How to get g++ to warn on unused member variables

Question

g++ generates warnings for unused local variables. Is it possible to have g++ warn for unused class member variables and/or global variables?

class Obj {
 public:
  Obj(int a, int b) : num1(a), num2(b) {}
  int addA(int i) {
    return i + num1;
  }

 private:
  int num1;
  int num2;
};

How do I get g++ to warn me that num2 is unused?

UPDATE: I am currently compiling with:

g++ -Wall -Wextra -pedantic *.cc -o myprogram 

Answer 1

Clang's -Wunused-private-field enables the warning you're asking for. On your code base, it shows:

$ clang -Wunused-private-field /tmp/nic.cpp  
/tmp/nic.cpp:10:22: warning: private field 'num2' is not used [-Wunused-private-field]
             int num2;
                 ^
1 warning generated.

Answer 2

I'm not aware of any such warning. Additionally I'll speculate that the reason it doesn't exist is because it can't be reliably generated in all cases, so they elected to not spend effort making it work for some subset of cases. For example, if the class friends another function that's in a library, the compiler would have no way of knowing if that library mutated any particular class attribute or not.