c++ initialization of static object in class declaration

Question

I have a C++ class (class1) with a static object of another class (class2) as a private member.

I know upon using the program I will have to initialize the static object, I can use a default constructor for this (undesired value).

Is it possible to initialize the static object to my desired value only once, and only if I create an object of the containing class (class1)?

Any help would be appreciated.

You can't use a class constructor to initialize a static member. Constructors are called every time a new instance of the object is created, but statics are only initialized once. — John Dibling
– John Dibling, Commented May 8, 2012 at 13:42
@SethCarnegie: In your code below, Bptr = new B(arguments, to, constructor); isn't an initialization. The initialization is B* A::Bptr = nullptr;. — John Dibling
– John Dibling, Commented May 8, 2012 at 13:46

Seth Carnegie · Accepted Answer · 2012-05-08 13:40:06Z

23

Yes.

// interface

class A {

    static B b;
};

// implementation

B A::b(arguments, to, constructor); // or B A::b = something;

However, it will be initialised even if you don't create an instance of the A class. You can't do it any other way unless you use a pointer and initialise it once in the constructor, but that's probably a bad design.

IF you really want to though, here's how:

// interface

class A {
    A() { 
        if (!Bptr)
            Bptr = new B(arguments, to, constructor);

        // ... normal code
    }

    B* Bptr;
};

// implementation

B* A::Bptr = nullptr;

However, like I said, that's most likely a bad design, and it has multithreading issues.

edited May 8, 2012 at 13:40

answered May 8, 2012 at 13:29

Seth Carnegie

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12 Comments

bryan sammon Over a year ago

ok I see what you mean. Yea, the pointer thing wont work cause I can gonna be making many objects of class A

Seth Carnegie Over a year ago

@bryansammon it doesn't matter how many you make, I will write an example.

bryan sammon Over a year ago

Thanks alot, you think thats a bad design habit?

Seth Carnegie Over a year ago

@bryansammon well, it of course depends on your situation, but I think there are more situations in which it's bad design than there are in which it's good design. I can't think of a situation in which you'd want a new seperate single object to appear after you create one of those objects. You get to make the final decision though.

Seth Carnegie Over a year ago

@bryansammon actually nevermind, that way wouldn't work, my bad.

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