c++ - using a static const int in a struct/class

Question

struct A {
    static const int a = 5;

    struct B {
       static const int b = a;
    };

 };

 int main() {
   return A::B::b;
 }

The above code compiles. However if you go by Effective C++ book by Scott Myers(pg 14); We need a definition for a in addition to the declaration. Can anyone explain why this is an exception?

Answer 1

C++ compilers allow static const integers (and integers only) to have their value specified at the location they are declared. This is because the variable is essentially not needed, and lives only in the code (it is typically compiled out).

Other variable types (such as static const char*) cannot typically be defined where they are declared, and require a separate definition.

For a tiny bit more explanation, realize that accessing a global variable typically requires making an address reference in the lower-level code. But your global variable is an integer whose size is this typically around the size of an address, and the compiler realizes it will never change, so why bother adding the pointer abstraction?