const object in c++

Question

I have a question on constant objects. In the following program:

class const_check{
    int a;
    public:
    const_check(int i);
    void print() const;
    void print2();
};

const_check::const_check(int i):a(i) {}

void const_check::print() const {
int a=19;
    cout<<"The value in a is:"<<a;
}

void const_check::print2() {
    int a=10;
    cout<<"The value in a is:"<<a;
}

int main(){
    const_check b(5);
    const const_check c(6);
    b.print2();
    c.print();
}

void print() is constant member function of the class const_check, so according to the definition of constants if any attempt to change int a it should result in an error but the program works fine for me.I think i am having some confusion here, can anybody tell me why the compiler is not flagging it as an error??

Answer 1

By writing

int a = 19;

inside print(), you are declaring a new local variable a. This has nothing to do with the int a that you declared inside the class const_check. The member variable is said to be shadowed by the local variable. And it's perfectly okay to declare local variables in a const function and modify them; the constness only applies to the fields of the object.

Try writing

a = 19;

instead, and see an error appear.

Answer 2

You're not changing the member variable a in either print() or print2(). You're declaring a new local variable a which shadows the member variable a.

Answer 3

You aren't changing the instance variable a you are creating a local variable a in each method.

Answer 4

Also, unless I am mistaken, you forgot to actually declare the member variable const to begin with.