Question

This is what I’m trying to do and I can’t:

#include <string>
using namespace std;
class A {
public:
  bool has() const { return get().length(); }
  string& get() { /* huge code here */ return s; }
private:
  string s;
};

The error I’m getting is:

passing ‘const A’ as ‘this’ argument of
‘std::string& A::get()’ discards qualifiers

I understand what the problem is, but how can I fix it? I really need has() to be const. Thanks.

Answer 1

Add a second overload of get():

string const & get() const { return s; }

That will be called on a const typed object of class A.

In practice, I prefer adding only const-typed accessors, and then keeping modifications entirely internal to the class or even avoid them entirely. For example, that means having a method DoUpdateLabel(){/*do something with s*/} rather than expose the internals. That has the nice side effect that you can avoid duplicating accessors in many cases.

If you absolutely must have modification via accessors and you also don’t want an extra const wrapper, you can use const_cast<>:

bool has() const { return const_cast<A*>(this)->get().length(); }

However, if get() has side-effects and has() is declared const, it’s questionable whether this is behavior you really want.