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• Newbie question: When to use extern "C" { //code } ?

  - by Russel

  Hello, Maybe I'm not understanding the differences between C and C++, but when and why do we need to use: extern "C" { ? Apparently its a "linkage convention"? I read about it briefly and noticed that all the .h header files included with MSVS surround their code with it. What type of code exactly is "C code" and NOT "C++ code"? I thought C++ included all C code? I'm guessing that this is not the case and that C++ is different and that standard features/functions exist in one or the other but not both (ie: printf is C and cout is C++), but that C++ is backwards compatible though the extern "C" declaration. Is this correct? My next question depends on the answer to the first, but I'll ask it here anyway: Since MSVS header files that are written in C are surrounded by extern "C" { ... }, when would you ever need to use this yourself in your own code? If your code is C code and you are trying to compile it in a C++ compiler, shouldn't it work without problem because all the standard h files you include will already have the extern "C" thing in them with the C++ compiler? Do you have to use this when compiling in C++ but linking to alteady built C libraries or something? Please help clarify this for me... Thanks! --Keith

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• Question about memory allocation when initializing char arrays in C/C++.

  - by Carlos Nunez

  Before anything, I apologize if this question has been asked before. I am programming a simple packet sniffer for a class project. For a little while, I ran into the issue where the source and destination of a packet appeared to be the same. For example, the source and destination of an Ethernet frame would be the same MAC address all of the time. I custom-made ether_ntoa(char *) because Windows does not seem to have ethernet.h like Linux does. Code snippet is below: char *ether_ntoa(u_char etheraddr[ETHER_ADDR_LEN]) { int i, j; char eout[32]; for(i = 0, j = 0; i < 5; i++) { eout[j++] = etheraddr[i] >> 4; eout[j++] = etheraddr[i] & 0xF; eout[j++] = ':'; } eout[j++] = etheraddr[i] >> 4; eout[j++] = etheraddr[i] & 0xF; eout[j++] = '\0'; for(i = 0; i < 17; i++) { if(eout[i] < 10) eout[i] += 0x30; else if(eout[i] < 16) eout[i] += 0x57; } return(eout); } I solved the problem by using malloc() to have the compiler assign memory (i.e. instead of char eout[32], I used char * eout; eout = (char *) malloc (32);). However, I thought that the compiler assigned different memory locations when one sized a char-array at compile time. Is this incorrect? Thanks! Carlos Nunez

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• How am I able to create A List<T> containing a generic Interface?

  - by Conrad Clark

  I have a List which must contain IInteract Objects. But IInteract is a generic interface which requires 2 type arguments. My main idea is iterate through a list of Objects and "Interact" one with another if they didn't interact yet. So i have this object List<IObject> WorldObjects = new List<IObject>(); and this one: private List<IInteract> = new List<IInteract>(); Except I can't compile the last line because IInteract requires 2 type arguments. But I don't know what the arguments are until I add them. I could add interactions between Objects of Type A and A... or Objects of Type B and C. I want to create "Interaction" classes which do something with the "acting" object and the "target" object, but I want them to be independent from the objects... so I could add an Interaction between for instance... "SuperUltraClass" and... an "integer". Am I using the wrong approach?

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• What happens when we combine RAII and GOTO ?

  - by Robert Gould

  I'm wondering, for no other purpose than pure curiosity (because no one SHOULD EVER write code like this!) about how the behavior of RAII meshes with the use of Goto (lovely idea isn't it). class Two { public: ~Two() { printf("2,"); } }; class Ghost { public: ~Ghost() { printf(" BOO! "); } }; void foo() { { Two t; printf("1,"); goto JUMP; } Ghost g; JUMP: printf("3"); } int main() { foo(); } When running the following code in VS2005 I get the following output: 1,2,3 BOO! However I imagined, guessed, hoped that 'BOO!' wouldn't actually appear as the Ghost should have never been instantiated (IMHO, because I don't know the actual expected behavior of this code). Any Guru out there knows what's up? Just realized that if I instantiate an explicit constructor for Ghost the code doesn't compile... class Ghost { public: Ghost() { printf(" HAHAHA! "); } ~Ghost() { printf(" BOO! "); } }; Ah, the mystery ...

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• Windows Service Installation

  - by Goober

  Scenario I have a server, that has NO Visual Studio Installed. It literally has a normal command prompt and nothing installed yet. We don't want to install anything (except the .Net framework which we have already done). We just want to install a bunch of C# Windows Services that we have written. So far I have been installing and running the windows service on my local machine using a "setup and deploy" project that I built into the application, which I could then use to install the service locally. Question How can I install the service on the server? I imagine it can be done from the command prompt only, but what else do I need? - If anything? and where do I put the files that I want to install BEFORE I install them? I imagine I will have to compile the application on my local machine in Visual Studio, then copy it over to the server, and then run an install utility to install it on the server? Any help would be greatly appreciated.

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• Test Driven Development with C++: How to test a class which depends on other classes?

  - by Nikhil

  Suppose I have a class A which depends on 3 other classes X, Y and Z, either A uses these through a reference or a pointer or say A is templated to be instantiated with X, Y and Z doesn't matter, the key is that in order to test A, I need to have X, Y and Z. So I need to have fakes for A, B and C. Suppose I write them. Now, how do I swap real and fake objects easily? I can see that this works very easily in the case of templates. In order to make it work when A depends on X, Y and Z through a reference or a pointer, I would need to have a base class say X_Interface from which I can inherit X_Real and X_Fake. So basically, I would end up in having 3 times the number of classes for every class that would need to have a fake. I am most likely missing something. There has to be a simpler way to do this. Having a base class X_Interface is also quite expensive as I will be using more space and making virtual calls. I guess I could use CRTP as I know whether its a X_Real or X_Fake at compile time but still there must be a better way.

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• Compiling code at runtime, loading into current appdomain.

  - by Richard Friend

  Hi Im compiling some code at runtime then loading the assembly into the current appdomain, however when i then try to do Type.GetType it cant find the type... Here is how i compile the code... public static Assembly CompileCode(string code) { Microsoft.CSharp.CSharpCodeProvider provider = new CSharpCodeProvider(); ICodeCompiler compiler = provider.CreateCompiler(); CompilerParameters compilerparams = new CompilerParameters(); compilerparams.GenerateExecutable = false; compilerparams.GenerateInMemory = false; foreach (Assembly assembly in AppDomain.CurrentDomain.GetAssemblies()) { try { string location = assembly.Location; if (!String.IsNullOrEmpty(location)) { compilerparams.ReferencedAssemblies.Add(location); } } catch (NotSupportedException) { // this happens for dynamic assemblies, so just ignore it. } } CompilerResults results = compiler.CompileAssemblyFromSource(compilerparams, code); if (results.Errors.HasErrors) { StringBuilder errors = new StringBuilder("Compiler Errors :\r\n"); foreach (CompilerError error in results.Errors) { errors.AppendFormat("Line {0},{1}\t: {2}\n", error.Line, error.Column, error.ErrorText); } throw new Exception(errors.ToString()); } else { AppDomain.CurrentDomain.Load(results.CompiledAssembly.GetName()); return results.CompiledAssembly; } } This bit fails after getting the type from the compiled assembly just fine, it does not seem to be able to find it using Type.GetType.... Assembly assem = RuntimeCodeCompiler.CompileCode(code); string typeName = String.Format("Peverel.AppFramework.Web.GenCode.ObjectDataSourceProxy_{0}", safeTypeName); Type t = assem.GetType(typeName); //This works just fine.. Type doesntWork = Type.GetType(t.AssemblyQualifiedName); Type doesntWork2 = Type.GetType(t.Name); ....

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• What limits scaling in this simple OpenMP program?

  - by Douglas B. Staple

  I'm trying to understand limits to parallelization on a 48-core system (4xAMD Opteron 6348, 2.8 Ghz, 12 cores per CPU). I wrote this tiny OpenMP code to test the speedup in what I thought would be the best possible situation (the task is embarrassingly parallel): // Compile with: gcc scaling.c -std=c99 -fopenmp -O3 #include <stdio.h> #include <stdint.h> int main(){ const uint64_t umin=1; const uint64_t umax=10000000000LL; double sum=0.; #pragma omp parallel for reduction(+:sum) for(uint64_t u=umin; u<umax; u++) sum+=1./u/u; printf("%e\n", sum); } I was surprised to find that the scaling is highly nonlinear. It takes about 2.9s for the code to run with 48 threads, 3.1s with 36 threads, 3.7s with 24 threads, 4.9s with 12 threads, and 57s for the code to run with 1 thread. Unfortunately I have to say that there is one process running on the computer using 100% of one core, so that might be affecting it. It's not my process, so I can't end it to test the difference, but somehow I doubt that's making the difference between a 19~20x speedup and the ideal 48x speedup. To make sure it wasn't an OpenMP issue, I ran two copies of the program at the same time with 24 threads each (one with umin=1, umax=5000000000, and the other with umin=5000000000, umax=10000000000). In that case both copies of the program finish after 2.9s, so it's exactly the same as running 48 threads with a single instance of the program. What's preventing linear scaling with this simple program?

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• Having some fun - what is a good way to include a secret key functionality and fire the KeyDown event?

  - by Sisyphus

  To keep myself interested, I try to put little Easter Eggs in my projects (mostly to amuse myself). I've seen some websites where you can type a series of letters "aswzaswz" and you get a "secret function" - how would I achieve this in C#? I've assigned a "secret function" in the past by using modifier keys bool showFunThing = (Control.ModifierKeys & Keys.Control) == Keys.Control; but wanted to get a bit more secretive (without the modifier keys) I just wanted the form to detect a certain word typed without any input ... I've built a method that I think should do it: private StringBuilder _pressedKeys = new StringBuilder(); protected override void OnKeyDown(KeyEventArgs e) { const string kWord = "fun"; char letter = (char)e.KeyValue; if (!char.IsLetterOrDigit(letter)) { return; } _pressedKeys.Append(letter); if (_pressedKeys.Length == kWord.Length) { if (_pressedKeys.ToString().ToLower() == kWord) { MessageBox.Show("Fun"); _pressedKeys.Clear(); } } base.OnKeyDown(e); } Now I need to wire it up but I can't figure out how I'm supposed to raise the event in the form designer ... I've tried this: this.KeyDown +=new System.Windows.Forms.KeyEventHandler(OnKeyDown); and a couple of variations on this but I'm missing something because it won't fire (or compile). It tells me that the OnKeyDown method is expecting a certain signature but I've got other methods like this where I haven't specified arguments. I fear that I may have got myself confused so I am turning to SO for help ... anyone?

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• Getting error when compiling debug mode: C++/CLI - error LNK2022

  - by Yochai Timmer

  I've got a CLI code wrapping a C++ DLL. When i try to compile it in debug mode, i get the following error: Error 22 error LNK2022: metadata operation failed (8013118D) : Inconsistent layout information induplicated types .... MSVCMRTD.lib (locale0_implib.obj) The weird thing is that on Release mode it compiles OK and works OK. The only difference i can see that causes the problem is when i change: Configuration Properties - C/C++ - Code Generation - Runtime Library When it's set to: Multi-threaded Debug DLL (/MDd) it throws the error. When it's set to: Multi-threaded DLL (/MD) it compiles fine. The same settings work for all the other DLLs in the project (CLI and C++) and they inherit the same properties. I'm using VS2010. So, how can i solve this ? And can I get some explanation to WHY this is happening ? Update: I've basically tried changing every option in the project's properties with no luck. I've read somewhere that this might be caused from duplicate declarations of a type of the same name. But in the CLI file i'm calling std::string etc. explicitly from std. Any other ideas ?

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• Boost link error when using "--layout=system" on VS2005

  - by Kevin

  I'm new to boost, and thought I'd try it out with some realistic deployment scenarios for the .dlls, so I used the following command to compile/install the libraries: .\bjam install --layout=system variant=debug runtime-link=shared link=shared --with-date_time --with-thread --with-regex --with-filesystem --includedir=<my include directory> --libdir=<my bin directory> > installlog.txt That seemed to work, but my simple program (taken right from the "Getting Started" page) fails: #include <boost/regex.hpp> #include <iostream> #include <string> // Place your functions after this line int main() { std::string line; boost::regex pat( "^Subject: (Re: |Aw: )*(.*)" ); while (std::cin) { std::getline(std::cin, line); boost::smatch matches; if (boost::regex_match(line, matches, pat)) std::cout << matches[2] << std::endl; } } This fails with the following linker error: fatal error LNK1104: cannot open file 'libboost_regex-vc80-mt-1_42.lib' I'm sure that both the .lib and the .dlls are in that directory, and named how I want them to be (ie: boost_regex.lib, etc, all unversioned, as the --layout=system says). So why is it looking for the versioned type of it? And how do I get it to look for the unversioned type of the library? I've tried this with more "normal" options, such as below: .\bjam stage --build-type=complete --with-date_time --with-thread --with-filesystem --with-regex > mybuildlog.txt And that works fine. I made sure my compiler saw the "stage\lib" directory, and it compiled and ran fine with nothing beyond having the environment looking into the right lib directory. But when I took those "testing" directories away, and wanted to use these others (unversioned), then it failed. I'm under VS2005 here on XP. Any ideas?

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• Excel 2010 64 bit can't create .net object

  - by aboes81

  I have a simple class library that I use in Excel. Here is a simplification of my class... using System; using System.Runtime.InteropServices; namespace SimpleLibrary { [ComVisible(true)] public interface ISixGenerator { int Six(); } public class SixGenerator : ISixGenerator { public int Six() { return 6; } } } In Excel 2007 I would create a macro enabled workbook and add a module with the following code: Public Function GetSix() Dim lib As SimpleLibrary.SixGenerator lib = New SimpleLibrary.SixGenerator Six = lib.Six End Function Then in Excel I could call the function GetSix() and it would return six. This no longer works in Excel 2010 64bit. I get a Run-time error '429': ActiveX component can't create object. I tried changing the platform target to x64 instead of Any CPU but then my code wouldn't compile unless I unchecked the Register for COM interop option, doing so makes it so my macro enable workbook cannot see SimpleLibrary.dll as it is no longer regsitered. Any ideas how I can use my library with Excel 2010 64 bit?

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• Dynamically invoke web service at runtime

  - by Ulrik Rasmussen

  So, our application needs support for dynamically calling web services which are unknown at compile time. The user should therefore be able to specify a URL to a WSDL, and specify some data bindings for the request and reply parameters. When Googling for answers, it seems like the way to do this is by actually compiling a web service proxy class at runtime, loading it, and invoking the methods using reflection. I think this seems like a rather clunky approach, given that I don't really need a strongly typed set of classes when I'm going to cast my data dynamically anyway. Dynamically compiling code for doing something that simple also just seems like The Wrong Way To Do It. Restricting ourself to the SOAP protocol, is there any library for C# that implements this protocol for dynamic use? I can imagine that it would be possible to generate runtime key/value data structures from the WSDL, which could be used to specify the request messages, as well as reading the replies. The library should then be able to send well-formed SOAP messages to the server, and parse the replies, without the programmer having to generate the XML manually (at least not the headers and other plumbing). I can't seem to find any library that actually does this. Is what I want to do really that esoteric, or have I just searched the wrong places? Thanks, Ulrik

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• LNK 1104 error to lib file - Continues despite removing includes and links

  - by user1556594

  A link error to a lib file popped up out of the blue in a c++ application of mine after code was working fine in my last session. Error 1 error LNK1104: cannot open file '..........\Program Files (x86)\FMOD SoundSystem\FMOD Programmers API Windows\api\lib\fmodex_vc.lib' I triple checked my project directories were set up correctly to link to the lib file, that the file existed in said directory and that it was a working version of the .lib. My next step was to remove the includes to the file and the links to bypass the error and work on the rest of my code until the problem was solved. The error remains, however, despite: Commenting out absolutely every include relating to the lib. Commenting out absolutely every line of code dependant on the includes. Removing the directory from VC++ Directories in the project properties. Checking the Additional Library Directories field was also clear of references. To my understanding this should have made the library and related code virtually non-existant to the compiler. What am I missing? The library itself is fmodex_vc.lib - part of the FMOD API for providing sound to interactive applications. Again, the application was working one session, but failed to compile the next. I hadn't touched the code since so this led me to believe some aspect of VS is at fault. I'd like to avoid the time involded in re-installing if possible as I'm on the clock for a review tomorrow evening and there are a few more things I'd like to smooth out before then. If necessary, however, I won't hesitate. Very much appreciate the help.

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• What would be different in Java if Enum declaration didn't have the recursive part

  - by atamur

  Please see http://stackoverflow.com/questions/211143/java-enum-definition and http://stackoverflow.com/questions/3061759/why-in-java-enum-is-declared-as-enume-extends-enume for general discussion. Here I would like to learn what exactly would be broken (not typesafe anymore, or requiring additional casts etc) if Enum class was defined as public class Enum<E extends Enum> I'm using this code for testing my ideas: interface MyComparable<T> { int myCompare(T o); } class MyEnum<E extends MyEnum> implements MyComparable<E> { public int myCompare(E o) { return -1; } } class FirstEnum extends MyEnum<FirstEnum> {} class SecondEnum extends MyEnum<SecondEnum> {} With it I wasn't able to find any benefits in this exact case. PS. the fact that I'm not allowed to do class ThirdEnum extends MyEnum<SecondEnum> {} when MyEnum is defined with recursion is a) not relevant, because with real enums you are not allowed to do that just because you can't extend enum yourself b) not true - pls try it in a compiler and see that it in fact is able to compile w/o any errors PPS. I'm more and more inclined to believe that the correct answer here would be "nothing would change if you remove the recursive part" - but I just can't believe that.

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• Why is the compiler caching my "random" and NULLED variables?

  - by alex gray

  I am confounded by the fact that even using different programs (on the same machine) to run /compile, and after nilling the vaues (before and after) the function.. that NO MATTER WHAT.. I'll keep getting the SAME "random" numbers… each and every time I run it. I swear this is NOT how it's supposed to work.. I'm going to illustrate as simply as is possible… #import <Foundation/Foundation.h> int main(int argc, char *argv[]) { int rPrimitive = 0; rPrimitive = 1 + rand() % 50; NSNumber *rObject = nil; rObject = [NSNumber numberWithInt:rand() % 10]; NSLog(@"%i %@", rPrimitive, rObject); rPrimitive = 0; rObject = nil; NSLog(@"%i %@", rPrimitive, rObject); return 0; } Run it in TextMate: i686-apple-darwin11-llvm-gcc-4.2 8 9 0 (null) Run it in CodeRunner: i686-apple-darwin11-llvm-gcc-4.2 8 9 0 (null) Run it a million times, if you'd like. You can gues what it will always be. Why does this happen? Why oh why is this "how it is"?

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• GNU Make - Dependencies on non program code

  - by Tim Post

  A requirement for a program I am writing is that it must be able to trust a configuration file. To accomplish this, I am using several kinds of hashing algorithms to generate a hash of the file at compile time, this produces a header with the hashes as constants. Dependencies for this are pretty straight forward, my program depends on config_hash.h, which has a target that produces it. The makefile looks something like this : config_hash.h: $(SH) genhash config/config_file.cfg > $(srcdir)/config_hash.h $(PROGRAM): config_hash.h $(PROGRAM_DEPS) $(CC) ... ... ... I'm using the -M option to gcc, which is great for dealing with dependencies. If my header changes, my program is rebuilt. My problem is, I need to be able to tell if the config file has changed, so that config_hash.h is re-generated. I'm not quite sure how explain that kind of dependency to GNU make. I've tried listing config/config_file.cfg as a dependency for config_hash.h, and providing a .PHONY target for config_file.cfg without success. Obviously, I can't rely on the -M switch to gcc to help me here, since the config file is not a part of any object code. Any suggestions? Unfortunately, I can't post much of the Makefile, or I would have just posted the whole thing.

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• CGContextDrawImage returning bad access

  - by Marcelo

  Hello guys, I've been trying to blend two UIImage for about 2 days now and I've been getting some BAD_ACCESS errors. First of all, I have two images that have the same orientation, basically I'm using the CoreGraphics to do the blending. One curious detail, everytime I modify the code, the first time I compile and run it on device, I get to do everything I want without any sort of trouble. Once I restart the application, I get error and the program shuts down. Can anyone give me a light? I tried accessing the baseImage sizes dynamically, but it gives me bad access too. Here's a snippet of how I'm doing the blending. UIGraphicsBeginImageContext(CGSizeMake(320, 480)); CGContextRef context = UIGraphicsGetCurrentContext(); CGContextTranslateCTM(context, 0, 480); CGContextScaleCTM(context, 1.0, -1.0); CGContextDrawImage(context, rect, [baseImage CGImage]); CGContextSetBlendMode(context, kCGBlendModeOverlay); CGContextDrawImage(context, rect, [tmpImage CGImage]); [transformationView setImage:UIGraphicsGetImageFromCurrentImageContext()]; UIGraphicsEndImageContext();

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• Invoking a function (main()) from a binary file in C

  - by Dhara Darji

  I have simple c program like, my_bin.c: #include <stdio.h> int main() { printf("Success!\n"); return 0; } I compile it with gcc and got executable: my_bin. Now I want to invoke main (or run this my_bin) using another C program. That I did with mmap and function pointer like this: #include <stdio.h> #include <fcntl.h> #include <sys/mman.h> int main() { void (*fun)(); int fd; int *map; fd = open("./my_bin", O_RDONLY); map = mmap(0, 8378, PROT_READ, MAP_SHARED, fd, 0); fun = map; fun(); return 0; } PS: I went through some tutorial, for how to read binary file and execute. But this gives Seg fault, any help appreciated! Thanks!

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• How can I implement an interface member in protected ?

  - by Nicolas Dorier

  Hi, I've been quite surprise when I saw the metadata of ReadOnlyObservableCollection in VS 2008... public class ReadOnlyObservableCollection<T> : ReadOnlyCollection<T>, INotifyCollectionChanged, INotifyPropertyChanged { // Summary: // Initializes a new instance of the System.Collections.ObjectModel.ReadOnlyObservableCollection<T> // class that serves as a wrapper for the specified System.Collections.ObjectModel.ObservableCollection<T>. // // Parameters: // list: // The collection to wrap. public ReadOnlyObservableCollection(ObservableCollection<T> list); // Summary: // Occurs when an item is added or removed. protected virtual event NotifyCollectionChangedEventHandler CollectionChanged; // // Summary: // Occurs when a property value changes. protected virtual event PropertyChangedEventHandler PropertyChanged; // Summary: // Raises the System.Collections.ObjectModel.ReadOnlyObservableCollection<T>.CollectionChanged // event. // // Parameters: // args: // The event data. protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs args); // // Summary: // Raises the System.Collections.ObjectModel.ReadOnlyObservableCollection<T>.PropertyChanged // event. // // Parameters: // args: // The event data. protected virtual void OnPropertyChanged(PropertyChangedEventArgs args); } As you can see, CollectionChanged, a member of INotifyCollectionChanged is implemented in protected... and I can't do that in my own class. .NET framework should not compile ! Does someone has an explanation of this mystery ?

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• Accept templated parameter of stl_container_type<string>::iterator

  - by Rodion Ingles

  I have a function where I have a container which holds strings (eg vector<string>, set<string>, list<string>) and, given a start iterator and an end iterator, go through the iterator range processing the strings. Currently the function is declared like this: template< typename ContainerIter> void ProcessStrings(ContainerIter begin, ContainerIter end); Now this will accept any type which conforms to the implicit interface of implementing operator*, prefix operator++ and whatever other calls are in the function body. What I really want to do is have a definition like the one below which explicitly restricts the amount of input (pseudocode warning): template< typename Container<string>::iterator> void ProcessStrings(Container<string>::iterator begin, Container<string>::iterator end); so that I can use it as such: vector<string> str_vec; list<string> str_list; set<SomeOtherClass> so_set; ProcessStrings(str_vec.begin(), str_vec.end()); // OK ProcessStrings(str_list.begin(), str_list.end()); //OK ProcessStrings(so_set.begin(), so_set.end()); // Error Essentially, what I am trying to do is restrict the function specification to make it obvious to a user of the function what it accepts and if the code fails to compile they get a message that they are using the wrong parameter types rather than something in the function body that XXX function could not be found for XXX class.

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• Dynamic loading a class in java with a different package name

  - by C. Ross

  Is it possible to load a class in Java and 'fake' the package name/canonical name of a class? I tried doing this, the obvious way, but I get a "class name doesn't match" message in a ClassDefNotFoundException. The reason I'm doing this is I'm trying to load an API that was written in the default package so that I can use it directly without using reflection. The code will compile against the class in a folder structure representing the package and a package name import. ie: ./com/DefaultPackageClass.class // ... import com.DefaultPackageClass; import java.util.Vector; // ... My current code is as follows: public Class loadClass(String name) throws ClassNotFoundException { if(!CLASS_NAME.equals(name)) return super.loadClass(name); try { URL myUrl = new URL(fileUrl); URLConnection connection = myUrl.openConnection(); InputStream input = connection.getInputStream(); ByteArrayOutputStream buffer = new ByteArrayOutputStream(); int data = input.read(); while(data != -1){ buffer.write(data); data = input.read(); } input.close(); byte[] classData = buffer.toByteArray(); return defineClass(CLASS_NAME, classData, 0, classData.length); } catch (MalformedURLException e) { throw new UndeclaredThrowableException(e); } catch (IOException e) { throw new UndeclaredThrowableException(e); } }

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• Where do I put javaassist code?

  - by DutrowLLC

  I have an application running on google app engine. I'm using restlets and I have a couple of layers set up including the restlet layer, the model layer, the business layer, and the data layer. I'm attempting to use javaassist to modify some classes, but I'm unsure where to actually put the code. I tried to put the code in the static initialization block: public class Person { String firstName; String getFirstName(){return null;} static{ ClassPool pool = ClassPool.getDefault(); try { CtClass CtPerson = pool.get("Person"); CtMethod CtGetFirstName = CtPerson.getDeclaredMethod("GetFirstName"); CtGetFirstName.setBody("return firstName;"); CtPerson.toClass(); } catch (Exception e) { e.printStackTrace(); } } } ...but that resulted in this error: javassist.CannotCompileException:.....attempted duplicate class definition...". I guess it makes sense that I can't edit the class file in the middle of its generation. I know the code works because I was able to run it correctly by simply putting it in a location that would run when I sent the program a command. (accessed a Restlet resource). The code ran fine if an instance of the class had not already been instantiated, however once I instantiated an instance of the affected class, the javaassist code failed. I assume I need to put this code somewhere that it will only run either: once after the program starts, directly before a class is instantiated for the first time, or even better, during compile time.

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• User defined literal arguments are not constexpr?

  - by Pubby

  I'm testing out user defined literals. I want to make _fac return the factorial of the number. Having it call a constexpr function works, however it doesn't let me do it with templates as the compiler complains that the arguments are not and cannot be constexpr. I'm confused by this - aren't literals constant expressions? The 5 in 5_fac is always a literal that can be evaluated during compile time, so why can't I use it as such? First method: constexpr int factorial_function(int x) { return (x > 0) ? x * factorial_function(x - 1) : 1; } constexpr int operator "" _fac(unsigned long long x) { return factorial_function(x); // this works } Second method: template <int N> struct factorial { static const unsigned int value = N * factorial<N - 1>::value; }; template <> struct factorial<0> { static const unsigned int value = 1; }; constexpr int operator "" _fac(unsigned long long x) { return factorial_template<x>::value; // doesn't work - x is not a constexpr }

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• Does cout need to be terminated with a semicolon ?

  - by Philippe Harewood

  I am reading Bjarne Stroustrup's Programming : Principles and Practice Using C++ In the drill section for Chapter 2 it talks about various ways to look at typing errors when compiling the hello_world program #include "std_lib_facilities.h" int main() //C++ programs start by executing the function main { cout << "Hello, World!\n", // output "Hello, World!" keep_window_open(); // wait for a character to be entered return 0; } In particular this section asks: Think of at least five more errors you might have made typing in your program (e.g. forget keep_window_open(), leave the Caps Lock key on while typing a word, or type a comma instead of a semicolon) and try each to see what happens when you try to compile and run those versions. For the cout line, you can see that there is a comma instead of a semicolon. This compiles and runs (for me). Is it making an assumption ( like in the javascript question: Why use semicolon? ) that the statement has been terminated ? Because when I try for keep_terminal_open(); the compiler informs me of the semicolon exclusion.

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