idiom (see
http://www.parashift.com/c++-faq-lit....html#faq-20.8)
that have already been mentioned, private constructors can be used to
disable the implicitly generated copy constructor (and assignment
operator) if the class is noncopyable. A singleton holder class itself
often uses this feature, e.g.:

template<class T>
class Singleton
{
public:
static T& Instance();
private:
// Disabled functions
Singleton();
Singleton( const Singleton& );
Singleton& operator=( const Singleton& );
Singleton* operator&();
~Singleton();
};

template<class T>
T& Singleton<T>::I nstance()
{
static T myObject;
return myObject;
}

class C
{
public:
void DoSomething() {/*...*/}
private:
friend class Singleton<C>;
C() {/*...*/}
~C() {/*...*/}

// Disabled functions for singleton usage
C( const C& );
C& operator=( const C& );
C& operator&();
};

typedef Singleton<C> theC;

void Foo()
{
theC::Instance( ).DoSomething() ;
}

Also, a protected constructor can be used to enforce proper
initialization, especially in the case that the constructor needs to
call a virtual function. This is generally superior to forcing the user
to call an Init() function, which can easily be forgotten, leaving the
object uninitialized. This example is drawn from Sutter and
Alexandrescu's _C++ Coding Standards_ (Item 49):

class B // Hierarchy root
{
protected:
B() { /*...*/ }

// Called right after construction
virtual void PostInitialize( ) { /*...*/ }

public:

// Interface for creating objects
template<class T>
static std::auto_ptr<T > Create()
{
std::auto_ptr<T > p( new T );
p->PostInitialize ();
return p;
}
};

// A derived class
class D : public B { /*...*/ };

// Creating an initialized D object
std::auto_ptr<D > p = D::Create<D>();

Cheers! --M

Apr 20 '06 #10

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