Why is this chaining method not working?

Question

It's a simple counter. The method add is being called to increment the private variable count by 1 by default. I am returning the Counter class from the function so that it may be chained, but when I look at the output, it gives me 1 when I expect it to be 3 because I called add three times.

#include <iostream>
#include <vector>

using std::cout;

class Counter {
    public:
        Counter() : count(0) {}

        Counter add() {
            ++count; return *this;
        }

        int getCount() {
            return count;
        }
    private:
        int count;
};

int main() {

    Counter counter;

    counter.add().add().add();

    cout << counter.getCount();

}

Answer 1

The whole idea of chaining idiom is based on accessing the same, original object in each chained call. This is usually achieved by returning a reference to the original object from each modifying method. This is how your add should have been declared

    Counter &add() { // <- note the `&`
        ++count; return *this;
    }

That way, each application of add in your chained expression will modify the same, original object.

In your original code, you return a temporary copy of the original object from add. So, each additional application of add (after the first one) works on a temporary copy, modifies that copy and produces yet another temporary copy. All those temporary copies disappear without a trace at the end of the full expression. For this reason, you never get to see the effects of any add calls besides the very first one.