How can I implement `my_class_instance(a, b) = value` operation - c++

Question

I have a class Flight and I need to implement two operations:
Flight s;
auto value = s(1, 2);
s(3, 4) = value;
I know how to implement first operation:
class Flight
{
public:
int operator()(int a, int b) {
return cache.for(a, b).get();
}
private:
// cache
}
But I don't know implement second operation. Something like this:
void operator()=(int a, int b, int value) {
cache.for(a, b).set(value);
}
Now I have an error operation is not accessible for expression like s(3, 4) = value;.
How can I implement this operation?

Answer 1

There is no ()= operator. It is two separate operators - operator(), and then operator= on the return value of operator().
Since you want to use separate get() and set() methods inside the cache, you will have to make your Flight::operator() return a helper proxy, and then you can implement proxy::operator int to call the get() method and proxy::operator= to call the set() method, eg:
class Flight
{
public:
class Proxy
{
private:
Flight &f;
int a, b;
public:
Proxy(Flight &f, int a, int b) : f(f), a(a), b(b) {}
operator int() {
return f.cache.For(a, b).get();
};
Proxy& operator=(int value) {
f.cache.For(a, b).set(value);
return *this;
}
};
friend class Proxy;
Proxy operator()(int a, int b) {
return Proxy{*this, a, b};
}
private:
// cache
};
Then your example will work as you want:
Flight s;
int value = s(1, 2); // <-- important - int, not auto!
s(3, 4) = value;
Live Demo