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#include <iostream>
using namespace std;
 
class c1
{
    public:
    class c2
    {
        protected:
        static void func();
    };
 
    public:
    static int a1;
    private:
    static int a2;
};
 
 
 
void c1::c2::func()
{
    int f1= c1::a1; //works
    int f2= c1::a2;
 
    c1::a1= 1;  //fails
    c1::a2= 2;
 
    printf("func"); 
}
 
int main() {
	// your code goes here
	return 0;
}

prog.cpp: In static member function ‘static void c1::c2::func()’:
prog.cpp:23:9: warning: unused variable ‘f1’ [-Wunused-variable]
     int f1= c1::a1; //works
         ^
prog.cpp:24:9: warning: unused variable ‘f2’ [-Wunused-variable]
     int f2= c1::a2;
         ^
/home/d91WcC/cc5NqOCF.o: In function `c1::c2::func()':
prog.cpp:(.text+0xc): undefined reference to `c1::a1'
prog.cpp:(.text+0x16): undefined reference to `c1::a2'
collect2: error: ld returned 1 exit status

Standard output is empty