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  • Why aren't operator conversions implicitly called for templated functions? (C++)

    - by John Gordon
    I have the following code: template <class T> struct pointer { operator pointer<const T>() const; }; void f(pointer<const float>); template <typename U> void tf(pointer<const float>); void g() { pointer<float> ptr; f(ptr); tf(ptr); } When I compile the code with gcc 4.3.3 I get a message (aaa.cc:17: error: no matching function for call to ‘tf(pointer<float>&)’) indicating that the compiler called 'operator pointer<const T>' for the non-templated function f(), but didn't for the templated function tf(). Why and is there any workaround short of overloading tf() with a const and non-const version? Thanks in advance for any help.

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  • Why `is_base_of` works with private inheritance?

    - by Alexey Malistov
    Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class B {}; class D : private B {}; //Exspression is true. int test[is_base_of<B,D>::value && !is_base_of<D,B>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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  • Is it guaranteed that new Integer(i) == i in Java?

    - by polygenelubricants
    Consider the following snippet: int i = 99999999; byte b = 99; short s = 9999; Integer ii = Integer.valueOf(9); // should be within cache System.out.println(new Integer(i) == i); // "true" System.out.println(new Integer(b) == b); // "true" System.out.println(new Integer(s) == s); // "true" System.out.println(new Integer(ii) == ii); // "false" It's obvious why the last line will ALWAYS prints "false": we're using == reference identity comparison, and a new object will NEVER be == to an already existing object. The question is about the first 3 lines: are those comparisons guaranteed to be on the primitive int, with the Integer auto-unboxed? Are there cases where the primitive would be auto-boxed instead, and reference identity comparisons are performed? (which would all then be false!)

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  • scala 2.8 implict java collections conversions

    - by nablik
    I have problem with JavaConversions with 2.8 beta: import scala.collection.JavaConversions._ class Utils(dbFile : File, sep: String) extends IUtils { (...) def getFeatures() : java.util.List[String] = csv.attributes.toList } And then exception: [INFO] Utils.scala:20: error: type mismatch; [INFO] found : List[String] [INFO] required: java.util.List[String] [INFO] def getFeatures() : java.util.List[String] = csv.attributes.toList [INFO]

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  • Why does Option not extend the Iterable trait directly?

    - by oxbow_lakes
    Option is implicitly convertible to an Iterable - but why does it not just just implement Iterable directly: def iterator = new Iterator[A] { var end = !isDefined def next() = { val n = if (end) throw new NoSuchElementException() else get end = true n } def hasNext = !end } EDIT: In fact it's even weider than that because in 2.8 Option does declare an iterator method: def iterator: Iterator[A] = if (isEmpty) Iterator.empty else Iterator.single(this.get)

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  • In SQLCMD mode, should CONNECT be an implicit batch separator?

    - by Greg Low
    Hi Folks, I've been working with SQLCMD mode again today and one thing about it always bites me. If I execute a script like: ::CONNECT SERVER1 SELECT @@VERSION; ::CONNECT SERVER2 SELECT @@VERSION; ::CONNECT SERVER3 SELECT @@VERSION; I'm sure I'm not the only person that would be surprised to see all three SELECT commands executed against SERVER3 and none executed against SERVER1 or SERVER2. If you think that's odd behavior, here's where to vote: https://connect.microsoft.com/SQLServer/feedback/details/611144/sqlcmd-connect-to-a-different-server-should-be-an-implicit-batch-separator#detail...(read more)

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  • Why does a Linq Cast<T> operation fail when I have an implicit cast defined?

    - by Ryan Versaw
    I've created two classes, with one of them having an implicit cast between them: public class Class1 { public int Test1; } public class Class2 { public int Test2; public static implicit operator Class1(Class2 item) { return new Class1{Test1 = item.Test2}; } } When I create a new list of one type and try to Cast<T> to the other, it fails with an InvalidCastException: List<Class2> items = new List<Class2>{new Class2{Test2 = 9}}; foreach (Class1 item in items.Cast<Class1>()) { Console.WriteLine(item.Test1); } This, however, works fine: foreach (Class1 item in items) { Console.WriteLine(item.Test1); } Why is the implicit cast not called when using Cast<T>?

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  • Why is this Scala example of implicit paremeter not working?

    - by Alex R
    simple REPL test... def g(a:Int)(implicit b:Int) = {a+b} Why do neither of these attempted usages work? 1. scala class A { var b:Int =8; var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A { var b:Int =8; var c = g(2) } 2. scala class A(var b:Int) { var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A(var b:Int) { var c = g(2) } ^ Thanks

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  • Implicit vs explicit getters/setters in AS3, which to use and why?

    - by James
    Since the advent of AS3 I have been working like this: private var loggy:String; public function getLoggy ():String { return loggy; } public function setLoggy ( loggy:String ):void { // checking to make sure loggy's new value is kosher etc... this.loggy = loggy; } and have avoided working like this: private var _loggy:String; public function get loggy ():String { return loggy; } public function set loggy ( loggy:String ):void { // checking to make sure loggy's new value is kosher etc... this.loggy = loggy; } I have avoided using AS3's implicit getters/setters partly so that I can just start typing "get.." and content assist will give me a list of all my getters, and likewise for my setters. I also dislike underscores in my code which turned me off the implicit route. Another reason is that I prefer the feel of this: whateverObject.setLoggy( "loggy's awesome new value!" ); to this: whateverObject.loggy = "loggy's awesome new value!"; I feel that the former better reflects what is actually happening in the code. I am calling functions, not setting values directly. After installing Flash Builder and the great new plugin SourceMate ( which helps to get some of the useful features that FDT is famous into FB ) I realized that when I use SourceMate's "generate getters and setters" feature it automatically sets my code up using the implicit route: private var _loggy:String; public function get loggy ():String { return loggy; } public function set loggy ( loggy:String ):void { // do whatever is needed to check to make sure loggy is an acceptable value this.loggy = loggy; } I figure that these SourceMate people must know what they are doing or they wouldn't be writing workflow enhancement plugins for coding in AS3, so now I am questioning my ways. So my question to you is: Can anyone give me a good reason why I should give up my explicit g/s ways, start using the implicit technique, and embrace those stinky little _underscores for my private vars? Or back me up in my reasons for doing things the way that I do?

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  • How to you solve the problem of implicit locking and parallel execution?

    - by Eonil
    Where the code is: function A() { lock() doSomething() unlock() } We can call A safely from multiple threads, but it never be executed in parallel . For parallel execution, we have to evade all of this code. But the problem is we never know the A is getting lock or not. If we have source code (maybe lucky case), we have to decode all code to know locking is happening or not. This sucks. But even worse is we normally have no source code. It's obvious this kind of hidden locks will become bottleneck of parallel execution even all the other parts are designed for parallel. And also, (1) With locks, execution cannot be parallel. (2) And I can't know whether the locks are used or not in any code. (3) Defensively, I can't make parallel anything! This facts drives me crazy. How do you solve this problem?

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  • What are the implications of having an "implicit declaration of function" warning in C?

    - by SiegeX
    As the question states, what exactly are the implications of having the 'implicit declaration of function' warning? We just cranked up the warning flags on gcc and found quite a few instances of these warnings and I'm curious what type of problems this may have caused prior to fixing them? Also, why is this a warning and not an error. How is gcc even able to successfully link this executable? As you can see in the example below, the executable functions as expected. Take the following two files for example: file1.c #include <stdio.h> int main(void) { funcA(); return 0; } file2.c #include <stdio.h> void funcA(void) { puts("hello world"); } Compile & Output $ gcc -Wall -Wextra -c file1.c file2.c file1.c: In function 'main': file1.c:3: warning: implicit declaration of function 'funcA' $ gcc -Wall -Wextra file1.o file2.o -o test.exe $ ./test.exe hello world

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  • Possible to create an implicit cast for an anonymous type to a dictionary?

    - by Ralph
    I wrote a method like this: using AttrDict = System.Collections.Generic.Dictionary<string, object>; using IAttrDict = System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string, object>>; static string HtmlTag(string tagName, string content = null, IAttrDict attrs = null) { var sb = new StringBuilder("<"); sb.Append(tagName); if(attrs != null) foreach (var attr in attrs) sb.AppendFormat(" {0}=\"{1}\"", attr.Key, attr.Value.ToString().EscapeQuotes()); if (content != null) sb.AppendFormat(">{0}</{1}>", content, tagName); else sb.Append(" />"); return sb.ToString(); } Which you can call like HtmlTag("div", "hello world", new AttrDict{{"class","green"}}); Not too bad. But what if I wanted to allow users to pass an anonymous type in place of the dict? Like HtmlTag("div", "hello world", new {@class="green"}); Even better! I could write the overload easily, but the problem is I'm going to have about 50 functions like this, I don't want to overload each one of them. I was hoping I could just write an implicit cast to do the work for me... public class AttrDict : Dictionary<string, object> { public static implicit operator AttrDict(object obj) { // conversion from anonymous type to AttrDict here } } But C# simply won't allow it: user-defined conversions to or from a base class are not allowed So what can I do?

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  • How to write a CXF client with implicit security header.

    - by Jayaprakash
    Hi everyone, I have to access a service which is developed using different framework. The client program is using CXF. I dont have access to the provider environment so I cannot use any config files (Cxf.xml, cxfservlet.xml etc..). I need to access the webservice with implicit security header (UsernameTokenType, oasis-200401-wss-wssecurity-secext-1.0.xsd) . Can anyone throw some light on this?..

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  • Should I add try/catch around when casting on an attribute of JSP implicit object?

    - by Michael Mao
    Hi all: Basically what I mean is like this: List<String[]> routes = (List<String[]>)application.getAttribute("routes"); For the above code, it tries to get an attribute named "routes" from the JSP implicit object - application. But as everyone knows, after this line of code, routes may very well contains a null - which means this application hasn't got an attribute named "routes". Is this "casting on null" good programming practice in Java or not? Basically I try to avoid exceptions such as java.io.InvalidCastException I reckon things like this are more as "heritage" of Java 1.4 when generic types were not introduced to this language. So I guess everything stored in application attributes as Objects (Similar to traditional ArrayList). And when you do "downcast", there might be invalid casts. What would you do in this case? Update: Just found that although in the implicit object application I did store a List of String arrays, when I do this : List<double[]> routes = (List<double[]>)application.getAttribute("routes"); It doesn't produce any error... And I felt not comfortable already... And even with this code: out.print(routes.get(0)); It does print out something strange : [Ljava.lang.String;@18b352f Am I printing a "pointer to String"? I can finally get an exception like this: out.print(routes.get(0)[1]); with error : java.lang.ClassCastException: [Ljava.lang.String; Because it was me to add the application attribute, I know which type should it be cast to. I feel this is not "good enough", what if I forget the exact type? I know there are still some cases where this sort of thing would happen, such as in JSP/Servlet when you do casting on session attributes. Is there a better way to do this? Before you say:"OMG, why you don't use Servlets???", I should justify my reason as this: - because my uni sucks, its server can only work with Servlets generated by JSP, and I don't really want to learn how to fix issues like that. look at my previous question on this , and this is uni homework, so I've got no other choice, forget all about war, WEB-INF,etc, but "code everything directly into JSP" - because the professors do that too. :)

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  • implicit argument passing of super from method defined by define_method() is not supported. Specify

    - by jaycode
    Most of you should already know Pragmatic book's "Agile web dev with rails" (third edition). On page 537 - 541 it has "Custom Form Builders" code as follows: class TaggedBuilder < ActionView::Helpers::FormBuilder # <p> # <label for="product_description">Description</label><br/> # <%= form.text_area 'description' %> #</p> def self.create_tagged_field(method_name) define_method(method_name) do |label, *args| @template.content_tag("p" , @template.content_tag("label" , label.to_s.humanize, :for => "#{@object_name}_#{label}") + "<br/>" + super) end end field_helpers.each do |name| create_tagged_field(name) end end This code doesn't work with Ruby 1.9.1. It returns error as follows: implicit argument passing of super from method defined by define_method() is not supported. Specify all arguments explicitly. (ActionView::TemplateError) My question is: What should I change in the code to fix this? Thank you

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  • What is the basic design idea behind the Scala for-loop implicit box/unboxing of numerical types?

    - by IODEV
    I'm trying to understand the behavior of Scala for-loop implicit box/unboxing of "numerical" types. Why does the two first fail but not the rest? 1) Fails: scala for (i:Long <- 0 to 10000000L) {} <console>:19: error: type mismatch;<br> found : Long(10000000L) required: Int for (i:Long <- 0 to 10000000L) {} ^ 2 Fails: scala for (i <- 0 to 10000000L) {} <console>:19: error: type mismatch; found : Long(10000000L) required: Int for (i <- 0 to 10000000L) {} ^ 3) Works: scala for (i:Long <- 0L to 10000000L) {} 4) Works: scala for (i <- 0L to 10000000L) {}

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  • Implicit declaration when using a function before it is defined in C, why can't the compiler figure this out?

    - by rolls
    As the title says, I know what causes this error but I want to know why the compiler gives it in this circumstance. Eg : main.c void test(){ test1(); } void test1(){ ... } Would give an implicit declaration warning as the compiler would reach the call to test1() before it has read its declaration, I can see the obvious problems with this (not knowing return type etc), but why can't the compiler do a simple pass to get all function declarations, then compile the code removing these errors? It just seems so simple to do and I don't believe I've seen similar warnings in other languages. Does anyone know if there is a specific purpose for this warning in this situation that I am overlooking?

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  • Why do Java and C# not have implicit conversions to boolean?

    - by Shaun
    Since I started Java it's been very aggravating for me that it doesn't support implicit conversions from numeric types to booleans, so you can't do things like: if (flags & 0x80) { ... } instead you have to go through this lunacy: if ((flags & 0x80) != 0) { ... } It's the same with null and objects. Every other C-like language I know including JavaScript allows it, so I thought Java was just moronic, but I've just discovered that C# is the same (at least for numbers, don't know about null/objects): http://msdn.microsoft.com/en-us/library/c8f5xwh7(VS.71).aspx Microsoft changed it on purpose from C++, so why? Clearly I'm missing something. Why change (what I thought was) the most natural thing in the world to make it longer to type? What on Earth is wrong with it?

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  • How is the implicit segment register of a near pointer determined?

    - by Daniel Trebbien
    In section 4.3 of Intel 64® and IA-32 Architectures Software Developer's Manual. Volume 1: Basic Architecture, it says: A near pointer is a 32-bit offset ... within a segment. Near pointers are used for all memory references in a flat memory model or for references in a segmented model where the identity of the segment being accessed is implied. This leads me to wondering: how is the implied segment register determined? I know that (%eip) and displaced (%eip) (e.g. -4(%eip)) addresses use %cs by default, and that (%esp) and displaced (%esp) addresses use %ss, but what about (%eax), (%edx), (%edi), (%ebp) etc., and can the implicit segment register depend also on the instruction that the memory address operand appears in?

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