C++ Initialize class static data member

Question

I have a class which has a number of static function to perform some calculation. However, before the calculation, I need to pass in a data to initialize some of the static data members. Currently I have an init(data) function and a clearResource() function which should be called before and after the use of the class. Is there a better way of doing that?

For example:

classA(){
 static int a;
 static init(int b) {
    a = b;
 }
 static functionA(){
   //perform something based on value of a;
    switch(a){
    }
 }

}

int main(){
  classA::init(5);
  classA::functionA();
 }

Thanks

How the static functions can be dependent on your object state? — yadab, Nov 16 '10 at 07:35
It's not clear whether you want to use `init` and `clearResource` for each object of your class or only once for all objects. — Alexey Malistov, Nov 16 '10 at 07:40
Well if you have a function that is dependent on state then you should `wrap the state and the function` into its own class that encapsulates all this information. Then the constructor/destructor of this new class will handle all the above automatically. — Martin York, Nov 16 '10 at 08:15
You should really revisit your design. Note that just the description is a clear indication that your program will never support multithreading (a static variable has to be modified in each thread that uses `functionA`, both before entering and after completion. If a second thread tries to call the function while the first thread is inside, it will change the *global* while in the middle of the operation. — David Rodríguez - dribeas, Nov 16 '10 at 09:00
Thanks for the reply @David: Could you elaborate a bit on the multithreading part? I do not quite understand it. I didn't need to modify the value of the int a and it only needs to be init once at the beginning of the programme. Would that still give problems? Thanks. — Leslieg, Nov 16 '10 at 09:18

score 2 · Answer 1 · answered Nov 16 '10 at 07:34

Avoid using static member functions : have your constructor initialize the data and the destructor clear the resources (see RAII). If the existing class cannot be changed, implement a helper class which calls init from its constructor and clearResource from its destructor.

score 2 · Answer 2 · answered Nov 16 '10 at 08:48

2

You can use this kind of design

class A()
{
public:
 static int a;
 static void functionA(int arg = A::a)
 {
  if(A::a != arg)
   A::a = arg;
  ...
 }
};

int A::a = 0;
int main()
{
 A::functionA();
}

answered Nov 16 '10 at 08:48

Mihran Hovsepyan

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Not nice by any means but fulfills the question requirements... I would ommit the default value if setting it is a requirement. – David Rodríguez - dribeas Nov 16 '10 at 08:58
How can you free "a" afterwards, using this solution, supposing it is a (class-)object and not a primitive data type? – Jan Rüegg Mar 08 '12 at 10:13

score 1 · Answer 3 · edited May 23 '17 at 12:29

1

You should apply the RAII concept: see this and this questions.

edited May 23 '17 at 12:29

Community

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answered Nov 16 '10 at 07:34

Simone

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vitaut · Answer 4 · 2010-11-16T08:05:00.687

1

Make the member functions and data non-static, initialize in a constructor and free resources in the destructor. This will guarantee the correct sequence of calls: initialize - perform operations - free resources in the client code.

edited Nov 16 '10 at 08:05

answered Nov 16 '10 at 07:35

vitaut

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No, no, don't ensure uniqueness and don't consider the Singleton pattern. – Puppy Nov 16 '10 at 07:52
there's usually little point in trying to enforce uniqueness, but many issues arise in the attempt... it's not worth it. – Matthieu M. Nov 16 '10 at 07:58
@DeadMG: It is unlikely that the OP needs uniqueness in this particular case so I removed the reference to Singleton. Although I wouldn't be so sure without seeing the actual (not example) code. – vitaut Nov 16 '10 at 08:02

score 1 · Answer 5 · answered Nov 16 '10 at 09:31

I'd avoid using static members in this case.

This is your problem. You have a class that does processing on some data. That data, for whatever reason, needs to be shared across all instances of this processing class. Ok then, we have a non-static solution!

class Data : boost::noncopyable
{
public:
    Data()
    {
        // initialise all of our data.
    }; // eo ctor
}; // eo class Data

Where you instantiate this class is up to you. It could be a member of an application class that is run at start up, or part of some root. It just needs to be accessible and does not need to be static nor a singleton.

class DataProcessor
{
private:
    Data& m_Data;

public:
    DataProcessor(Data& _data) : m_Data(_data)
    {
    }; // eo ctor
}; // eo class DataProcessor

C++ Initialize class static data member

5 Answers5

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