• 0

C++ (STL): remove specific object from list


Question

Hello,

I have learned that I can create a linked list of objects, like this:

list< MyClass* > = myList;

I also know that if it were a list of primitive types, such as a list<int>, I could remove a specific element by calling

remove(myList.begin(), myList.end(), 8);

which for instance would remove any element that is 8.

However let's say MyClass has an attribute name_. I have a list of objects of type MyClass, and I'd like to create a function that finds and removes the object that has that name.

Stub:

void remove(string &name) const

{

remove(myList.begin(), myList.end(), ???);

}

The thing is, if I supply the string &name as an argument to the remove algorithm, it will attempt to compare MyClass objects with strings, which doesn't make sense. What I'd like it to do is compare the name_ attribute of its MyClass objects with the string &name.

I thought of supplying a boolean function that would return element->name_ == name, but the function would need two arguments (list<MyClass*> element, string &name) and I don't think that would work.

As you see I'm stuck.

10 answers to this question

Recommended Posts

  • 0

The way I have done this (which is probably wrong as I taught it to myself) is to use a std::map.

This way you would do something like:

 std::map&lt; std::string name, MyClass* &gt; MyList;
MyList["ClassName"] = PtrToClass;
MyList.erase( the name of the class (the std::name), in this case "ClassName" );

Even if this is wrong hopefully someone can show me how to do it right.

  • 0

for a list:

std::list&lt;MyClass*&gt; MyList

a remove function could be like:

void remove(string &amp;name) const
{
  std::list&lt;MyClass*&gt;::iterator iter;
  for (iter = MyList.begin(); iter != MyList.end(); iter++)
  {
	if ((*iter)-&gt;name_.equalis(name))
	{
	  MyList.erase(iter);
	  break;
	}
  }
}

  • 0

Lant's suggestion is probably the correct one if changing the container is practical which would depend on the particular circumstances.

dev's suggestion is probably the easier (and by no means wrong) if you have to stick with list<>.

The more STL solution would be to use one of the supplied algorithms, in this case remove_if would seem appropriate. The link shows some sample code which uses 'compose1' and 'bind2nd' to construct the 'predicate' function. This is also the STL way of doing things, i.e. you are reusing generic components instead of writing lots of special case code.

If all this 'predicate' stuff is a bit confusing try a different approach. Give your class an overloaded equality operator with another class of the same type which only compares their name_ attribute. Then pass a dummy instance of such a class to remove with the name_ attribute set to the desired value to remove.

As you can gather there are really a load of approaches and this can make C++ seem overwhelming at first. Just remember no one way is necessarily the best. But usually the professionals prefer the solution which reuses existing generic code rather than the individual approach.

  • 0

Ok I'm starting to make some sense of out bind2nd and compose1. Really cool stuff.

Another important question I have about the STL: let's say I have a vector of pointers to dynamic objects, for example:

std::vector< MyClass* > myVector;

MyClass * object1 = new MyClass();

MyClass * object2 = new MyClass();

myVector.push_back(object1);

myVector.push_back(object2);

When I call myVector.clear(), the cplusplus reference says that it will call the destructors for each of the elements. The thing is, I'm not sure if I need to call delete for each of these pointers or not.

  • 0
  Eoin said:
dev's suggestion is probably the easier (and by no means wrong) if you have to stick with list<>.

i tend to go for more verbose code due to me favouring the C way of doing things over C++. Only times i use C++ parts is when i want a list of things and don't want to make my own linked list, or i need classes with subclasses. Most of the time i avoid the STL due it not liking threading very well (in my experience anyway)

  Dr_Asik said:
Ok I'm starting to make some sense of out bind2nd and compose1. Really cool stuff.

Another important question I have about the STL: let's say I have a vector of pointers to dynamic objects, for example:

std::vector< MyClass* > myVector;

MyClass * object1 = new MyClass();

MyClass * object2 = new MyClass();

myVector.push_back(object1);

myVector.push_back(object2);

When I call myVector.clear(), the cplusplus reference says that it will call the destructors for each of the elements. The thing is, I'm not sure if I need to call delete for each of these pointers or not.

afaik you will need to delete the objects, i wasn't aware that calling clear() would call the destructors of the objects but apparently it does (Y)

  • 0
  Dr_Asik said:
When I call myVector.clear(), the cplusplus reference says that it will call the destructors for each of the elements. The thing is, I'm not sure if I need to call delete for each of these pointers or not.

Yeah I think you still need to call delete. I'm pretty sure the destructors only get called for objects stored by value

std::vector< MyClass > myVector;

MyClass object1;

myVector.push_back(object1);

If you need to store them as pointers consider using a smart point like boost::shared_ptr<>.

  • 0

Hm I'm really puzzled at how I am supposed to deallocate memory when erasing specific elements from a list. I thought about using auto_ptr (I can't use boost now), but the cplusplus reference advises against this:

Warning: It is generally a bad idea to put auto_ptr objects inside C++ STL containers. C++ containers can do funny things with the data inside them, including frequent reallocation (when being copied, for instance). Since calling the destructor of an auto_ptr object will free up the memory associated with that object, any C++ container reallocation will cause any auto_ptr objects to become invalid.

http://www.cppreference.com/cppmisc/auto_ptr.html

So what should I do?

  • 0

Well you can still manually call delete whenever you remove a element from a container, but ideally you do want a smart pointer to do this automatically.

If Boost IS an option then use shared_ptr as I suggested or the pointer containers. Or if you have access to tr1 the tr1::shared_ptr is the boost::shared_ptr basicially.

Otherwise you could try finding another reference counted smart pointer. The problem is inside STL containers objects can get copied as the container grows so if you used auto_ptr then at times there could exist two auto_ptr's to the same object. You need to use a reference counted smart pointer in this case as it ensures the object is deleted only when there are no smart pointers left pointing to it.

I know Scott Meyers published a reference counted smart pointer. Haven't used it myself but I imagine it should work fine. Of course it's published in one of his books but maybe you can find a listing of the code if you look around his online articles. His site maintains a list.

Others surely exist out there too but ideally you should use the boost or tr1 shared_ptr if you can.

  • 0

Well, as I really have to stick to absolute basic stuff, the STL, and not use any sort of loops, I have developped a very heavy and unelegant solution. Basically I selectively copy the elements that must not be removed into a new container. But as the containers contain pointers to these elements, I have to really copy objects, a costly process. Then I call delete on every pointer of the old container, erase the container, and assign the new container to the old container. Then the new container is discarded.

This topic is now closed to further replies.
  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Posts

    • "What's that? Microsoft recommends Intel CPUs? My next computer must have Intel then." - Nobody
    • Apple warns Australia against joining EU in mandating iPhone app sideloading by David Uzondu Apple has issued a warning to Australia, essentially telling the country not to follow the European Union's lead in making iPhone app sideloading a requirement. This communication comes as the Australian federal government considers new rules that could force Apple to open up its iOS ecosystem, much like what happened in Europe with recent legislation. Since iOS 17.4 and iPadOS 18, users in the EU have been able to get apps from outside the official App Store, a direct consequence of the DMA designating Apple as a "gatekeeper". The Australian government floated a proposal in a paper released late last year. The paper suggested "designating" digital platforms like Apple's App Store. Being designated this way means these platforms would have to follow new rules intended to keep them from limiting competition. The government pointed to Apple's in-app payment system, which usually comes with a commission, and the lack of sideloading as likely targets for regulation. Right now, apps like Netflix and Spotify can't let users subscribe through their iOS apps without giving Apple a big cut, and they're not even allowed to tell users where to find a better deal. Apple, in its response to this Australian paper, stated that Australia should not use the EU's Digital Markets Act "as a blueprint". The company's core argument is that the changes mandated by the EU's DMA, which came into full effect in March 2024, introduce serious security and privacy risks for users. Apple claims that allowing sideloading and alternative app stores effectively opens the door for malware, fraud, scams, and other harmful content. The tech company also highlighted specific concerns from its European experience, alleging that its compliance there has led to users being able to install pornography apps and apps that facilitate copyright infringement, things its curated App Store aims to prevent. Apple maintains that its current review process is vital for user protection, and that its often criticized 30% commission applies mainly to the highest earning apps, with most developers paying a lower 15% rate or nothing. It is worth noting that Apple's implementation of DMA requirements in the EU, which includes a "Core Technology Fee" for apps distributed outside its store, has already drawn scrutiny from European Commission officials who question if these measures truly comply with the DMA's idea of keeping the market fair. The Australian government has not yet detailed its next steps in this process, and the Treasury still needs to publish the full submissions to its proposal paper, including Apple's complete arguments. Source: The Guardian
    • Oh the Chinese crybabies who couldn't accept the fact that they lost GOTY to Astro Bot? Not interested
    • Download this SQL Essentials For Dummies eBook (worth $10) for free by Steven Parker FOR DUMMIES is a trademark of John Wiley & Sons, Inc. A right-to-the-point guide on all the key topics of SQL programming SQL Essentials For Dummies is your quick reference to all the core concepts of SQL—a valuable common standard language used in relational databases. This useful guide is straightforward—with no excess review, wordy explanations, or fluff—so you get what you need, fast. Great for a brush-up on the basics or as an everyday desk reference, this book is one you can rely on. Strengthen your understanding of the basics of SQL Review what you've already learned or pick up key skills Use SQL to create, manipulate, and control relational databases Jog your memory on the essentials as you work and get clear answers to your questions Perfect for supplementing classroom learning, reviewing for a certification, and staying knowledgeable on the job, SQL Essentials For Dummies is the convenient, direct, and digestible reference you've been looking for. Claim your complimentary eBook worth $10 for free, before the offer ends on June 17. How to get it Please ensure you read the terms and conditions to claim this offer. Complete and verifiable information is required in order to receive this free offer. If you have previously made use of these free offers, you will not need to re-register. While supplies last! Download the SQL Essentials For Dummies eBook (worth $10) for free Offered by Wiley, view other free resources The below offers are also available for free for a limited time: Winxvideo AI V3.0 Lifetime License for PC ($69.95 Value) FREE – Expires 6/8 Aiarty Image Enhancer for PC/Mac ($85 Value) FREE – Expires 6/8 Solutions Architect's Handbook, Third Edition ($42.99 Value) FREE – Expires 6/10 AI and Innovation ($21 Value) FREE – Expires 6/11 Unruly: Fighting Back when Politics, AI, and Law Upend [...] ($18 Value) FREE - Expires 6/17 SQL Essentials For Dummies ($10 Value) FREE – Expires 6/17 Continuous Testing, Quality, Security, and Feedback ($27.99 Value) FREE – Expires 6/18 VideoProc Converter AI v7.5 for FREE (worth $78.90) – Expires 6/18 Macxvideo AI ($39.95 Value) Free for a Limited Time – Expires 6/22 The Ultimate Linux Newbie Guide – Featured Free content Python Notes for Professionals – Featured Free content Learn Linux in 5 Days – Featured Free content Quick Reference Guide for Cybersecurity – Featured Free content We post these because we earn commission on each lead so as not to rely solely on advertising, which many of our readers block. It all helps toward paying staff reporters, servers and hosting costs. Other ways to support Neowin The above deal not doing it for you, but still want to help? Check out the links below. Check out our partner software in the Neowin Store Buy a T-shirt at Neowin's Threadsquad Subscribe to Neowin - for $14 a year, or $28 a year for an ad-free experience Disclosure: An account at Neowin Deals is required to participate in any deals powered by our affiliate, StackCommerce. For a full description of StackCommerce's privacy guidelines, go here. Neowin benefits from shared revenue of each sale made through the branded deals site.
    • No one is surprised. It goes both ways. Warhammer remake is not on the PS5, but Xbox/PC and Game Pass day 1 and it is NOT a Microsoft owned game (Sega). 100% Microsoft money deal. Timed of course. Sega wins because they get the Microsoft money now then when the deal ends they get the much larger PS audience to make money from. As far as Black Myth is concerned, the Chinese gaming platform market is very much PC then PS then everything else. Consoles were banned in China for years, then allowed and the PS is the console of choice. The release of Black Myth actually saw a PS5 console sale spike because of Chinese gamer's buying PS5's just to play this game. With the ever shrinking Xbox console market I can see a point where 3rd party games try to get Sony exclusive money because the ROI for Xbox console versions of those games is a shrinking endeavor.
  • Recent Achievements

    • One Month Later
      MoyaM earned a badge
      One Month Later
    • One Month Later
      qology earned a badge
      One Month Later
    • One Year In
      Frinco90 earned a badge
      One Year In
    • Apprentice
      Frinco90 went up a rank
      Apprentice
    • Week One Done
      theevergreentree earned a badge
      Week One Done
  • Popular Contributors

    1. 1
      +primortal
      448
    2. 2
      +FloatingFatMan
      247
    3. 3
      snowy owl
      234
    4. 4
      ATLien_0
      202
    5. 5
      Xenon
      146
  • Tell a friend

    Love Neowin? Tell a friend!