static member initialization

[Muthuveerappan]

Hi,

My understanding was that all static data members in a class needs to be initialized.

In one of the programs, I forgot to initialize a static composite data member (what i mean by composite type is a a class type) and the compiler didn't throw any error.

It did throw an error when i didn't initialize a static primitive data member

class A
{};

class B
{
    public:
        static void f1();
    private:
        static int n1;
        static A a1;
};


void B :: f1()
{
    n1 = 10; //throws an error
    A a2;
    a2 = a1; //does not throw an error though it is not initialized
}

int main()
{
    return(0);
}

error:

hyper71: singleton> g++ test.cpp -o test
/tmp/cciEM614.o: In function `B::f1()':
test.cpp:(.text+0x6): undefined reference to `B::n1'
collect2: ld returned 1 exit status

Questions:
========
1) Are the static composite (class type) data members initialized using their default constructors ? And is that the reason why the compilers don't throw an error.

2) The error thrown is "undefined reference to `B::n1'". Not sure i fully understand the error message, I thought it should have thrown an error related to not initializing.

3) Is class data type referred as composite data types ? - not sure if there is another term for this ...

Thanks,
Muthu

[Skizmo]

Static info

[Philip Nicoletti]

Static member variables need to be "defined" , not initialized. That is why you get
an undefined reference.

I would think that both of the static variables should have given the undefined
reference error. Try actually using the function ...

int main()
{
    B b;
    b.f1();
    
    return(0);
}

[Muthuveerappan]

Thanks Skizmo and Philip, I am just a bit confused with definition vs initialization.

The reason I am confused is that because the below mentioned program compiles successfully (a1 is not defined / initialized):

I have pasted the complete code just to be sure - gcc version 4.2.1

class A
{};

class B
{
    public:
        static void f1();
    private:
        static int n1; 
        static A a1; 
};


void B :: f1()
{
    //n1 = 10; //throws an error
    A a2; 
    a2 = a1; //does not throw an error though it is not initialized
}

int main()
{
    B b;
    b.f1();

    return(0);
}

[laserlight]

The reason I am confused is that because the below mentioned program compiles successfully (a1 is not defined / initialized):

You're probably observing the effects of undefined behaviour.

If I uncomment the "throws an error" and compile with the Comeau online compiler, the program compiles successfully. If I add an int member variable to A, Comeau still does not complain. But g++ then fails to compile the program.