Is it possible to create constructors that return the reference to the old items, based on parameters?

Question

Let's say there's a class:

class X {
    public:
        static X& get_instance(int foo, int bar);
    private:
        X();
        int foo, bar;
}

If I wrote this correctly then get_instance is a named constructor. Now here's what I'm trying to do;

Let's create an instance of X;

X& first = X::get_instance(3, 5);
X& second = X::get_instance(4, 6);
X& third = X::get_instance(3, 5);

I want to write get_instance in such a way that, when it is called with the same parameters twice, instead of creating a new instance, it should just return a reference to the old one. When the instances created with parameters (3, 5) all get deleted, then creating a new instance with (3, 5) create a new instance.

I imagined this would be possible by overloading the equality test operator and keeping track of every object the constructor has created so far. But this is C++, and there has to be a better way to do it.

This sounds like the "flyweight pattern". If so, look at [boost flyweight](http://www.boost.org/doc/libs/1_63_0/libs/flyweight/doc/index.html) — PaulMcKenzie, Mar 11 '17 at 00:37

score 1 · Accepted Answer · edited May 23 '17 at 12:09

Herb Sutter's favorite 10-liner, adjusted for slightly different arguments and removing the lock:

static std::shared_ptr<X> X::get_instance(int foo, int bar) {
    static std::map<std::pair<int, int>, std::weak_ptr<X>> cache;

    auto key = std::make_pair(foo, bar);     
    auto sp = cache[key].lock();
    if (!sp) cache[key] = sp = std::shared_ptr<X>(new X(foo, bar)); 
    return sp;
}

Can't use make_shared since the constructor is private, unless you use one of the tricks suggested on this question.

Is it possible to create constructors that return the reference to the old items, based on parameters?

1 Answers1