CODEWITHSUNDEEP
c++static-variablesinstance-methods
Davide Valdo
Davide Valdo

Reputation: 779

Static variables in instance methods

Let's say I have this program:

class Foo {
 public:
    unsigned int bar () {
        static unsigned int counter = 0;
        return counter++;
    }
};

int main ()
{
    Foo a;
    Foo b;
}

(Of course this example makes no sense since I'd obviously declare "counter" as a private attribute, but it's just to illustrate the problem).

I'd like to know how C++ behaves in this kind of situation: will the variable "counter" in the bar() method be the same for every instance?

Upvotes: 12

Views: 6498

Answers (5)

Winder
Winder

Reputation: 1984

Your example was a couple lines away from being something you could compile and test:

#include <iostream>
using namespace std;
class Foo {
 public:
    unsigned int bar () {
        static unsigned int counter = 0;
        return counter++;
    }
};

int main ()
{
    Foo a;
    Foo b;

    for (int i=0; i < 10; i++)
      cout<<i<<". "<<a.bar()<<" / "<<b.bar()<<endl;
}

The output looks like this:

0. 1 / 0
1. 3 / 2
2. 5 / 4
3. 7 / 6
4. 9 / 8
5. 11 / 10
6. 13 / 12
7. 15 / 14
8. 17 / 16
9. 19 / 18

So yes, the counter is shared across all instances.

Upvotes: 1

sud03r
sud03r

Reputation: 19799

You just need to grasp two things:

  1. Static variables are stored in static area of the executing program (which is same as that of global variable).
  2. Scope is limited by the general rules of parentheses.Additionally static variables have internal linkage.

Upvotes: 0

Greg Bacon
Greg Bacon

Reputation: 139711

From The C++ Programming Language (2nd edition), page 200, by Bjarne Stroustrup:

Don't use static except inside [plain] functions (§7.1.2) and classes (§10.2.4).

Upvotes: 1

Timo Geusch
Timo Geusch

Reputation: 24351

Yes, counter will be shared across all instances of objects of type Foo in your executable. As long as you're in a singlethreaded environment, it'll work as expected as a shared counter.

In a multithreaded environment, you'll have interesting race conditions to debug :).

Upvotes: 10

dcp
dcp

Reputation: 55467

By "be the same for every instance" you mean there will be one instance of this variable shared across each class instance, then yes, that's correct. All instances of the class will use that same variable instance.

But keep in mind that with class variables you have to take things like multi-threading into account in many cases, which is a whole different topic.

Upvotes: 2

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