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#include <iostream>
using namespace std;
 
struct A
{
	int member = 5;
 
	void f() const
	{
		member++; // Won't work (modifiying this->member)
		g(); // Won't work (g() is not const)
	}
 
	void g() 
	{
	    // Both works, of course
		const_g();
		nonconst_g(); 
	}
 
	void const_g() const { }
	void nonconst_g() { }
};
 
int main() {
	A a;
	a.f();
	a.g(); 
 
	const A b;
	b.f(); // Works : f() won't modify b;
	b.g(); // Won't work : g() isn't const and might modify b;
	return 0;
}

prog.cpp: In member function 'void A::f() const':
prog.cpp:10:9: error: increment of member 'A::member' in read-only object
   member++; // Won't work (modifiying this->member)
         ^
prog.cpp:11:5: error: passing 'const A' as 'this' argument discards qualifiers [-fpermissive]
   g(); // Won't work (g() is not const)
     ^
prog.cpp:14:7: note:   in call to 'void A::g()'
  void g() 
       ^
prog.cpp: In function 'int main()':
prog.cpp:32:6: error: passing 'const A' as 'this' argument discards qualifiers [-fpermissive]
  b.g(); // Won't work : g() isn't const and might modify b;
      ^
prog.cpp:14:7: note:   in call to 'void A::g()'
  void g() 
       ^

Standard output is empty