static members and consts

Question

class a
{
protected:
  const int _ID;

public:
  a::a(int id){};
  a::top(int num);
};

class b : public a
{
  static int ok;
  b::b(int id):a(id){};
  a::top(ok);
}

int main()
{
  int t=5;
  b opj=b(t);
}

first why i get this compile error that solved only when i remove the const

non-static const member ‘const int Student::_ID’, can't use default assignment operator - instantiated from ‘void std::vector::_M_insert_aux(__gnu_cxx::__normal_iterator, const _Tp&) [with _Tp = Student, _Alloc = std::allocator]’

second

i have anther problem

undefined reference to b::ok

Answer 1

Second first: b::ok has been declared, but not defined. Someplace, (preferably b.cpp), you need to add:

  int b::ok;

As for your first problem, _ID is const, it value cannot be changed -- but, you never give it a value to start with. You have to assign it an initial value:

  protected:
      const int _ID = 1234;

Now, you really should defined _ID as well, as we did with b::ok, but since it is const, the compiler may let you get away without doing that (some conditions apply).

Answer 2

This code has too may reasons to report compile errors.

• In C++ you are not allowed to use qualified names when declaring class methods, so your a::a and a::top in the definition of class a are flat-out illegal.
• What method declarations without a return type are supposed to mean is not clear either.
• A declaration of a::top in b makes no sense either. What is it doing there? And what was the intent of having a::top(ok) in the middle of the class definition?

In other words, you code makes no sense whatsoever. It is not even a remotely C++ code. If you feed this into a C++ compiler the results will be pretty much as meaningful as if you fed Shakespeare's "Hamlet" into a C++ compiler. It is not possible to say what's wrong with the code, when everything is wrong. If this is some other language, please, tag it accordingly instead of [C++].

Answer 3

Since _ID is const, it has to be initialized in the constructor/base member initialization list. Sort of like this:

a::a(int id) : _ID(5) {};

However, I got a ton of errors when I tried to build this. Are you sure you pasted the whole thing?

Answer 4

To answer your second question first: ok is a static member and needs to be initialized as such. Include a line similar to:

int b::ok = 0;

...in the implementation file for your class b.

As for the _ID member: You declared _ID to be a constant, but you did not define it, i.e. you did not provide a value for the constant. Since it is constant and you are not allowed to change it later - how do you expect _ID to take a value then?

Also, there are some other "funny" things going on in your source code that make me advise you to (re-)read an introductory text.