write to a const reference of ostream

Question

I am reading C++ primer. I encountered the following code:

#include <iostream>
#include <string>

using namespace std;

class PrintString {
public:
    PrintString(ostream &o = cout, char c = ' '): os(o), sep(c) {}
    void operator() (const string &s) const { os << s << sep; }
private:
    ostream &os;
    char sep;
};


int main() {
    const PrintString printer;

    printer("ABC");

    return 0;
}

This code works, but I don't know why. Below is what I think, it would be great if anyone could help to point out where I am wrong...

Here, 'printer' is a const PrintString object, so its data members are const, so 'printer.os' is a const reference to cout. Therefore, we should not be able to write to 'printer.os' since writing to cout changes it.

Thanks in advance!

Answer 1

The reference isn't being modified, only what it refers to. This works the same with a pointer. If you had a pointer to int data member (int*), using it in a const member function would make its type int* const. You aren't able to change the pointer itself, but you can change what it points to. For example:

struct Foo
{
    int a;
    int* p = &a;

    void foo() const
    {
        p = new int; // ERROR! Not allowed to modify const pointer
        *p = 100; // OK, it's a pointer to a non-const int
    }
};

So when using os, you're only modifying the object that it refers to, not the reference itself.