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• LINQ extention SelectMany in 3.5 vs 4.0?

  - by Moberg

  Hi When I saw Darins suggestion here .. IEnumerable<Process> processes = new[] { "process1", "process2" } .SelectMany(Process.GetProcessesByName); ( http://stackoverflow.com/questions/3059667/process-getprocessesbyname/3059733#3059733 ) .. I was a bit intrigued and I tried it in VS2008 with .NET 3.5 - and it did not compiling unless I changed it to .. IEnumerable<Process> res = new string[] { "notepad", "firefox", "outlook" } .SelectMany(s => Process.GetProcessesByName(s)); Having read some Darins answers before I suspected that it was me that were the problem, and when I later got my hands on a VS2010 with.NET 4.0 - as expected - the original suggestion worked beautifully. My question is : What have happend from 3.5 to 4.0 that makes this (new syntax) possible? Is it the extentionmethods that have been extended(hmm) or new rules for lambda syntax or? I've tried to search but my google-fu was not strong enough. Please forgive if the question is a bit naive and note that I've taged it as beginner :)

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• Using stringstream instead of `sscanf` to parse a fixed-format string

  - by John Dibling

  I would like to use the facilities provided by stringstream to extract values from a fixed-format string as a type-safe alternative to sscanf. How can I do this? Consider the following specific use case. I have a std::string in the following fixed format: YYYYMMDDHHMMSSmmm Where: YYYY = 4 digits representing the year MM = 2 digits representing the month ('0' padded to 2 characters) DD = 2 digits representing the day ('0' padded to 2 characters) HH = 2 digits representing the hour ('0' padded to 2 characters) MM = 2 digits representing the minute ('0' padded to 2 characters) SS = 2 digits representing the second ('0' padded to 2 characters) mmm = 3 digits representing the milliseconds ('0' padded to 3 characters) Previously I was doing something along these lines: string s = "20101220110651184"; unsigned year = 0, month = 0, day = 0, hour = 0, minute = 0, second = 0, milli = 0; sscanf(s.c_str(), "%4u%2u%2u%2u%2u%2u%3u", &year, &month, &day, &hour, &minute, &second, &milli ); The width values are magic numbers, and that's ok. I'd like to use streams to extract these values and convert them to unsigneds in the interest of type safety. But when I try this: stringstream ss; ss << "20101220110651184"; ss >> setw(4) >> year; year retains the value 0. It should be 2010. How do I do what I'm trying to do? I can't use Boost or any other 3rd party library, nor can I use C++0x.

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• Disable MSBuild output of "Processing /ORDER options..."

  - by Jippers

  The output file from our project build has gone from 6MB to over 75MB in text. Diff'ing the last good build and the first time it blew up, there's a section in the output file like this in the latest: Processing /ORDER options External code objects not listed in the /ORDER file: ?onCallDisconnected@CallStateConnected@CallImpl@space@@UAEXV?$shared_ptr@VCallImpl@space@@@boost@@V?$shared_ptr@VGenericCall@space@@@5@K@Z ; framework.lib(CallStates.obj) ??_DBoolSetting@space@@QAEXXZ ; framework.lib(SettingValueImpl.obj) ...... continues for ~50MB ??$?0U?$pair@$$CBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@J@std@@@?$allocator@U_Node@?$_Tree_nod@V?$_Tmap_traits@V?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@JU?$less@V?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@J@std@@@2@$0A@@std@@@std@@@std@@QAE@ABV?$allocator@U?$pair@$$CBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@J@std@@@1@@Z ; CallStatistics.obj Finished processing /ORDER options I'm not sure how this got in there, but anyone know how to turn it off?

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• MAC : How to check if the file is still being copied in cpp?

  - by Peda Pola

  In my current project, we had a requirement to check if the file is still copying. We have already developed a library which will give us OS notification like file_added , file_removed , file_modified, file_renamed on a particular folder along with the corresponding file path. The problem here is that, lets say if you add 1 GB file, it is giving multiple notification such as file_added , file_modified, file_modified as the file is being copied. Now i decided to surpass these notifications by checking if the file is copying or not. Based on that i will ignore events. I have written below code which tells if the file is being copied or not which takes file path as input parameter. Details:- In Mac, while file is being copied the creation date is set as some date less than 1970. Once it is copied the date is set to current date. Am using this technique. Based on this am deciding that file is being copied. Problem:- when we copy file in the terminal it is failing. Please advice me any approach. bool isBeingCopied(const boost::filesystem::path &filePath) { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; bool isBeingCopied = false; if([[[[NSFileManager defaultManager] attributesOfItemAtPath:[NSString stringWithUTF8String:filePath.string().c_str()] error:nil] fileCreationDate] timeIntervalSince1970] < 0) { isBeingCopied = true; } [pool release]; return isBeingCopied; }

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• c++ specialized overload?

  - by acidzombie24

  -edit- i am trying to close the question. i solved the problem with boost::is_base_and_derived In my class i want to do two things. 1) Copy int, floats and other normal values 2) Copy structs that supply a special copy function (template T copyAs(); } the struct MUST NOT return int's unless i explicitly say ints. I do not want the programmer mistaking the mistake by doing int a = thatClass; -edit- someone mention classes dont return anything, i mean using the operator Type() overload. How do i create my copy operator in such a way i can copy both 1) ints, floats etc and the the struct restricted in the way i mention in 2). i tried doing template <class T2> T operator = (const T2& v); which would cover my ints, floats etc. But how would it differentiate from structs? so i wrote T operator = (const SomeGenericBase& v); The idea was the GenericBase would be unsed instead then i can do v.Whatever. But that backfires bc the functions i want wouldnt exist, unless i use virtual, but virtual templates dont exist. Also i would hate to use virtual I think the solution is to get rid of ints and have it convert to something that can do .as(). So i wrote something up but now i have the same problem, how does that differentiate ints and structs that have the .as() function template?

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• Comparison of Java and .NET technologies/frameworks

  - by Paul Sasik

  I work in a shop that is a mix of mostly Java and .NET technologists. When discussing new solutions and architectures we often encounter impedance in trying to compare the various technologies, frameworks, APIs etc. in use between the two camps. It seems that each camp knows little about the other and we end up comparing apples to oranges and forgetting about the bushels. While researching the topic I found this: Java -- .Net rough equivalents It's a nice list but it's not quite exhaustive and is missing the key .NET 3.0 technologies and a few other tidbits. To complete that list: what are the near/rough equivalents (or a combination of technologies) in Java to the following in .NET? WCF WPF Silverlight = JavaFx WF = jBMP (Java Business Process Management) Generics Lambda expressions = lambdaJ project or Closures Linq (not Linq-to-SQL) ...have i missed anything else? Note that I omitted technologies that are already covered in the linked article. I would also like to hear feedback on whether the linked article is accurate. Thanks. (Will CW if requested.)

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• access properties of current model in has_many declaration

  - by seth.vargo

  Hello, I didn't exactly know how to pose this question other than through example... I have a class we will call Foo. Foo :has_many Bar. Foo has a boolean attribute called randomize that determines the order of the the Bars in the :has_many relationship: class CreateFoo < ActiveRecord::Migration def self.up create_table :foos do |t| t.string :name t.boolean :randomize, :default => false end end end   class CreateBar < ActiveRecord::Migration def self.up create_table :bars do |t| t.string :name t.references :foo end end end   class Bar < ActiveRecord::Base belongs_to :foo end   class Foo < ActiveRecord::Base # this is the line that doesn't work has_many :bars, :order => self.randomize ? 'RAND()' : 'id' end How do I access properties of self in the has_many declaration? Things I've tried and failed: creating a method of Foo that returns the correct string creating a lambda function crying Is this possible? UPDATE The problem seems to be that the class in :has_many ISN'T of type Foo: undefined method `randomize' for #<Class:0x1076fbf78> is one of the errors I get. Note that its a general Class, not a Foo object... Why??

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• Rationale in selecting Hash Key type

  - by Amrish

  Guys, I have a data structure which has 25 distinct keys (integer) and a value. I have a list of these objects (say 50000) and I intend to use a hash table to store/retrieve them. I am planning to take one of these approaches. Create a integer hash from these 25 integer keys and store it on a hash table. (Yeah! I have some means to handle collisions) Make a string concatenation on the individual keys and use it as a hash key for the hash table. For example, if the key values are 1,2,4,6,7 then the hash key would be "12467". Assuming that I have a total of 50000 records each with 25 distinct keys and a value, then will my second approach be a overkill when it comes to the cost of string comparisons it needs to do to retrieve and insert a record? Some more information! Each bucket in the hash table is a balanced binary tree. I am using the boost library's hash_combine method to create the hash from the 25 keys.

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• Interface Builder vs Cocos 2D - how choice the best for your app.

  - by baDa

  Hello everyone! I was a flash developer for 3 years, and in the last 5 months, i begin the iphone development, i do 2 applications with interface builder for clients, and now i really want to do a little game, is quite simple, one match 3! I made the engine in interface builder, and seens good to me! But after i read some posts, i really want to try it in the cocos2D! So, in 2 days i rewrite all my first engine for cocos2D, very annoying upsidedown coordinates but ok, i really do! But the performance side by side with interface builder version is really scare! Many Many slow downs at the cocos2d side! And the animation seens bugged to me! I really scare! I really don't know what is the best choice for a simple game. And i want some opinions: Using cocos2d when need some physics? When we have many objects at screen? What is the performance boost i have with cocos2D? I have how to share this 2 applications with you guys?! Without your UID?!

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• Prototyping Qt/C++ in Python

  - by tstenner

  I want to write a C++ application with Qt, but build a prototype first using Python and then gradually replace the Python code with C++. Is this the right approach, and what tools (bindings, binding generators, IDE) should I use? Ideally, everything should be available in the Ubuntu repositories so I wouldn't have to worry about incompatible or old versions and have everything set up with a simple aptitude install. Is there any comprehensive documentation about this process or do I have to learn every single component, and if yes, which ones? Right now I have multiple choices to make: Qt Creator, because of the nice auto completion and Qt integration. Eclipse, as it offers support for both C++ and Python. Eric (haven't used it yet) Vim PySide as it's working with CMake and Boost.Python, so theoretically it will make replacing python code easier. PyQt as it's more widely used (more support) and is available as a Debian package. Edit: As I will have to deploy the program to various computers, the C++-solution would require 1-5 files (the program and some library files if I'm linking it statically), using Python I'd have to build PyQt/PySide/SIP/whatever on every platform and explain how to install Python and everything else.

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• C#. Saving information about events and event handlers and removing handlers using this information

  - by Philipp

  I have an object which is creating several event handlers using lambda expressions, and I need to remove all handlers created in that object in one time. I'm trying to create some unified static object which will 'know' which handler relates to which object and event if event handler was created through this static object. I tried something like code below, but I don't understand how to save events and event handlers objects, to be able remove handlers in one time. class Program { static void Main(string[] args) { var EventSender = new EventSender(); var EventReceiver = new EventReceiver(EventSender); EventSender.Invoke(" some var "); LinkSaver.RemoveEvent(EventReceiver); // ?? Console.ReadKey(); } } public class ObjLink{ public object Event; public object Action; } public static class LinkSaver { public static void SetEvent<T>(object obj, Action<T> Event, T Action) { Event(Action); var objLink = new ObjLink{Event = Event, Action = Action}; if (!Links.ContainsKey(obj)) Links.Add(obj, new List<ObjLink>{objLink}); else Links[obj].Add(objLink); } static Dictionary<object,List<ObjLink>> Links = new Dictionary<object, List<ObjLink>>(); public static void RemoveEvent(object obj){ foreach(var objLink in Links[obj]){ // objLink.Event -= objLink.Action; ??? } } } public class EventReceiver { public EventReceiver(EventSender obj) { LinkSaver.SetEvent<EventSender.TestDelegate>(this, obj.SetEvent, str => Console.WriteLine(str + " test event!")); } } public class EventSender { public void Invoke(string var) { if (eventTest != null) eventTest(var); } public void SetEvent(TestDelegate Action) { eventTest += Action; } public delegate void TestDelegate(string var); private event TestDelegate eventTest; // by the way public void RemoveFromEvent() { foreach (var handler in eventTest.GetInvocationList()) eventTest -= (TestDelegate)handler; } }

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• Templates, Function Pointers and C++0x

  - by user328543

  One of my personal experiments to understand some of the C++0x features: I'm trying to pass a function pointer to a template function to execute. Eventually the execution is supposed to happen in a different thread. But with all the different types of functions, I can't get the templates to work. #include `<functional`> int foo(void) {return 2;} class bar { public: int operator() (void) {return 4;}; int something(int a) {return a;}; }; template <class C> int func(C&& c) { //typedef typename std::result_of< C() >::type result_type; typedef typename std::conditional< std::is_pointer< C >::value, std::result_of< C() >::type, std::conditional< std::is_object< C >::value, std::result_of< typename C::operator() >::type, void> >::type result_type; result_type result = c(); return result; } int main(int argc, char* argv[]) { // call with a function pointer func(foo); // call with a member function bar b; func(b); // call with a bind expression func(std::bind(&bar::something, b, 42)); // call with a lambda expression func( [](void)->int {return 12;} ); return 0; } The result_of template alone doesn't seem to be able to find the operator() in class bar and the clunky conditional I created doesn't compile. Any ideas? Will I have additional problems with const functions?

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• Is it possible to load an entire SQL Server CE database into RAM?

  - by DanM

  I'm using LinqToSql to query a small SQL Server CE database. I've noticed that any operations involving sub-properties are disappointingly slow. For example, if I have a Customer table that is referenced by an Order table via a foreign key, LinqToSql will automatically create an EntitySet<Order> property. This is a nice convenience, allowing me to do things like Customer.Order.Where(o => o.ProductName = "Stopwatch"), but for some reason, SQL Server CE hangs up pretty bad when I try to do stuff like this. One of my queries, which isn't really that complicated takes 3-4 seconds to complete. I can get the speed up to acceptable, even fast, if I just grab the two tables individually and convert them to List<Customer> and List<Order>, then join then manually with my own query, but this is throwing out a lot of the appeal of LinqToSql. So, I'm wondering if I can somehow get the whole database into RAM and just query that way, then occasionally save it. Is this possible? How? If not, is there anything else I can do to boost the performance? Note: My database in its initial state is about 250K and I don't expect it to grow to more than 1-2Mb. So, loading the data into RAM certainly wouldn't be a problem from a memory point of view.

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• Reordering fields in Django model

  - by Alex Lebedev

  I want to add few fields to every model in my django application. This time it's created_at, updated_at and notes. Duplicating code for every of 20+ models seems dumb. So, I decided to use abstract base class which would add these fields. The problem is that fields inherited from abstract base class come first in the field list in admin. Declaring field order for every ModelAdmin class is not an option, it's even more duplicate code than with manual field declaration. In my final solution, I modified model constructor to reorder fields in _meta before creating new instance: class MyModel(models.Model): # Service fields notes = my_fields.NotesField() created_at = models.DateTimeField(auto_now_add=True) updated_at = models.DateTimeField(auto_now=True) class Meta: abstract = True last_fields = ("notes", "created_at", "updated_at") def __init__(self, *args, **kwargs): new_order = [f.name for f in self._meta.fields] for field in self.last_fields: new_order.remove(field) new_order.append(field) self._meta._field_name_cache.sort(key=lambda x: new_order.index(x.name)) super(TwangooModel, self).__init__(*args, **kwargs) class ModelA(MyModel): field1 = models.CharField() field2 = models.CharField() #etc ... It works as intended, but I'm wondering, is there a better way to acheive my goal?

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• Tree iterator, can you optimize this any further?

  - by Ron

  As a follow up to my original question about a small piece of this code I decided to ask a follow up to see if you can do better then what we came up with so far. The code below iterates over a binary tree (left/right = child/next ). I do believe there is room for one less conditional in here (the down boolean). The fastest answer wins! The cnt statement can be multiple statements so lets make sure this appears only once The child() and next() member functions are about 30x as slow as the hasChild() and hasNext() operations. Keep it iterative <-- dropped this requirement as the recursive solution presented was faster. This is C++ code visit order of the nodes must stay as they are in the example below. ( hit parents first then the children then the 'next' nodes). BaseNodePtr is a boost::shared_ptr as thus assignments are slow, avoid any temporary BaseNodePtr variables. Currently this code takes 5897ms to visit 62200000 nodes in a test tree, calling this function 200,000 times. void processTree (BaseNodePtr current, unsigned int & cnt ) { bool down = true; while ( true ) { if ( down ) { while (true) { cnt++; // this can/will be multiple statesments if (!current->hasChild()) break; current = current->child(); } } if ( current->hasNext() ) { down = true; current = current->next(); } else { down = false; current = current->parent(); if (!current) return; // done. } } }

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• LINQ to SQL: NOTing a prebuilt expression

  - by ck

  I'm building a library of functions for one of my core L2S classes, all of which return a bool to allow checking for certain situations. Example: Expression<Func<Account, bool>> IsSomethingX = a => a.AccountSupplementary != null && a.AccountSupplementary.SomethingXFlag != null && a.AccountSupplementary.SomethingXFlag.Value; Now to query where this is not true, I CAN'T do this: var myAccounts= context.Accounts .Where(!IsSomethingX); // does not compile However, using the syntax from the PredicateBuilder class, I've come up with this: public static IQueryable<T> WhereNot<T>(this IQueryable<T> items, Expression<Func<T, bool>> expr1) { var invokedExpr = Expression.Invoke(expr1, expr1.Parameters.Cast<Expression>()); return items.Where(Expression.Lambda<Func<T, bool>> (Expression.Not(invokedExpr), expr1.Parameters)); } var myAccounts= context.Accounts .WhereNot(IsSomethingX); // does compile which actually produces the correct SQL. Does this look like a good solution, and is there anything I need to be aware of that might cause me problems in future?

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• Member function overloading/template specialization issue

  - by Ferruccio

  I've been trying to call the overloaded table::scan_index(std::string, ...) member function without success. For the sake of clarity, I have stripped out all non-relevant code. I have a class called table which has an overloaded/templated member function named scan_index() in order to handle strings as a special case. class table : boost::noncopyable { public: template <typename T> void scan_index(T val, std::function<bool (uint recno, T val)> callback) { // code } void scan_index(std::string val, std::function<bool (uint recno, std::string val)> callback) { // code } }; Then there is a hitlist class which has a number of templated member functions which call table::scan_index(T, ...) class hitlist { public: template <typename T> void eq(uint fieldno, T value) { table* index_table = db.get_index_table(fieldno); // code index_table->scan_index<T>(value, [&](uint recno, T n)->bool { // code }); } }; And, finally, the code which kicks it all off: hitlist hl; // code hl.eq<std::string>(*fieldno, p1.to_string()); The problem is that instead of calling table::scan_index(std::string, ...), it calls the templated version. I have tried using both overloading (as shown above) and a specialized function template (below), but nothing seems to work. After staring at this code for a few hours, I feel like I'm missing something obvious. Any ideas? template <> void scan_index<std::string>(std::string val, std::function<bool (uint recno, std::string val)> callback) { // code }

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• C++/Qt - Memory allocation question

  - by HardCoder1986

  Hello! I recently started investigating Qt for myself and have the following question: Suppose I have some QTreeWidget* widget. At some moment I want to add some items to it and this is done via the following call: QList<QTreeWidgetItem*> items; // Prepare the items QTreeWidgetItem* item1 = new QTreeWidgetItem(...); QTreeWidgetItem* item2 = new QTreeWidgetItem(...); items.append(item1); items.append(item2); widget->addTopLevelItems(items); So far it looks ok, but I don't actually understand who should control the objects' lifetime. I should explain this with an example: Let's say, another function calls widget->clear();. I don't know what happens beneath this call but I do think that memory allocated for item1 and item2 doesn't get disposed here, because their ownage wasn't actually transfered. And, bang, we have a memory leak. The question is the following - does Qt have something to offer for this kind of situation? I could use boost::shared_ptr or any other smart pointer and write something like shared_ptr<QTreeWidgetItem> ptr(new QTreeWidgetItem(...)); items.append(ptr.get()); but I don't know if the Qt itself would try to make explicit delete calls on my pointers (which would be disastrous since I state them as shared_ptr-managed). How would you solve this problem? Maybe everything is evident and I miss something really simple?

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• Creating serializeable unique compile-time identifiers for arbitrary UDT's.

  - by Endiannes

  I would like a generic way to create unique compile-time identifiers for any C++ user defined types. for example: unique_id<my_type>::value == 0 // true unique_id<other_type>::value == 1 // true I've managed to implement something like this using preprocessor meta programming, the problem is, serialization is not consistent. For instance if the class template unique_id is instantiated with other_type first, then any serialization in previous revisions of my program will be invalidated. I've searched for solutions to this problem, and found several ways to implement this with non-consistent serialization if the unique values are compile-time constants. If RTTI or similar methods, like boost::sp_typeinfo are used, then the unique values are obviously not compile-time constants and extra overhead is present. An ad-hoc solution to this problem would be, instantiating all of the unique_id's in a separate header in the correct order, but this causes additional maintenance and boilerplate code, which is not different than using an enum unique_id{my_type, other_type};. A good solution to this problem would be using user-defined literals, unfortunately, as far as I know, no compiler supports them at this moment. The syntax would be 'my_type'_id; 'other_type'_id; with udl's. I'm hoping somebody knows a trick that allows implementing serialize-able unique identifiers in C++ with the current standard (C++03/C++0x), I would be happy if it works with the latest stable MSVC and GNU-G++ compilers, although I expect if there is a solution, it's not portable.

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• STI and accepts_nested_attributes_for in rails

  - by ryanshackintosh

  I have models as follows: class Entity < ActiveRecord::Base has_many :addresses accepts_nested_attributes_for :addresses, :reject_if => lambda { |a| a[:label].blank?} , :allow_destroy => true end class Client < Entity before_save :set_type private def set_type self.type = "Client" end end class Address < ActiveRecord::Base belongs_to :entity end I have recently implemented accepts_nested_attributes_for on the /clients/new form, as follows: <% form_for(@client, :html => {:class => 'form'}) do |f| -%> <%= f.label :name %> <%= f.text_field :name -%> <%= f.label :phone %> <%= f.text_field :phone %> <% f.fields_for :addresses do |a| %> <%= a.label :street %> <%= a.text_field :street%> <%= a.label :city %> <%= a.text_field :city %> <% end %> <% end %> And my controller as follows: class ClientsController < ApplicationController before_filter :load_client , :except => [:index, :new, :create, :render_clients] def new @client = Client.new @client.addresses.build end def create @client = Client.new(params[:client]) if @client.save flash[:notice] = 'Client has been successfully added' redirect_to @client else render :action => 'new' end end The issue is that when the record is saved it gives an error stating: "Entity can't be blank" I assume it is something to do with the fact that a 'Client' and not an 'Entity' is being added. Can anyone point me in the right direction?

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• What wrapper class in C++ should I use for automated resource management?

  - by Vilx-

  I'm a C++ amateur. I'm writing some Win32 API code and there are handles and weirdly compositely allocated objects aplenty. So I was wondering - is there some wrapper class that would make resource management easier? For example, when I want to load some data I open a file with CreateFile() and get a HANDLE. When I'm done with it, I should call CloseHandle() on it. But for any reasonably complex loading function there will be dozens of possible exit points, not to mention exceptions. So it would be great if I could wrap the handle in some kind of wrapper class which would automatically call CloseHandle() once execution left the scope. Even better - it could do some reference counting so I can pass it around in and out of other functions, and it would release the resource only when the last reference left scope. The concept is simple - but is there something like that in the standard library? I'm using Visual Studio 2008, by the way, and I don't want to attach a 3rd party framework like Boost or something.

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• How can I write a function template for all types with a particular type trait?

  - by TC

  Consider the following example: struct Scanner { template <typename T> T get(); }; template <> string Scanner::get() { return string("string"); } template <> int Scanner::get() { return 10; } int main() { Scanner scanner; string s = scanner.get<string>(); int i = scanner.get<int>(); } The Scanner class is used to extract tokens from some source. The above code works fine, but fails when I try to get other integral types like a char or an unsigned int. The code to read these types is exactly the same as the code to read an int. I could just duplicate the code for all other integral types I'd like to read, but I'd rather define one function template for all integral types. I've tried the following: struct Scanner { template <typename T> typename enable_if<boost::is_integral<T>, T>::type get(); }; Which works like a charm, but I am unsure how to get Scanner::get<string>() to function again. So, how can I write code so that I can do scanner.get<string>() and scanner.get<any integral type>() and have a single definition to read all integral types? Update: bonus question: What if I want to accept more than one range of classes based on some traits? For example: how should I approach this problem if I want to have three get functions that accept (i) integral types (ii) floating point types (iii) strings, respectively.

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• How string accepting interface should look like?

  - by ybungalobill

  Hello, This is a follow up of this question. Suppose I write a C++ interface that accepts or returns a const string. I can use a const char* zero-terminated string: void f(const char* str); // (1) The other way would be to use an std::string: void f(const string& str); // (2) It's also possible to write an overload and accept both: void f(const char* str); // (3) void f(const string& str); Or even a template in conjunction with boost string algorithms: template<class Range> void f(const Range& str); // (4) My thoughts are: (1) is not C++ish and may be less efficient when subsequent operations may need to know the string length. (2) is bad because now f("long very long C string"); invokes a construction of std::string which involves a heap allocation. If f uses that string just to pass it to some low-level interface that expects a C-string (like fopen) then it is just a waste of resources. (3) causes code duplication. Although one f can call the other depending on what is the most efficient implementation. However we can't overload based on return type, like in case of std::exception::what() that returns a const char*. (4) doesn't work with separate compilation and may cause even larger code bloat. Choosing between (1) and (2) based on what's needed by the implementation is, well, leaking an implementation detail to the interface. The question is: what is the preffered way? Is there any single guideline I can follow? What's your experience?

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• Is it possible to cache all the data in a SQL Server CE database using LinqToSql?

  - by DanM

  I'm using LinqToSql to query a small, simple SQL Server CE database. I've noticed that any operations involving sub-properties are disappointingly slow. For example, if I have a Customer table that is referenced by an Order table, LinqToSql will automatically create an EntitySet<Order> property. This is a nice convenience, allowing me to do things like Customer.Order.Where(o => o.ProductName = "Stopwatch"), but for some reason, SQL Server CE hangs up pretty bad when I try to do stuff like this. One of my queries, which isn't really that complicated takes 3-4 seconds to complete. I can get the speed up to acceptable, even fast, if I just grab the two tables individually and convert them to List<Customer> and List<Order>, then join then manually with my own query, but this is throwing out a lot of what makes LinqToSql so appealing. So, I'm wondering if I can somehow get the whole database into RAM and just query that way, then occasionally save it. Is this possible? How? If not, is there anything else I can do to boost the performance besides resorting to doing all the joins manually? Note: My database in its initial state is about 250K and I don't expect it to grow to more than 1-2Mb. So, loading the data into RAM certainly wouldn't be a problem from a memory point of view. Update Here are the table definitions for the example I used in my question: create table Order ( Id int identity(1, 1) primary key, ProductName ntext null ) create table Customer ( Id int identity(1, 1) primary key, OrderId int null references Order (Id) )

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• Build OpenGL model in parallel?

  - by Brendan Long

  I have a program which draws some terrain and simulates water flowing over it (in a cheap and easy way). Updating the water was easy to parallelize using OpenMP, so I can do ~50 updates per second. The problem is that even with a small amounts of water, my draws per second are very very low (starts at 5 and drops to around 2 once there's a significant amount of water). It's not a problem with the video card because the terrain is more complicated and gets drawn so quickly that boost::timer tells me that I get infinity draws per second if I turn the water off. It may be related to memory bandwidth though (since I assume the model stays on the card and doesn't have to be transfered every time). What I'm concerned about is that on every draw, I'm calling glVertex3f() about a million times (max size is 450*600, 4 vertices each), and it's done entirely sequentially because Glut won't let me call anything in parallel. So.. is if there's some way of building the list in parallel and then passing it to OpenGL all at once? Or some other way of making it draw this faster? Am I using the wrong method (besides the obvious "use less vertices")?

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