我的特定基于代理的模型（ABM）的Java或C ++？

Question

I unfortunately need to develop an agent-based model. My background is C++; I'm decent but not a professional programmer. My goal is to determine whether, my background aside for the moment, the following kind of algorithm would be faster or dramatically easier to write in C++ or Java.

1. My agents will be of class Host . Their private member variables include their infection and immune statuses (type int ) with respect to different strains. (In C++, I might use an unordered_map or vector to hold this information, depending on the number of strains.) I plan to keep track of all hosts in a vector, vector< Host *> hosts .
2. The program will need to know at any time all the particular hosts infected with a particular strain or with immunity to a particular strain. For each strain, I could thus maintain two separate structures, eg, vector< Host *> immune and vector< Host *> infectious (I might make each two-dimensional, indexed by strain and then host).
3. Hosts can die. It seems like this creates a mess in C++, in that I would have to find the right individual to kill in host and search through the other structures ( immune and infectious ) to find all pointers to this object. Is this true? Is there a dramatically better way to do this in C++ than what I have here?

Thanks in advance for any help.

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I should add that if I use C++, I will use smart pointers. That said, I still don't see a slick way to delete all pointers to an object when the object needs to go. (When a host dies, I want to delete it from memory.)

I realize there's a lot to learn in Java. I'm hoping someone with more perspective on the differences between the languages, and who can understand what I need to do (above), can tell me if one language will obviously be more efficient than another.

Answer 1

I can't answer your all questions, but

I'm under the impression that Java will delete all these pointers implicitly if I delete the underlying object.

In Java you don't delete an object; instead, it gets effectively deleted when the reference count to it goes to zero. However, you may want to utilize weak references here; this way the object disappears when the strong reference count goes to zero.

Answer 2

I'm under the impression that Java will delete all these pointers implicitly if I delete the underlying object. Is this true?

Nope. You actually have it backwards; if you delete all the pointers, Java will delete the underlying object. So you'll still need to search through all three of your data structures ( hosts , immune , and infectious ) to kill that particular host.

However, this "search" will be fast and simple if you use the right data structures; a HashSet will do the job very nicely.

    private HashSet<Host> hosts;
    private HashSet<Host> immune;
    private HashSet<Host> infectious;

    public void killHost(Host deadManWalking) {
        hosts.remove(deadManWalking);
        immune.remove(deadManWalking);
        infectious.remove(deadManWalking);
    }

It's really that simple, and will take place in O(lg n) time. (Though you will have to override the equals and hashCode methods in your implementation of Host ; this is not technically challenging.)

My memories of C++ are too hazy for me to give any sort of authoritative comparison between the two languages; I did a ton of C++ work in college, haven't touched it since. Will C++ code run faster? Done right and assuming you don't have any memory leaks, I'd suspect it would, though Java's rep as a slow language is mostly a holdover from its youth; it's pretty decent these days. Easier to write? Well, give that you'd be learning the language, probably not. But the learning curve from C++ to Java is pretty gentle, and I personally don't miss C++ at all. Once you know the languages, Java is, in my opinion, vastly easier to work with. YMMV, natch, but it may well be worth the effort for you.

Answer 3

Actually, your impression is basicly backwards: Java will assume an object (the host in this case) is dead when there's no longer any pointer to give access to that object. At that point it'll clean up the object (automatically).

At a guess, however, there's one collection that "owns" the hosts, and would be responsible for deleting a host when it dies. The other pointers to the host don't own it. If that's the case, then in C++ you'd normally handle this by having the "owning" collection contain a shared_ptr to the host, and the other collections contain weak_ptr s to the host. To use the object via a weak_ptr , you have to first convert that to a shared_ptr that you can dereference to get to the host itself. If, however, the object has been deleted, the attempt at converting the weak_ptr to a shared_ptr will fail, and you'll know the host is dead (and you can then delete your reference to it).