Question

I want to have a simple class i can call to get a unique number whilst the program is running – i can do the below with a dynamic allocation, and then just delete when not needed, but i still wanted to get a static version too.
Strangely, the code below (which is seemingly straightforward) throws some strange comiple errors (appended below).

Any ideas whats going on ? is this an incorrect use of static ?

class Id_gen {
    private:
 //adding static here stops the code from compiling:
 static int curr_id;

    public:

 Id_gen() {curr_id = 1; cout<<"debug:constructed"; }
 int get_id() {curr_id++; return curr_id; };
};

int main () {


    Id_gen bGen;
    cout << bGen.get_id() <<endl;

return 0;    
}

running g++ (linux 64):

c++2.cpp:(.text._ZN6Id_genC1Ev[Id_gen::Id_gen()]+0xe): undefined reference to `Id_gen::curr_id'
/tmp/cc766N6p.o: In function `Id_gen::get_id()':
c++2.cpp:(.text._ZN6Id_gen6get_idEv[Id_gen::get_id()]+0xa): undefined reference to `Id_gen::curr_id'
c++2.cpp:(.text._ZN6Id_gen6get_idEv[Id_gen::get_id()]+0x13): undefined reference to `Id_gen::curr_id'
c++2.cpp:(.text._ZN6Id_gen6get_idEv[Id_gen::get_id()]+0x19): undefined reference to `Id_gen::curr_id'

Answer 1

Add the initialization/definition of the static member as:

int Id_gen::curr_id = 0;

after the class definition.

EDIT: As mentioned in the comment by @sbi : Initialization is optional, the linker requires the definition only.