CODEWITHSUNDEEP
c++
Josh Morrison
Josh Morrison

Reputation: 7636

C++ static member, a little confused here

    //static member in classes
#include <iostream>
using namespace std;

class CDummy {
public:
    static int n;
    CDummy() {n++;};
    ~CDummy(){n--;};
};

int CDummy::n =0;

int main(){
    CDummy a;
    CDummy b[5];
    CDummy *c = new CDummy;
    cout << a.n << endl;
    delete c;
    cout << CDummy::n << endl;
    return 0;
}

The result is 7, 6. Can anybody explain it for me? and I don't understand this "CDummy b[5];". People never use syntax like this in C, right? what is this here? Thank you!

Upvotes: 0

Views: 152

Answers (5)

Neera
Neera

Reputation: 1587

n is a static member variable in class CDummy. n is associated with all objects of the class rather than each object instance of the class. n is incremented as each object instance gets created and decremented as each object instance is destroyed. In the main function we create object a (n=1), followed by array of objects b of size 5 (n=6) followed by pointer to object c (n=7). So the first cout statement output 7. As object pointed to by c is destroyed n is decremented to 6. So the second cout statement outputs 6. Please refer to this link for details on static members in a class

Upvotes: 0

Jonathan Wood
Jonathan Wood

Reputation: 67195

CDummy b[5] is just an array of 5 CDummy objects.

A static member means there is only one instance of that member no matter how many instances are made of the class.

The class simply increments n from the constructor whenever a new instance is initialized, and decrements n from the destructor whenever the instance is destroyed. It's basically tracking how many instances of this class are currently active.

Upvotes: 0

bcsanches
bcsanches

Reputation: 2372

static on this situation mean that the variable n is the same for all instances of CDummy, in your case a, b and c (after you allocate the object using new) all share the same n. Thats why you see this value.

CDummy b[5] -> this declares an array of CDummy, on this case 5 CDummys.

Upvotes: 0

ClosureCowboy
ClosureCowboy

Reputation: 21541

CDummy b[5] is an array of five CDummy objects. CDummy a is simply a single instance of CDummy, and so is CDummy c.

Every time a CDummy is created, the constructor is called.

Let's add that up: 5 + 1 + 1 = 7. That's why n was initially 7. When c was deleted, n-- was executed, and n became 6.

Upvotes: 1

James McNellis
James McNellis

Reputation: 355019

CDummy b[5];

This declares an array of five CDummy objects. It ends up calling the CDummy default constructor five times (once for each object in the array).

You create seven CDummy objects: a, five in the array b, and the one pointed to by c. n then has a value of 7. Then you destroy one CDummy object (the one pointed to by c) and n has a value of 6. The remaining six CDummy objects are destroyed when they go out of scope when the main function returns.

Upvotes: 2

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