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• C#: at design time, how can I reliably determine the type of a variable that is declared using var?

  - by Cheeso

  I'm working on a completion (intellisense) facility for C# in emacs. The idea is, if a user types a fragment, then asks for completion via a particular keystroke combination, the completion facility will use .NET reflection to determine the possible completions. Doing this requires that the type of the thing being completed, be known. If it's a string, it has a set of known methods; if it's an Int32, it has a separate set of methods, and so on. Using semantic, a code lexer/parser package available in emacs, I can locate the variable declarations, and their types. Given that, it's straightforward to use reflection to get the methods and properties on the type, and then present the list of options to the user. The problem arrives when the code uses var in the declaration. How can I reliably determine the actual type used, when the variable is declared with the var keyword? Just to be clear, I don't need to determine it at runtime. I want to determine it at "Design time". So far the best idea I have is: extract the declaration statement, eg var foo = "a string value"; concatenate a statement foo.GetType(); dynamically compile the resulting C# fragment it into a new assembly load the assembly into a new AppDomain, run the framgment and get the return type. unload and discard the assembly This sounds awfully heavyweight, for each completion request in the editor. Any better ideas out there?

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• Porting Perl to C++ `print "\x{2501}" x 12;`

  - by jippie

  I am porting a program from Perl to C++ as a learning objective. I arrived at a routine that draws a table with commands like the following: Perl: print "\x{2501}" x 12; And it draws 12 times a '?' ("box drawings heavy horizontal"). Now I figured out part of the problem already: Perl: \x{}, \x00 Hexadecimal escape sequence; C++: \unnnn To print a single Unicode character: C++: printf( "\u250f\n" ); But does C++ have a smart equivalent for the 'x' operator or would it come down to a for loop? UPDATE Let me include the full source code I am trying to compile with the proposed solution. The compiler does throw an errors: g++ -Wall -Werror project.cpp -o project project.cpp: In function ‘int main(int, char**)’: project.cpp:38:3: error: ‘string’ is not a member of ‘std’ project.cpp:38:15: error: expected ‘;’ before ‘s’ project.cpp:39:3: error: ‘cout’ is not a member of ‘std’ project.cpp:39:16: error: ‘s’ was not declared in this scope #include <stdlib.h> #include <stdint.h> #include <stdio.h> #include <string.h> int main ( int argc, char *argv[] ) { if ( argc != 2 ) { fprintf( stderr , "usage: %s matrix\n", argv[0] ); exit( 2 ); } else { //std::string s(12, "\u250f" ); std::string s(12, "u" ); std::cout << s; } }

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• I have a bunch of template parameters that I want to hide from my users. How can I do this?

  - by Alex

  I have a superclass which is defined in terms of a few internal types it uses. Subclassing is performed as so: template <class InternalType1, class InternalType2> class Super { ... } class Sub : Super <interalTypeClass1, interalTypeClass2> { ... } But when I want to write a function that takes a pointer to the superclass, this happens : template <class InternalType1, class InternalType2> void function(Super<InternalType1, InternalType2>* in) { ... } The user really shouldn't know anything about the inside classes, and should really just concern himself with the use of the function. Some of these template lists become very very large, and expecting the user to pass them every time is wasteful, in my opinion. Any suggestions? EDIT: The function needs to know the internal types in use, so unless there is a way to access template types at compile time, I think there is no solution? Potential solution: Have each class do the following: #define SubTemplateArgs <SubTypeName, SubInternalType1, SubInternalType2> ?

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• Why it's can be compiled in GNU/C++, can't compiled in VC++2010 RTM?

  - by volnet

  #include #include #include #include "copy_of_auto_ptr.h" #ifdef _MSC_VER #pragma message("#include ") #include // http://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html#Diagnostic-Pragmas #endif /* case 1-4 is the requirement of the auto_ptr. which form http://ptgmedia.pearsoncmg.com/images/020163371X/autoptrupdate/auto_ptr_update.html */ /* case 1. (1) Direct-initialization, same type, e.g. */ std::auto_ptr source_int() { // return std::auto_ptr(new int(3)); std::auto_ptr tmp(new int(3)); return tmp; } /* case 2. (2) Copy-initialization, same type, e.g. */ void sink_int(std::auto_ptr p) { std::cout source_derived() { // return std::auto_ptr(new Derived()); std::auto_ptr tmp(new Derived()); return tmp; } /* case 4. (4) Copy-initialization, base-from-derived, e.g. */ void sink_base( std::auto_ptr p) { p-go(); } int main(void) { /* // auto_ptr */ // case 1. // auto_ptr std::auto_ptr p_int(source_int()); std::cout p_derived(source_derived()); p_derived-go(); // case 4. // auto_ptr sink_base(source_derived()); return 0; } In Eclipse(GNU C++.exe -v gcc version 3.4.5 (mingw-vista special r3)) it's two compile error: Description Resource Path Location Type initializing argument 1 of void sink_base(std::auto_ptr<Base>)' from result ofstd::auto_ptr<_Tp::operator std::auto_ptr<_Tp1() [with _Tp1 = Base, _Tp = Derived]' auto_ptr_ref_research.cpp auto_ptr_ref_research/auto_ptr_ref_research 190 C/C++ Problem Description Resource Path Location Type no matching function for call to `std::auto_ptr::auto_ptr(std::auto_ptr)' auto_ptr_ref_research.cpp auto_ptr_ref_research/auto_ptr_ref_research 190 C/C++ Problem But it's right in VS2010 RTM. Questions: Which compiler stand for the ISO C++ standard? The content of case 4 is the problem "auto_ptr & auto_ptr_ref want to resolve?"

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• Can the size of a structure change after compiled?

  - by Sarah Altiva

  Hi, suppose you have the following structure: #include <windows.h> // BOOL is here. #include <stdio.h> typedef struct { BOOL someBool; char someCharArray[100]; int someIntValue; BOOL moreBools, anotherOne, yetAgain; char someOthercharArray[23]; int otherInt; } Test; int main(void) { printf("Structure size: %d, BOOL size: %d.\n", sizeof(Test), sizeof(BOOL)); } When I compile this piece of code in my machine (32-bit OS) the output is the following: Structure size: 148, BOOL size: 4. I would like to know if, once compiled, these values may change depending on the machine which runs the program. E.g.: if I ran this program in a 64-bit machine, would the output be the same? Or once it's compiled it'll always be the same? Thank you very much, and forgive me if the answer to this question is obvious...

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• C++ Operator Ambiguity

  - by Scott

  Forgive me, for I am fairly new to C++, but I am having some trouble regarding operator ambiguity. I think it is compiler-specific, for the code compiled on my desktop. However, it fails to compile on my laptop. I think I know what's going wrong, but I don't see an elegant way around it. Please let me know if I am making an obvious mistake. Anyhow, here's what I'm trying to do: I have made my own vector class called Vector4 which looks something like this: class Vector4 { private: GLfloat vector[4]; ... } Then I have these operators, which are causing the problem: operator GLfloat* () { return vector; } operator const GLfloat* () const { return vector; } GLfloat& operator [] (const size_t i) { return vector[i]; } const GLfloat& operator [] (const size_t i) const { return vector[i]; } I have the conversion operator so that I can pass an instance of my Vector4 class to glVertex3fv, and I have subscripting for obvious reasons. However, calls that involve subscripting the Vector4 become ambiguous to the compiler: enum {x, y, z, w} Vector4 v(1.0, 2.0, 3.0, 4.0); glTranslatef(v[x], v[y], v[z]); Here are the candidates: candidate 1: const GLfloat& Vector4:: operator[](size_t) const candidate 2: operator[](const GLfloat*, int) <built-in> Why would it try to convert my Vector4 to a GLfloat* first when the subscript operator is already defined on Vector4? Is there a simple way around this that doesn't involve typecasting? Am I just making a silly mistake? Thanks for any help in advance.

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• Is there any need for me to use wstring in the following case

  - by Yan Cheng CHEOK

  Currently, I am developing an app for a China customer. China customer are mostly switch to GB2312 language in their OS encoding. I need to write a text file, which will be encoded using GB2312. I use std::ofstream file I compile my application under MBCS mode, not unicode. I use the following code, to convert CString to std::string, and write it to file using ofstream std::string Utils::ToString(CString& cString) { /* Will not work correctly, if we are compiled under unicode mode. */ return (LPCTSTR)cString; } To my surprise. It just works. I thought I need to at least make use of wstring. I try to do some investigation. Here is the MBCS.txt generated. I try to print a single character named ? (its value is 0xBDC5) When I use CString to carry this character, its length is 2. When I use Utils::ToString to perform conversion to std::string, the returned string length is 2. I write to file using std::ofstream My question is : When I exam MBCS.txt using a hex editor, the value is displayed as BD (LSB) and C5 (MSB). But I am using little endian machine. Isn't hex editor should show me C5 (LSB) and BD (MSB)? I check from wikipedia. GB2312 seems doesn't specific endianness. It seems that using std::string + CString just work fine for my case. May I know in what case, the above methodology will not work? and when I should start to use wstring?

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• Nested bind expressions

  - by user328543

  This is a followup question to my previous question. #include <functional> int foo(void) {return 2;} class bar { public: int operator() (void) {return 3;}; int something(int a) {return a;}; }; template <class C> auto func(C&& c) -> decltype(c()) { return c(); } template <class C> int doit(C&& c) { return c();} template <class C> void func_wrapper(C&& c) { func( std::bind(doit<C>, std::forward<C>(c)) ); } int main(int argc, char* argv[]) { // call with a function pointer func(foo); func_wrapper(foo); // error // call with a member function bar b; func(b); func_wrapper(b); // call with a bind expression func(std::bind(&bar::something, b, 42)); func_wrapper(std::bind(&bar::something, b, 42)); // error // call with a lambda expression func( [](void)->int {return 42;} ); func_wrapper( [](void)->int {return 42;} ); return 0; } I'm getting a compile errors deep in the C++ headers: functional:1137: error: invalid initialization of reference of type ‘int (&)()’ from expression of type ‘int (*)()’ functional:1137: error: conversion from ‘int’ to non-scalar type ‘std::_Bind(bar, int)’ requested func_wrapper(foo) is supposed to execute func(doit(foo)). In the real code it packages the function for a thread to execute. func would the function executed by the other thread, doit sits in between to check for unhandled exceptions and to clean up. But the additional bind in func_wrapper messes things up...

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• How to write curiously recurring templates with more than 2 layers of inheritance?

  - by Kyle

  All the material I've read on Curiously Recurring Template Pattern seems to one layer of inheritance, ie Base and Derived : Base<Derived>. What if I want to take it one step further? #include <iostream> using std::cout; template<typename LowestDerivedClass> class A { public: LowestDerivedClass& get() { return *static_cast<LowestDerivedClass*>(this); } void print() { cout << "A\n"; } }; template<typename LowestDerivedClass> class B : public A<LowestDerivedClass> { public: void print() { cout << "B\n"; } }; class C : public B<C> { public: void print() { cout << "C\n"; } }; int main() { C c; c.get().print(); // B b; // Intentionally bad syntax, // b.get().print(); // to demonstrate what I'm trying to accomplish return 0; } How can I rewrite this code to compile without errors (and output "C\nB\n")?

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• C++ Iterator Pipelining Designs

  - by Kirakun

  Suppose we want to apply a series of transformations, int f1(int), int f2(int), int f3(int), to a list of objects. A naive way would be SourceContainer source; TempContainer1 temp1; transform(source.begin(), source.end(), back_inserter(temp1), f1); TempContainer2 temp2; transform(temp1.begin(), temp1.end(), back_inserter(temp2), f2); TargetContainer target; transform(temp2.begin(), temp2.end(), back_inserter(target), f3); This first solution is not optimal because of the extra space requirement with temp1 and temp2. So, let's get smarter with this: int f123(int n) { return f3(f2(f1(n))); } ... SourceContainer source; TargetContainer target; transform(source.begin(), source.end(), back_inserter(target), f123); This second solution is much better because not only the code is simpler but more importantly there is less space requirement without the intermediate calculations. However, the composition f123 must be determined at compile time and thus is fixed at run time. How would I try to do this efficiently if the composition is to be determined at run time? For example, if this code was in a RPC service and the actual composition--which can be any permutation of f1, f2, and f3--is based on arguments from the RPC call.

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• Extract information from a Func<bool, T> or alike lambda

  - by Syska

  I''m trying to build a generic cache layer. ICacheRepository Say I have the following: public class Person { public int PersonId { get; set; } public string Firstname { get; set; } public string Lastname { get; set; } public DateTime Added { get; set; } } And I have something like this: list.Where(x => x.Firstname == "Syska"); Here I want to extract the above information, to see if the query supplied the "PersonId" which it did not, so I dont want to cache it. But lets say I run a query like this: list.Where(x => x.PersonId == 10); Since PersonId is my key ... I want to cache it. with the key like "Person_10" and I later can fetch it from the cache. I know its possible to extract the information with Expression<Func<>> but there seems to be a big overhead of doing this (when running compile and extract the Constant values etc. and a bunch of cache to be sure to parse right) Are there a framework for this? Or some smart/golden way of doing this ?

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• Class<T> and static method Class.forName() drive me crazy.

  - by matt

  Hi, this code doesn't compile. I'm wondering what I am doing wrong: private static Importable getRightInstance(String s) throws Exception { Class<Importable> c = Class.forName(s); Importable i = c.newInstance(); return i; } where Importable is an interface and the string s is the name of an implementing class. The compiler says: ./Importer.java:33: incompatible types found : java.lang.Class<capture#964 of ?> required: java.lang.Class<Importable> Class<Importable> c = Class.forName(format(s)); thanks for any help! All the solutions Class<? extends Importable> c = Class.forName(s).asSubclass(Importable.class); and Class<? extends Importable> c = (Class<? extends Importable>) Class.forName(s); and Class<?> c = Class.forName(format(s)); Importable i = (Importable)c.newInstance(); give this error: Exception in thread "main" java.lang.IncompatibleClassChangeError: class C1 has interface Importable as super class where C1 is effectively a class implementing Importable, one of those i want to cast to Importable.

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• Xcode Unit Testing - Accessing Resources from the application's bundle?

  - by Ben Scheirman

  I'm running into an issue and I wanted to confirm that I'm doing things the correct way. I can test simple things with my SenTestingKit tests, and that works okay. I've set up a Unit Test Bundle and set it as a dependency on the main application target. It successfully runs all tests whenever I press cmd+B. Here's where I'm running into issues. I have some XML files that I need to load from the resources folder as part of the application. Being a good unit tester, I want to write unit tests around this to make sure that they are loading properly. So I have some code that looks like this: NSString *filePath = [[NSBundle mainBundle] pathForResource:@"foo" ofType:@"xml"]; This works when the application runs, but during a unit test, mainBundle points to the wrong bundle, so this line of code returns nil. So I changed it up to utilize a known class like this: NSString *filePath = [[NSBundle bundleForClass:[Config class]] pathForResource:@"foo" ofType:@"xml"]; This doesn't work either, because in order for the test to even compile code like this, it Config needs to be part of the Unit Test Target. If I add that, then the bundle for that class becomes the Unit Test bundle. (Ugh!) Am I approaching this the wrong way?

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• Code Analysis Warning CA1004 with generic method

  - by Vaccano

  I have the following generic method: // Load an object from the disk public static T DeserializeObject<T>(String filename) where T : class { XmlSerializer xmlSerializer = new XmlSerializer(typeof(T)); try { TextReader textReader = new StreamReader(filename); var result = (T)xmlSerializer.Deserialize(textReader); textReader.Close(); return result; } catch (FileNotFoundException) { } return null; } When I compile I get the following warning: CA1004 : Microsoft.Design : Consider a design where 'MiscHelpers.DeserializeObject(string)' doesn't require explicit type parameter 'T' in any call to it. I have considered this and I don't know a way to do what it requests with out limiting the types that can be deserialized. I freely admit that I might be missing an easy way to fix this. But if I am not, then is my only recourse to suppress this warning? I have a clean project with no warnings or messages. I would like to keep it that way. I guess I am asking "why this is a warning?" At best this seems like it should be a message. And even that seems a bit much. Either it can or it can't be fixed. If it can't then you are just stuck with the warning with no recourse but suppressing it. Am I wrong?

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• Is app stability unusually hard with Java?

  - by wrp

  Java's extensive libraries and tool support are appealing, but I've never tried developing with it because most Java applications I've used have been extremely buggy. This has me puzzled, because I hear that Java is the dominant platform for enterprise development. Maybe it's fine for server-side stuff but not desktop applications. I'm not talking about things like the long compile/startup times or the random latencies due to garbage collection. This is about things just going wrong. Some of the most common problems I see are: corrupted icons corrupted fonts, in menus usually and editing areas sometimes inaccurate framing of GUI elements dialogs sometimes popping up blank Maybe the problems are mostly with Swing. I've rarely used a Java application long enough to find deeper issues. I can think of a few possible explanations for what I've experienced: It's possible to write stable apps with Java, just harder. Java apps are always buggy and enterprise users just put up with it. Server-side apps are fine because just Swing is buggy. I'm living under a curse and need the services of a good witchdoctor.

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• How good idea is it to use code contracts in Visual Studio 2010 Professional (ie. no static checking

  - by Lasse V. Karlsen

  I create class libraries, some which are used by others around the world, and now that I'm starting to use Visual Studio 2010 I'm wondering how good idea it is for me to switch to using code contracts, instead of regular old-style if-statements. ie. instead of this: if (String.IsNullOrWhiteSpace(fileName)) throw new ArgumentNullException("fileName"); (yes, I know, if it is whitespace, it isn't strictly null) use this: Contract.Requires(!String.IsNullOrWhiteSpace(fileName)); The reason I'm asking is that I know that the static checker is not available to me, so I'm a bit nervous about some assumptions that I make, that the compiler cannot verify. This might lead to the class library not compiling for someone that downloads it, when they have the static checker. This, coupled with the fact that I cannot even reproduce the problem, would make it tiresome to fix, and I would gather that it doesn't speak volumes to the quality of my class library if it seemingly doesn't even compile out of the box. So I have a few questions: Is the static checker on by default if you have access to it? Or is there a setting I need to switch on in the class library (and since I don't have the static checker, I won't) Are my fears unwarranted? Is the above scenario a real problem? Any advice would be welcome.

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• XML parsing problem

  - by Albinoswordfish

  I'm having this strange XML parsing problem. I have this XML string I'm trying to parse <?xml version="1.0"?> <response status="success"> <lot>32342</lot> </response> I'm using XPath with Java in order to do this. I'm using the Xpath expression "/response/@status" to find the text "success". However whenever I evaluate this expression I get an empty string. However I am able to successfully parse this string using "/response/@type" <?xml version="1.0"?> <response type="success"> <lot>32342</lot> </response> So why would simply changing the name of the attribute change the return string to nothing? is = new InputSource(testWOcreateStrGood); xPathexpressionSuccess = xPath.compile("/response/@status"); responseStr = xPathexpressionSuccess.evaluate(is); reponseStr is the string I posted earlier with the "status" attribute

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• Can't access media assets in an SWC

  - by Sold Out Activist

  I'm having trouble accessing content in an SWC. The main project compiles without error, but the assets (sound, music) aren't displayed or played. My workflow: i. Using Flash CS5 1. Create MAsset.fla 2. Import sounds, art 3. Assign class names, check export in frame 1 4a. Write out the classes in the actions panel in frame 1 4b. OR. Add a document class and write out the classes there 5. Export SWC. Filesize is similar to what it is when I directly import assets in the main project library. 6. Create Project.fla and document class Project.as 7. Import SWC into main project through the Actionscript panel. 8. Add code, which calls the class names from the SWC (e.g. DCL_UI_MOUSE) 9. Compile. No compiler errors, but nothing doing. And the resulting SWF filesize doesn't reflect anything more than the compiled code from the main project. Regarding step 4, if I just write the class name in the root timeline or document class, the compiler error will go away and the asset appear to be compiled in the SWC. But I have also tried: var asset0000:DCL_UI_MOUSE; And: var asset0000:DCL_UI_MOUSE = new DCL_UI_MOUSE(); Regardless the assets don't make it into the final SWF. What am I doing wrong?

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• What is the "right" way to make a modular ASP.NET page?

  - by Wayne Werner

  Hi, I'm writing an internal program using ASP.NET (Visual Studio 2008). The basic premise is that folks will connect to the page through their web browser, put in some data (which will be validated and sanitized of course), click "submit" and then a query will be run on a database. I told you that story so I can tell you this story - I want to make my program modular for the likelihood that this program will be updated later on, and probably extended. My initial thought is to compile the database handler as a dll and then use its functionality on my page (see my little diagram). But I can't figure out how to hook the dll into my page - My Google-fu has failed which leads me to my questions: 1) Is creating a dll the "right" solution? If so, continue to question 2, if not, what should I be doing? Any resources/tutorial links would be appreciated. 2) How do I attach a dll? Visual Studio tells me I can't Imports from an .aspx page, and I've tried <%@ Import namespace="MyDllName" % and a few other variations, none of which worked. Thanks!

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• Output error in comparing characters from two strings

  - by Andrew Martin

  I'm stuck with my a piece of code I'm creating. My IDE is Eclipse and when I use its debugging feature, to trace what's happening on each line, it outputs perfectly. However, when I click the "run" project, it just outputs a blank screen: public static void compareInterests(Client[] clientDetails) { int interests = 0; for (int p = 0; p < numberOfClients; p++) { for (int q = 0; q < numberOfClients; q++) { String a = clientDetails[p].getClientInterests(); String b = clientDetails[q].getClientInterests(); int count = 0; while (count < a.length()) { if (a.charAt(count) == b.charAt(count)) interests++; count++; } if ((interests >= 3) && (clientDetails[p].getClientName() != clientDetails[q].getClientName())) System.out.print (clientDetails[p].getClientName() + " is compatible with " + clientDetails[q].getClientName()); interests = 0; } } } The code is designed to import an object array which contains information on a client's name and a client's interests. The client's interests are stored in the format "01010", where each 1 means they are interested in that activity, each 0 means they are not. My code compares each character of every client's string with every other client's string and outputs the results for all client's that don't have the same name and have three or more interests in common. When I run this code through Java's debugger, it outputs fine - but when I click run project or compile, I just get a blank screen. Any ideas?

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• OcaIDE doesn't see JoCaml tools

  - by Surikator

  I'm having a problem while using OcaIDE in ocamlbuild mode. I'm trying to compile my own JoCaml sources. According to the JoCaml manual (bottom of page), to use ocamlbuild with JoCaml, I just need to add the -use-jocaml argument to ocamlbuild. Indeed, if I go to the root of my project and write ocamlbuild -use-jocaml foo.native it generates my executable just fine. However, in OcaIDE I get /bin/sh: jocamldep: command not found In OcaIDE, the -use-jocaml flag is passed in the "Other Flags" box (in Project Properties). And that certainly is working, as the complaint is precisely that it doesn't find jocaml stuff. The puzzling thing is that jocaml is installed and can be accessed from any random terminal window. For example, running jocamldep -modules foo.ml > foo.ml.depends on my project does generate the desired dependency file. So, it would seem I would have to configure OcaIDE and tell it where JoCaml executables are or something. This is done for OCaml, for example. But there is no place to do that for JoCaml. And it's really strange that, if jocamldep/jocamlc/etc are all accessible from anywhere, OcaIDE wouldn't be able to pick them. Any ideas? (I am aware I can do an ocamlbuild plugin and pass the flag in a "myocamlbuild.ml" file. I'll probably use that a latter stage after I get familiar with ocamlbuild plugins. But here the question is about OcaIDE. EDIT: Actually, ocamlbuild plugins don't seem to be a solution as, although there is an option -use-jocaml in ocamlbuild to enforce jocaml use (and it works fine), the plugin system doesn't support it, i.e. jocaml is not in the list of options.)

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• Ways to make (relatively) safe assumptions about the type of concrete subclasses?

  - by Kylotan

  I have an interface (defined as a abstract base class) that looks like this: class AbstractInterface { public: bool IsRelatedTo(const AbstractInterface& other) const = 0; } And I have an implementation of this (constructors etc omitted): class ConcreteThing { public: bool IsRelatedTo(const AbstractInterface& other) const { return m_ImplObject.has_relationship_to(other.m_ImplObject); } private: ImplementationObject m_ImplObject; } The AbstractInterface forms an interface in Project A, and the ConcreteThing lives in Project B as an implementation of that interface. This is so that code in Project A can access data from Project B without having a direct dependency on it - Project B just has to implement the correct interface. Obviously the line in the body of the IsRelatedTo function cannot compile - that instance of ConcreteThing has an m_ImplObject member, but it can't assume that all AbstractInterfaces do, including the other argument. In my system, I can actually assume that all implementations of AbstractInterface are instances of ConcreteThing (or subclasses thereof), but I'd prefer not to be casting the object to the concrete type in order to get at the private member, or encoding that assumption in a way that will crash without a diagnostic later if this assumption ceases to hold true. I cannot modify ImplementationObject, but I can modify AbstractInterface and ConcreteThing. I also cannot use the standard RTTI mechanism for checking a type prior to casting, or use dynamic_cast for a similar purpose. I have a feeling that I might be able to overload IsRelatedTo with a ConcreteThing argument, but I'm not sure how to call it via the base IsRelatedTo(AbstractInterface) method. It wouldn't get called automatically as it's not a strict reimplementation of that method. Is there a pattern for doing what I want here, allowing me to implement the IsRelatedTo function via ImplementationObject::has_relationship_to(ImplementationObject), without risky casts? (Also, I couldn't think of a good question title - please change it if you have a better one.)

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• Java Inheritance doubt in parameterised collection

  - by Gala101

  It's obvious that a parent class's object can hold a reference to a child, but does this not hold true in case of parameterised collection ?? eg: Car class is parent of Sedan So public void doSomething(Car c){ ... } public void caller(){ Sedan s = new Sedan(); doSomething(s); } is obviously valid But public void doSomething(Collection<Car> c){ ... } public void caller(){ Collection<Sedan> s = new ArrayList<Sedan>(); doSomething(s); } Fails to compile Can someone please point out why? and also, how to implement such a scenario where a function needs to iterate through a Collection of parent objects, modifying only the fields present in parent class, using parent class methods, but the calling methods (say 3 different methods) pass the collection of three different subtypes.. Ofcourse it compiles fine if I do as below: public void doSomething(Collection<Car> c){ ... } public void caller(){ Collection s = new ArrayList<Sedan>(); doSomething(s); }

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• Python to C/C++ const char question

  - by tsukemonoki

  I am extending Python with some C++ code. One of the functions I'm using has the following signature: int PyArg_ParseTupleAndKeywords(PyObject *arg, PyObject *kwdict, char *format, char **kwlist, ...); (link: http://docs.python.org/release/1.5.2p2/ext/parseTupleAndKeywords.html) The parameter of interest is kwlist. In the link above, examples on how to use this function are given. In the examples, kwlist looks like: static char *kwlist[] = {"voltage", "state", "action", "type", NULL}; When I compile this using g++, I get the warning: warning: deprecated conversion from string constant to ‘char*’ So, I can change the static char* to a static const char*. Unfortunately, I can't change the Python code. So with this change, I get a different compilation error (can't convert char** to const char**). Based on what I've read here, I can turn on compiler flags to ignore the warning or I can cast each of the constant strings in the definition of kwlist to char *. Currently, I'm doing the latter. What are other solutions? Sorry if this question has been asked before. I'm new.

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• scala 2.8 CanBuildFrom

  - by oxbow_lakes

  Following on from another question I asked, I wanted to understand a bit more about the Scala method TraversableLike[A].map whose signature is as follows: def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That Notice a few things about this method: it takes a function turning each A in the traversable into a B it returns That and takes an implicit argument of type CanBuildFrom[Repr, B, That] I can call this as follows: > val s: Set[Int] = List("Paris", "London").map(_.length) s: Set[Int] Set(5,6) What I cannot quite grasp is how the fact that That is bound to B (i.e. it is some collection of B's) is being enforced by the compiler. The type parameters look to be independent in both the signature above and in the signature of the trait CanBuildFrom itself: trait CanBuildFrom[-From, -Elem, +To] How is the scala compiler ensuring that That cannot be forced into something which does not make sense? > val s: Set[String] = List("Paris", "London").map(_.length) //will not compile EDIT - this question of course boils down to: How does the compiler decide what implicit CanBuildFrom objects are in scope for the call?

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