Re: Singleton --- Just Look and give Suggestion's

James Kanze <>
Sat, 7 Mar 2009 02:39:18 -0800 (PST)
On Mar 5, 6:15 pm, srdgame <> wrote:

=E4=BA=8E Thu, 05 Mar 2009 08:16:45 -0800=EF=BC=8CPallav singh=E5=86=99=



My perferred Singleton way, If I want to make class A to be
singleton, I will declear it as:
class A : public Singleton< A >

With a semicolon after the }, of course:-).

This is more or less the standard idiom, of course.

namespace srdgame

template <class T>
class Singleton
    static T* get_instance()
        return &(get_singleton());
    static T& get_singleton()
        static T instance;
        return instance;
    Singleton(const Singleton& sig);
    Singleton& operator = (const Singleton& sig);

This has the disadvantage that the singleton will be destructed
some time when static objects are destructed. This usually
isn't what is wanted.

My own singleton uses a policy to control this:

// DestructionPolicy:
// ==================
//! Determines whether the destructor will be called on exit or
//! not. (Unless there is a very strong reason to call it, it is
//! recommended that it not be called, so as to avoid order of
//! destruction issues.)
enum DestructionPolicy
} ;

// Singleton:
// ==========
//! Makes a singleton for UserClass. If UserClass should always
//! be a singleton, it can derived from this template class,
//! declaring this template class as friend and its constructor as
//! private.
//! This implementation is thread safe if and only if the first
//! call to instance takes place before threading starts. This
//! template ensures that the constructor is called at least once
//! during static initialization; on normal implementations, this
//! means that the singleton will be thread safe if threading is
//! not started before main.
template< typename UserClass, DestructionPolicy dtorPolicy =
neverDestruct >
class Singleton
    static UserClass& instance() ;

    static UserClass* ourInstance ;

    template< DestructionPolicy discrimPolicy >
    class Discrim {} ;
    static UserClass* createInstance( Discrim< neverDestruct > ) ;
    static UserClass* createInstance( Discrim< destructOnExit > ) ;
} ;

template< typename UserClass, DestructionPolicy dtorPolicy >
UserClass* Singleton< UserClass, dtorPolicy >::
        = &Singleton< UserClass, dtorPolicy >::instance() ;

template< typename UserClass, DestructionPolicy dtorPolicy >
Singleton< UserClass, dtorPolicy >::instance()
    if ( ourInstance == NULL ) {
        ourInstance = createInstance( Discrim< dtorPolicy >() ) ;
    return *ourInstance ;

template< typename UserClass, DestructionPolicy dtorPolicy >
Singleton< UserClass, dtorPolicy >::createInstance(
    Discrim< neverDestruct > )
    return new UserClass ;

template< typename UserClass, DestructionPolicy dtorPolicy >
Singleton< UserClass, dtorPolicy >::createInstance(
    Discrim< destructOnExit > )
    static UserClass theOneAndOnly ;
    return &theOneAndOnly ;

Like yours, it's meant to be derived from. This seemed so
obvious to me that I forgot to document it. (One should
probably also assign a UserClass* to a Singleton< UserClass >*
somewhere in the code, so that it won't compile *unless*
UserClass derives from it.)

James Kanze (GABI Software)
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