c++ 초보 질문입니다.

// test.cpp
class A
{
public:
    void ConstFunc() const {} // 상수 멤버함수
    void NonConstFunc() {}    // 비상수 멤버함수
 
    const bool operator() (int input) { // 상수를 반환하는, 비상수 멤버함수
        return input > 0;
    }
 
    bool operator() () const { // 비상수를 반환하는, 상수 멤버함수
        ConstFunc(); // OK
        NonConstFunc(); // compile error
        return true;
    }
};
 
int main()
{
    A a;
    int i1 = a(10);
    int i2 = a(10)++; // error
}
 
$ g++ test.cpp
test.cpp: In member function ‘bool A::operator()() const’:
test.cpp:15:22: error: passing ‘const A’ as ‘this’ argument of ‘void A::NonConstFunc()’ discards qualifiers [-fpermissive]
test.cpp: In function ‘int main()’:
test.cpp:24:19: error: lvalue required as increment operand
test.cpp:28:18: error: passing ‘const A’ as ‘this’ argument of ‘void A::NonConstFunc()’ discards qualifiers [-fpermissive]