Unresolved external symbol on static class members

in my case, I declared one static variable in .h file, like

//myClass.h
class myClass
{
static int m_nMyVar;
static void myFunc();
}

and in myClass.cpp, I tried to use this m_nMyVar. It got LINK error like:

error LNK2001: unresolved external symbol "public: static class... The link error related cpp file looks like:

//myClass.cpp
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}

So I add below code on the top of myClass.cpp

//myClass.cpp
int myClass::m_nMyVar; //it seems redefine m_nMyVar, but it works well
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}

then LNK2001 is gone.

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.
}

If you are using C++ 17 you can just use the inline specifier (see https://stackoverflow.com/a/11711082/55721)

---

If using older versions of the C++ standard, you must 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)