Unresolved external symbol on static class members


Question

Very simply put:

I have a class that consists mostly of static public members, so I can group similar functions together that still have to be called from other classes/functions.

Anyway, I have defined two static unsigned char variables in my class public scope, when I try to modify these values in the same class' constructor, I am getting an "unresolved external symbol" error at compilation.

class test 
{
public:
    static unsigned char X;
    static unsigned char Y;

    ...

    test();
};

test::test() 
{
    X = 1;
    Y = 2;
}

I'm new to C++ so go easy on me. Why can't I do this?

1
121
5/26/2018 10:52:33 AM

Accepted Answer

You forgot to add the definitions to match your declarations of X and Y

unsigned char test::X;
unsigned char test::Y;

somewhere. You might want to also initialize a static member

unsigned char test::X = 4;

and again, you do that in the definition (usually in a CXX file) not in the declaration (which is often in a .H file)

134
10/12/2008 7:49:10 AM

Static data members declarations in the class declaration are not definition of them. To define them you should do this in the .CPP file to avoid duplicated symbols.

The only data you can declare and define is integral static constants. (Values of enums can be used as constant values as well)

You might want to rewrite your code as:

class test {
public:
  const static unsigned char X = 1;
  const static unsigned char Y = 2;
  ...
  test();
};

test::test() {
}

If you want to have ability to modify you static variables (in other words when it is inappropriate to declare them as const), you can separate you code between .H and .CPP in the following way:

.H :

class test {
public:

  static unsigned char X;
  static unsigned char Y;

  ...

  test();
};

.CPP :

unsigned char test::X = 1;
unsigned char test::Y = 2;

test::test()
{
  // constructor is empty.
  // We don't initialize static data member here, 
  // because static data initialization will happen on every constructor call.
}

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