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  • Mutable class as a child of an immutable class

    - by deamon
    I want to have immutable Java objects like this (strongly simplyfied): class Immutable { protected String name; public Immutable(String name) { this.name = name; } public String getName() { return name; } } In some cases the object should not only be readable but mutable, so I could add mutability through inheritance: public class Mutable extends Immutable { public Mutable(String name) { super(name); } public void setName(String name) { super.name = name; } } While this is technically fine, I wonder if it conforms with OOP and inheritance that mutable is also of type immutable. I want to avoid the OOP crime to throw UnsupportedOperationException for immutable object, like the Java collections API does. What do you think? Any other ideas?

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  • Overriding GetHashCode in a mutable struct - What NOT to do?

    - by Kyle Baran
    I am using the XNA Framework to make a learning project. It has a Point struct which exposes an X and Y value; for the purpose of optimization, it breaks the rules for proper struct design, since its a mutable struct. As Marc Gravell, John Skeet, and Eric Lippert point out in their respective posts about GetHashCode() (which Point overrides), this is a rather bad thing, since if an object's values change while its contained in a hashmap (ie, LINQ queries), it can become "lost". However, I am making my own Point3D struct, following the design of Point as a guideline. Thus, it too is a mutable struct which overrides GetHashCode(). The only difference is that mine exposes and int for X, Y, and Z values, but is fundamentally the same. The signatures are below: public struct Point3D : IEquatable<Point3D> { public int X; public int Y; public int Z; public static bool operator !=(Point3D a, Point3D b) { } public static bool operator ==(Point3D a, Point3D b) { } public Point3D Zero { get; } public override int GetHashCode() { } public override bool Equals(object obj) { } public bool Equals(Point3D other) { } public override string ToString() { } } I have tried to break my struct in the way they describe, namely by storing it in a List<Point3D>, as well as changing the value via a method using ref, but I did not encounter they behavior they warn about (maybe a pointer might allow me to break it?). Am I being too cautious in my approach, or should I be okay to use it as is?

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  • C++ 'mutable' keyword

    - by Rob
    A while ago I came across some code that marked a member variable of a class with the 'mutable' keyword. As far as I can see it simply allows you to modify a variable in a 'const' method: class Foo { private: mutable bool done_; public: void doSomething() const { ...; done_ = true; } }; Is this the only use of this keyword or is there more to it than meets the eye? I have since used this technique in a class, marking a boost::mutex as mutable allowing const functions to lock it for thread-safety reasons, but, to be honest, it feels like a bit of a hack.

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  • Mutable Records in F#

    - by MarkPearl
    I’m loving my expert F# book – today I thought I would give a post on using mutable records as covered in Chapter 4 of Expert F#. So as they explain the simplest mutable data structures in F# are mutable records. The whole concept of things by default being immutable is a new one for me from my C# background. Anyhow… lets look at some C# code first. using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace MutableRecords { public class DiscreteEventCounter { public int Total { get; set; } public int Positive { get; set; } public string Name { get; private set; } public DiscreteEventCounter(string name) { Name = name; } } class Program { private static void recordEvent(DiscreteEventCounter s, bool isPositive) { s.Total += 1; if (isPositive) s.Positive += 1; } private static void reportStatus (DiscreteEventCounter s) { Console.WriteLine("We have {0} {1} out of {2}", s.Positive, s.Name, s.Total); } static void Main(string[] args) { var longCounter = new DiscreteEventCounter("My Discrete Counter"); recordEvent(longCounter, true); recordEvent(longCounter, true); reportStatus(longCounter); Console.ReadLine(); } } } Quite simple, we have a class that has a few values. We instantiate an instance of the class and perform increments etc on the instance. Now lets look at an equivalent F# sample. namespace EncapsulationNS module Module1 = open System type DiscreteEventCounter = { mutable Total : int mutable Positive : int Name : string } let recordEvent (s: DiscreteEventCounter) isPositive = s.Total <- s.Total+1 if isPositive then s.Positive <- s.Positive+1 let reportStatus (s: DiscreteEventCounter) = printfn "We have %d %s out of %d" s.Positive s.Name s.Total let newCounter nm = { Total = 0; Positive = 0; Name = nm } // // Using it... // let longCounter = newCounter "My Discrete Counter" recordEvent longCounter (true) recordEvent longCounter (true) reportStatus longCounter System.Console.ReadLine() Notice in the type declaration of the DiscreteEventCounter we had to explicitly declare that the total and positive value holders were mutable. And that’s it – a very simple example of mutable types.

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  • Inserting mutable pairs into a mutable list

    - by Romelus
    How can I push a mutable pair onto a stack such that i'm only creating one stack. I have some code that works but creates lists within lists within lists.... Here is what I believe should work but throws an error. (define func (arg1 arg2 arg3) // Where arg3 is an empty list (mappend (mcons arg1 arg2) arg3)) The above code complains and says: "mcar: expects argument of type ; given ... Can anyone show me how I can get a result that looks like so,: (list (arg1 arg2) (arg# arg#) ...)

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  • Why are mutable structs evil?

    - by divo
    Following the discussions here on SO I already read several times the remark that mutable structs are evil (like in the answer to this question). What's the actual problem with mutability and structs?

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  • boost::serialization with mutable members

    - by redmoskito
    Using boost::serialization, what's the "best" way to serialize an object that contains cached, derived values in mutable members, such that cached members aren't serialized, but on deserialization, they are initialized the their appropriate default. A definition of "best" follows later, but first an example: class Example { public: Example(float n) : num(n), sqrt_num(-1.0) {} float get_num() const { return num; } // compute and cache sqrt on first read float get_sqrt() const { if(sqrt_num < 0) sqrt_num = sqrt(num); return sqrt_num; } template <class Archive> void serialize(Archive& ar, unsigned int version) { ... } private: float num; mutable float sqrt_num; }; On serialization, only the "num" member should be saved. On deserialization, the sqrt_num member must be initialized to its sentinel value indicating it needs to be computed. What is the most elegant way to implement this? In my mind, an elegant solution would avoid splitting serialize() into separate save() and load() methods (which introduces maintenance problems). One possible implementation of serialize: template <class Archive> void serialize(Archive& ar, unsigned int version) { ar & num; sqrt_num = -1.0; } This handles the deserialization case, but in the serialization case, the cached value is killed and must be recomputed. Also, I've never seen an example of boost::serialize that explicitly sets members inside of serialize(), so I wonder if this is generally not recommended. Some might suggest that the default constructor handles this, for example: int main() { Example e; { std::ifstream ifs("filename"); boost::archive::text_iarchive ia(ifs); ia >> e; } cout << e.get_sqrt() << endl; return 0; } which works in this case, but I think fails if the object receiving the deserialized data has already been initialized, as in the example below: int main() { Example ex1(4); Example ex2(9); cout << ex1.get_sqrt() << endl; // outputs 2; cout << ex2.get_sqrt() << endl; // outputs 3; // the following two blocks should implement ex2 = ex1; // save ex1 to archive { std::ofstream ofs("filename"); boost::archive::text_oarchive oa(ofs); oa << ex1; } // read it back into ex2 { std::ifstream ifs("filename"); boost::archive::text_iarchive ia(ifs); ia >> ex2; } // these should be equal now, but aren't, // since Example::serialize() doesn't modify num_sqrt cout << ex1.get_sqrt() << endl; // outputs 2; cout << ex2.get_sqrt() << endl; // outputs 3; return 0; } I'm sure this issue has come up with others, but I have struggled to find any documentation on this particular scenario. Thanks!

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  • Should this immutable struct be a mutable class?

    - by ChaosPandion
    I showed this struct to a fellow programmer and they felt that it should be a mutable class. They felt it is inconvenient not to have null references and the ability to alter the object as required. I would really like to know if there are any other reasons to make this a mutable class. [Serializable] public struct PhoneNumber : ICloneable, IEquatable<PhoneNumber> { private const int AreaCodeShift = 54; private const int CentralOfficeCodeShift = 44; private const int SubscriberNumberShift = 30; private const int CentralOfficeCodeMask = 0x000003FF; private const int SubscriberNumberMask = 0x00003FFF; private const int ExtensionMask = 0x3FFFFFFF; private readonly ulong value; public int AreaCode { get { return UnmaskAreaCode(value); } } public int CentralOfficeCode { get { return UnmaskCentralOfficeCode(value); } } public int SubscriberNumber { get { return UnmaskSubscriberNumber(value); } } public int Extension { get { return UnmaskExtension(value); } } public PhoneNumber(ulong value) : this(UnmaskAreaCode(value), UnmaskCentralOfficeCode(value), UnmaskSubscriberNumber(value), UnmaskExtension(value), true) { } public PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber) : this(areaCode, centralOfficeCode, subscriberNumber, 0, true) { } public PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber, int extension) : this(areaCode, centralOfficeCode, subscriberNumber, extension, true) { } private PhoneNumber(int areaCode, int centralOfficeCode, int subscriberNumber, int extension, bool throwException) { value = 0; if (areaCode < 200 || areaCode > 989) { if (!throwException) return; throw new ArgumentOutOfRangeException("areaCode", areaCode, @"The area code portion must fall between 200 and 989."); } else if (centralOfficeCode < 200 || centralOfficeCode > 999) { if (!throwException) return; throw new ArgumentOutOfRangeException("centralOfficeCode", centralOfficeCode, @"The central office code portion must fall between 200 and 999."); } else if (subscriberNumber < 0 || subscriberNumber > 9999) { if (!throwException) return; throw new ArgumentOutOfRangeException("subscriberNumber", subscriberNumber, @"The subscriber number portion must fall between 0 and 9999."); } else if (extension < 0 || extension > 1073741824) { if (!throwException) return; throw new ArgumentOutOfRangeException("extension", extension, @"The extension portion must fall between 0 and 1073741824."); } else if (areaCode.ToString()[1] - 48 > 8) { if (!throwException) return; throw new ArgumentOutOfRangeException("areaCode", areaCode, @"The second digit of the area code cannot be greater than 8."); } else { value |= ((ulong)(uint)areaCode << AreaCodeShift); value |= ((ulong)(uint)centralOfficeCode << CentralOfficeCodeShift); value |= ((ulong)(uint)subscriberNumber << SubscriberNumberShift); value |= ((ulong)(uint)extension); } } public object Clone() { return this; } public override bool Equals(object obj) { return obj != null && obj.GetType() == typeof(PhoneNumber) && Equals((PhoneNumber)obj); } public bool Equals(PhoneNumber other) { return this.value == other.value; } public override int GetHashCode() { return value.GetHashCode(); } public override string ToString() { return ToString(PhoneNumberFormat.Separated); } public string ToString(PhoneNumberFormat format) { switch (format) { case PhoneNumberFormat.Plain: return string.Format(@"{0:D3}{1:D3}{2:D4} {3:#}", AreaCode, CentralOfficeCode, SubscriberNumber, Extension).Trim(); case PhoneNumberFormat.Separated: return string.Format(@"{0:D3}-{1:D3}-{2:D4} {3:#}", AreaCode, CentralOfficeCode, SubscriberNumber, Extension).Trim(); default: throw new ArgumentOutOfRangeException("format"); } } public ulong ToUInt64() { return value; } public static PhoneNumber Parse(string value) { var result = default(PhoneNumber); if (!TryParse(value, out result)) { throw new FormatException(string.Format(@"The string ""{0}"" could not be parsed as a phone number.", value)); } return result; } public static bool TryParse(string value, out PhoneNumber result) { result = default(PhoneNumber); if (string.IsNullOrEmpty(value)) { return false; } var index = 0; var numericPieces = new char[value.Length]; foreach (var c in value) { if (char.IsNumber(c)) { numericPieces[index++] = c; } } if (index < 9) { return false; } var numericString = new string(numericPieces); var areaCode = int.Parse(numericString.Substring(0, 3)); var centralOfficeCode = int.Parse(numericString.Substring(3, 3)); var subscriberNumber = int.Parse(numericString.Substring(6, 4)); var extension = 0; if (numericString.Length > 10) { extension = int.Parse(numericString.Substring(10)); } result = new PhoneNumber( areaCode, centralOfficeCode, subscriberNumber, extension, false ); return result.value == 0; } public static bool operator ==(PhoneNumber left, PhoneNumber right) { return left.Equals(right); } public static bool operator !=(PhoneNumber left, PhoneNumber right) { return !left.Equals(right); } private static int UnmaskAreaCode(ulong value) { return (int)(value >> AreaCodeShift); } private static int UnmaskCentralOfficeCode(ulong value) { return (int)((value >> CentralOfficeCodeShift) & CentralOfficeCodeMask); } private static int UnmaskSubscriberNumber(ulong value) { return (int)((value >> SubscriberNumberShift) & SubscriberNumberMask); } private static int UnmaskExtension(ulong value) { return (int)(value & ExtensionMask); } } public enum PhoneNumberFormat { Plain, Separated }

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  • Why the "mutable default argument fix" syntax is so ugly, asks python newbie

    - by Cawas
    Now following my series of "python newbie questions" and based on another question. Go to http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables and scroll down to "Default Parameter Values". There you can find the following: def bad_append(new_item, a_list=[]): a_list.append(new_item) return a_list def good_append(new_item, a_list=None): if a_list is None: a_list = [] a_list.append(new_item) return a_list So, question here is: why is the "good" syntax over a known issue ugly like that in a programming language that promotes "elegant syntax" and "easy-to-use"? Why not just something in the definition itself, that the "argument" name is attached to a "localized" mutable object like: def better_append(new_item, a_list=[].local): a_list.append(new_item) return a_list I'm sure there would be a better way to do this syntax, but I'm also almost positive there's a good reason to why it hasn't been done. So, anyone happens to know why?

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  • Problem with custom Equality and GetHashCode in a mutable object

    - by Shimmy
    Hello! I am using Entity Framework in my application. I implemented with the partial class of an entity the IEquatable<T> interface: Partial Class Address : Implements IEquatable(Of Address) 'Other part generated Public Overloads Function Equals(ByVal other As Address) As Boolean _ Implements System.IEquatable(Of Address).Equals If ReferenceEquals(Me, other) Then Return True Return AddressId = other.AddressId End Function Public Overrides Function Equals(ByVal obj As Object) As Boolean If obj Is Nothing Then Return MyBase.Equals(obj) If TypeOf obj Is Address Then Return Equals(DirectCast(obj, Address)) Else Return False End Function Public Overrides Function GetHashCode() As Integer Return AddressId.GetHashCode End Function End Class Now in my code I use it this way: Sub Main() Using e As New CompleteKitchenEntities Dim job = e.Job.FirstOrDefault Dim address As New Address() job.Addresses.Add(address) Dim contains1 = job.Addresses.Contains(address) 'True e.SaveChanges() Dim contains2 = job.Addresses.Contains(address) 'False 'The problem is that I can't remove it: Dim removed = job.Addresses.Remoeve(address) 'False End Using End Sub Note (I checked in the debugger visualizer) that the EntityCollection class stores its entities in HashSet so it has to do with the GetHashCode function, I want it to depend on the ID so entities are compared by their IDs. The problem is that when I hit save, the ID changes from 0 to its db value. So the question is how can I have an equatable object, being properly hashed. Please help me find what's wrong in the GetHashCode function (by ID) and what can I change to make it work. Thanks a lot.

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  • How to implement IEquatable<T> when mutable fields are part of the equality - Problem with GetHashCo

    - by Shimmy
    Hello! I am using Entity Framework in my application. I implemented with the partial class of an entity the IEquatable<T> interface: Partial Class Address : Implements IEquatable(Of Address) 'Other part generated Public Overloads Function Equals(ByVal other As Address) As Boolean _ Implements System.IEquatable(Of Address).Equals If ReferenceEquals(Me, other) Then Return True Return AddressId = other.AddressId End Function Public Overrides Function Equals(ByVal obj As Object) As Boolean If obj Is Nothing Then Return MyBase.Equals(obj) If TypeOf obj Is Address Then Return Equals(DirectCast(obj, Address)) Else Return False End Function Public Overrides Function GetHashCode() As Integer Return AddressId.GetHashCode End Function End Class Now in my code I use it this way: Sub Main() Using e As New CompleteKitchenEntities Dim job = e.Job.FirstOrDefault Dim address As New Address() job.Addresses.Add(address) Dim contains1 = job.Addresses.Contains(address) 'True e.SaveChanges() Dim contains2 = job.Addresses.Contains(address) 'False 'The problem is that I can't remove it: Dim removed = job.Addresses.Remoeve(address) 'False End Using End Sub Note (I checked in the debugger visualizer) that the EntityCollection class stores its entities in HashSet so it has to do with the GetHashCode function, I want it to depend on the ID so entities are compared by their IDs. The problem is that when I hit save, the ID changes from 0 to its db value. So the question is how can I have an equatable object, being properly hashed. Please help me find what's wrong in the GetHashCode function (by ID) and what can I change to make it work. Thanks a lot.

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  • Why did Matz choose to make Strings mutable by default in Ruby?

    - by Seth Tisue
    It's the reverse of this question: http://stackoverflow.com/questions/93091/why-cant-strings-be-mutable-in-java-and-net Was this choice made in Ruby only because operations (appends and such) are efficient on mutable strings, or was there some other reason? (If it's only efficiency, that would seem peculiar, since the design of Ruby seems otherwise to not put a high premium on faciliating efficient implementation.)

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  • C# How can I tell if an IEnumerable is Mutable?

    - by Logan
    I want a method to update certain entries of an IEnumerable. I found that doing a foreach over the entries and updating the values failed as in the background I was cloning the collection. This was because my IEnumerable was backed by some LINQ-SQL queries. By changing the method to take a List I have changed this behavior, Lists are always mutable and hence the method changes the actual objects in the list. Rather than demand a List is passed is there a Mutable interface I can use? // Will not behave as consistently for all IEnumerables public void UpdateYesterday (IEnumerable<Job> jobs) { foreach (var job : jobs) { if (job.date == Yesterday) { job.done = true; } } }

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  • Clean way to use mutable implementation of Immutable interfaces for encapsulation

    - by dsollen
    My code is working on some compost relationship which creates a tree structure, class A has many children of type B, which has many children of type C etc. The lowest level class, call it bar, also points to a connected bar class. This effectively makes nearly every object in my domain inter-connected. Immutable objects would be problematic due to the expense of rebuilding almost all of my domain to make a single change to one class. I chose to go with an interface approach. Every object has an Immutable interface which only publishes the getter methods. I have controller objects which constructs the domain objects and thus has reference to the full objects, thus capable of calling the setter methods; but only ever publishes the immutable interface. Any change requested will go through the controller. So something like this: public interface ImmutableFoo{ public Bar getBar(); public Location getLocation(); } public class Foo implements ImmutableFoo{ private Bar bar; private Location location; @Override public Bar getBar(){ return Bar; } public void setBar(Bar bar){ this.bar=bar; } @Override public Location getLocation(){ return Location; } } public class Controller{ Private Map<Location, Foo> fooMap; public ImmutableFoo addBar(Bar bar){ Foo foo=fooMap.get(bar.getLocation()); if(foo!=null) foo.addBar(bar); return foo; } } I felt the basic approach seems sensible, however, when I speak to others they always seem to have trouble envisioning what I'm describing, which leaves me concerned that I may have a larger design issue then I'm aware of. Is it problematic to have domain objects so tightly coupled, or to use the quasi-mutable approach to modifying them? Assuming that the design approach itself isn't inherently flawed the particular discussion which left me wondering about my approach had to do with the presence of business logic in the domain objects. Currently I have my setter methods in the mutable objects do error checking and all other logic required to verify and make a change to the object. It was suggested that this should be pulled out into a service class, which applies all the business logic, to simplify my domain objects. I understand the advantage in mocking/testing and general separation of logic into two classes. However, with a service method/object It seems I loose some of the advantage of polymorphism, I can't override a base class to add in new error checking or business logic. It seems, if my polymorphic classes were complicated enough, I would end up with a service method that has to check a dozen flags to decide what error checking and business logic applies. So, for example, if I wanted to have a childFoo which also had a size field which should be compared to bar before adding par my current approach would look something like this. public class Foo implements ImmutableFoo{ public void addBar(Bar bar){ if(!getLocation().equals(bar.getLocation()) throw new LocationException(); this.bar=bar; } } public interface ImmutableChildFoo extends ImmutableFoo{ public int getSize(); } public ChildFoo extends Foo implements ImmutableChildFoo{ private int size; @Override public int getSize(){ return size; } @Override public void addBar(Bar bar){ if(getSize()<bar.getSize()){ throw new LocationException(); super.addBar(bar); } My colleague was suggesting instead having a service object that looks something like this (over simplified, the 'service' object would likely be more complex). public interface ImmutableFoo{ ///original interface, presumably used in other methods public Location getLocation(); public boolean isChildFoo(); } public interface ImmutableSizedFoo implements ImmutableFoo{ public int getSize(); } public class Foo implements ImmutableSizedFoo{ public Bar bar; @Override public void addBar(Bar bar){ this.bar=bar; } @Override public int getSize(){ //default size if no size is known return 0; } @Override public boolean isChildFoo return false; } } public ChildFoo extends Foo{ private int size; @Override public int getSize(){ return size; } @Override public boolean isChildFoo(); return true; } } public class Controller{ Private Map<Location, Foo> fooMap; public ImmutableSizedFoo addBar(Bar bar){ Foo foo=fooMap.get(bar.getLocation()); service.addBarToFoo(foo, bar); returned foo; } public class Service{ public static void addBarToFoo(Foo foo, Bar bar){ if(foo==null) return; if(!foo.getLocation().equals(bar.getLocation())) throw new LocationException(); if(foo.isChildFoo() && foo.getSize()<bar.getSize()) throw new LocationException(); foo.setBar(bar); } } } Is the recommended approach of using services and inversion of control inherently superior, or superior in certain cases, to overriding methods directly? If so is there a good way to go with the service approach while not loosing the power of polymorphism to override some of the behavior?

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  • If immutable objects are good, why do people keep creating mutable objects?

    - by Vinoth Kumar
    If immutable objects are good,simple and offers benefits in concurrent programming why do programmers keep creating mutable objects? I have four years of experience in Java programming and as I see it, the first thing people do after creating a class is generate getters and setters in the IDE (thus making it mutable). Is there a lack of awareness or can we get away with using mutable objects in most scenarios?

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  • Impossible to be const-correct when combining data and it's lock?

    - by Graeme
    I've been looking at ways to combine a piece of data which will be accessed by multiple threads alongside the lock provisioned for thread-safety. I think I've got to a point where I don't think its possible to do this whilst maintaining const-correctness. Take the following class for example: template <typename TType, typename TMutex> class basic_lockable_type { public: typedef TMutex lock_type; public: template <typename... TArgs> explicit basic_lockable_type(TArgs&&... args) : TType(std::forward<TArgs...>(args)...) {} TType& data() { return data_; } const TType& data() const { return data_; } void lock() { mutex_.lock(); } void unlock() { mutex_.unlock(); } private: TType data_; mutable TMutex mutex_; }; typedef basic_lockable_type<std::vector<int>, std::mutex> vector_with_lock; In this I try to combine the data and lock, marking mutex_ as mutable. Unfortunately this isn't enough as I see it because when used, vector_with_lock would have to be marked as mutable in order for a read operation to be performed from a const function which isn't entirely correct (data_ should be mutable from a const). void print_values() const { std::lock_guard<vector_with_lock>(values_); for(const int val : values_) { std::cout << val << std::endl; } } vector_with_lock values_; Can anyone see anyway around this such that const-correctness is maintained whilst combining data and lock? Also, have I made any incorrect assumptions here?

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  • F# ref-mutable vars vs object fields

    - by rwallace
    I'm writing a parser in F#, and it needs to be as fast as possible (I'm hoping to parse a 100 MB file in less than a minute). As normal, it uses mutable variables to store the next available character and the next available token (i.e. both the lexer and the parser proper use one unit of lookahead). My current partial implementation uses local variables for these. Since closure variables can't be mutable (anyone know the reason for this?) I've declared them as ref: let rec read file includepath = let c = ref ' ' let k = ref NONE let sb = new StringBuilder() use stream = File.OpenText file let readc() = c := stream.Read() |> char // etc I assume this has some overhead (not much, I know, but I'm trying for maximum speed here), and it's a little inelegant. The most obvious alternative would be to create a parser class object and have the mutable variables be fields in it. Does anyone know which is likely to be faster? Is there any consensus on which is considered better/more idiomatic style? Is there another option I'm missing?

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  • Cocoa - Enumerate mutable array, removing objects

    - by Ward
    Hey there, I have a mutable array that contains mutable dictionaries with strings for the keys latitude, longitude and id. Some of the latitude and longitude values are the same and I want to remove the duplicates from the array so I only have one object per location. I can enumerate my array and using a second enumeration review each object to find objects that have different ids, but the same latitude and longitude, but if I try to remove the object, I'm muting the array during enumeration. Is there any way to remove objects from an array while enumerating so I only enumerate the current set of objects as the array is updated? Hope this question makes sense. Thanks, Howie

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  • iPhone Foundation - performance implications of mutable and xxxWithCapacity:0

    - by Adam Eberbach
    All of the collection classes have two versions - mutable and immutable, such as NSArray and NSMutableArray. Is the distinction merely to promote careful programming by providing a const collection or is there some performance hit when using a mutable object as opposed to immutable? Similarly each of the collection classes has a method xxxxWithCapacity, like [NSMutableArray arrayWithCapacity:0]. I often use zero as the argument because it seems a better choice than guessing wrongly how many objects might be added. Is there some performance advantage to creating a collection with capacity for enough objects in advance? If not why isn't the function something like + (id)emptyArray?

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