Question

The D Programming Language Version 2 has a nifty method to overload an expression like this:

classInstance[someName] = someValue;

Or as D Function defined in this little example:

ref Map opIndexAssign(ref const(ValueT) value, ref const(NameT) name)
{
    this.insert(name, value);
    return this;
}

Is this possible in C++ (ideally without using the STL)? If so, how?

Answer 1

Normally, you would use a proxy object as the return type of operator[]; that object will have a custom operator= defined. The vector<bool> specialization in the C++ Standard Library uses a proxy to get the behavior you are looking for. The proxy-based solution isn’t as transparent as the D version, though. The code is something like:

class proxy;

class my_map {
  public:
  proxy operator[](const key_type& k);
  // Rest of class
};

class proxy {
  my_map& m;
  key_type k;
  friend class my_map;
  proxy(my_map& m, const key_type& k): m(m), k(k) {}

  public:
  operator value_type() const {return m.read(k);}
  proxy& operator=(const value_type& v) {m.write(k, v); return *this;}
};

proxy my_map::operator[](const key_type& k) {
  return proxy(*this, k);
}