CODEWITHSUNDEEP
c++functionclassstaticmember
Tomás Arturo Herrera Castro
Tomás Arturo Herrera Castro

Reputation: 293

making static variables in member functions independent for each instance

Given the class:

class A {
    Public:
        void foo() {
            static int i;
            i++;
        }
};

How would you change it to prevent i from changing between instances following this example:

A o1, o2, o3;
o1.foo(); // i = 1
o2.foo(); // i = 1
o3.foo(); // i = 1
o1.foo(); // i = 2

i.e. allocate memory for i on every instance.

EDIT:

Yes, you can add i as an instance variable, but what if you need these counters in various (independent) functions ? I am looking to limit the scope of the variable only to the function ("in member functions"). It would seem awkward to add variables such as i, c, counter, counter_2 to the class if you need various counters, would it not ?

Upvotes: 3

Views: 1768

Answers (2)

Red.Wave
Red.Wave

Reputation: 4201

In circumstances where declaring data members become costly(need for sparse members that are not used so often), An instance indepent collection - normally an associative one - may come in handy. Knowing nothing more about the OP's intention, std::map family of classes can be used as first speculation. We need to have one counter per visited object in A::foo, but not for unvisited instances(i.e. A instances not calling A::foo). This was the the simplest first solution I came up with:

void A::foo(){
    static std::map<A*,std::size_t> i;
    ++i[this];
    //...
};

Upon call to std::map::operator[] on an object not in the map, the associated value is default constructed in a memory location already zeroed by the allocator( in short words 1st-timers are automatically initialized to 0).

Upvotes: 2

Bathsheba
Bathsheba

Reputation: 234665

class A
{
public:
    int i = 0;
    void foo(){
        ++i;
    }
};

is the normal way: i is now a member variable of the class. Clearly you don't want to use static.

Upvotes: 5

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