unresolved external symbol error when define the class

Question

I could not understand why when declaration a static variable causes a external symbol error. Who can describe the differences between below cases?

1) OK

class TrainComposition
{
public:
    int wagons;
    ...
}

2) Error (unresolved external symbol)

class TrainComposition
{
public:
    static int wagons;
    ...
}

3) Error (unresolved external symbol)

class TrainComposition
{
 static int wagons; 
 public:
    ...
}

Possible duplicate of [Unresolved external symbol on static class members](https://stackoverflow.com/questions/195207/unresolved-external-symbol-on-static-class-members) — Asesh, Jun 23 '17 at 04:08

score 0 · Accepted Answer · answered Jun 23 '17 at 05:37

2) and 3) are of course the same, the visibility does not change anything on the problem.

You declare your variable, but you don't define it. In other words, in your .cpp file you should add

int TrainComposition::wagons = 0;

This is most often not necessary for static members that are const, but sometimes even then.

1 Answers1