CODEWITHSUNDEEP
c++
M-Gregoire
M-Gregoire

Reputation: 818

Use of static private attributes

I'm having trouble understanding the use of private static attributes in a class :

-> private means the attributes will only be accessible from the class itself if I'm correct

-> static indicates the attributes belong to the class itself and not the object and, if I'm still correct, which permit to access it without creating an object

So, I can't imagine any use of a private static attribute.

Thanks in advance for any help :)

Kenshin

Upvotes: 0

Views: 1255

Answers (4)

Jason C
Jason C

Reputation: 40406

Here's another example that might expand your imagination:

class NotAThreadSafeExample {
public:
    NotAThreadSafeExample () { ++ debugReferenceCount; }
    ~NotAThreadSafeExample () { -- debugReferenceCount; }
    static int getDebugReferenceCount () { return debugReferenceCount; }
private:
    static int debugReferenceCount;
};

Or:

class Example {
private:
    static const int FIXED_COLUMN_WIDTH = 32;
};

Or maybe even something like this, where appropriate (atomicIncrement left to your imagination, e.g. InterlockedIncrement() on Windows or __sync_add_and_fetch() with GCC; integral type record_id_t chosen accordingly):

class Record {
public:
    Record () : id(atomicIncrement(&nextId)) { }
    record_id_t getId () const { return id; }
private:
    static volatile record_id_t nextId;
    record_id_t id;
};

Or any other use you can imagine for a static variable that is not accessible outside of a class or its friends.

Upvotes: 1

Mike Seymour
Mike Seymour

Reputation: 254741

You said it yourself: if you want a variable associated with the class but not part of any object (static), that can only be accessed within the class itself (private).

As a concrete example, here's a class that counts instances of itself:

class countable {
private:
    static unsigned count;
public:
    countable() {++count;}
    countable(const countable&) {++count;}
    ~countable() {--count;}

    static unsigned instance_count() {return count;}
};

Upvotes: 6

barak manos
barak manos

Reputation: 30146

Here is an example that might expand your imagination:

class Singleton
{
   public:
       static Singleton* getInstance();
       ~Singleton();
   private:
       Singleton();
       static Singleton* instance;
};

Upvotes: 3

Vinicius Kamakura
Vinicius Kamakura

Reputation: 7778

A friend class or function could access this static member.

Upvotes: 0

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