Access static constant of class - c++

Question

I'm trying to create static constant that will be named the same in different classes and can be accessed from other classes
I'm trying to access to the constant from other classes like that B::num
My code:
main.cpp
...
Ptr ptr;
A *a = C::make(ptr);
A.h
class A { //pure virtual
public:
virtual void func()=0;
virtual void func2()=0;
protected:
Ptr ptr;
}
B.h
class B : public A {
public:
B();
static unsigned char const num[2];
...
}
B.cpp
#include "B.h"
constexpr unsigned char B::num[]={0x4,0x5};
B::B(){..}
C.h
#include "A.h"
#include "B.h"
class C {
public:
static A* make(const Ptr &);
}
C.cpp
A *C::make(const Ptr &ptr){
if(!memcmp(ptr.memory,B::num,sizeof(B::num))){
...
}
}
errors
B.h file - error: from previous declaration 'B::num'
B.cpp file - error: redeclaration B::num differs in 'constexpr'
What is the problem?
Thanks.

Answer 1

First problem : you defined an unrelated, global a. The one you're trying to define is scoped inside the class(es).
static constexpr unsigned char class1::a[2] = {0x4,0x5};
// ^^^^^^^^
Second problem : defintions belong in .cpp files, not headers.

Answer 2

Here is an example of static const a in 2 classes :
#include <iostream>
class class1 {
public:
static unsigned char const a[2];
};
class class2 {
public:
static unsigned char const a[2];
};
constexpr unsigned char class1::a[] = {0x4,0x5};
constexpr unsigned char class2::a[] = {0x4,0x5};
int main() {
std::cout << static_cast<unsigned>(class1::a[0])
<< static_cast<unsigned>(class2::a[1]) << std::endl;
return 0;
}
Note:
the declaration contains static and const and should be in a .h file
the definition constains constexpr and should be in a .cpp