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• Possible to push a hash value only if it is unique?

  - by Structure

  In the example code below, assuming that the value $keyvalue is constantly changing, I am attempting to use a single line (or something similarly contained) to test and see if the current $keyvalue already exists. If it does, then do nothing. If it does not, then push it. This line would reside within a while statement which is why it needs to be contained within a few lines. Preserving order does not matter as long as there are no duplicate values. my $key = "numbers"; my $keyvalue = 1; my %hash = ($key => '1'); push (@{$hash{$key}}, $keyvalue) unless exists $hash{$key}; I am not getting any errors with use strict; use warnings;, but at the same time this is not working. In the example above, I would expect that since the default value is 1 that the $keyvalue would not be pushed as it is also 1. Perhaps I have gotten myself all turned around... Are there adjustments to get this to work or any alternatives that can be used instead to accomplish the same thing?

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• Is new int[10]() valid c++?

  - by Naveen

  While trying to answer this question I found that the code int* p = new int[10](); compiles fine with VC9 compiler and initializes the integers to 0. So my questions are: First of all is this valid C++ or is it a microsoft extension? Is it guaranteed to initialize all the elements of the array? Also, is there any difference if I do new int; or new int();? Does the latter guarantee to initialize the variable?

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• how to specify a pointer to an overloaded function?

  - by davka

  I want to pass an overloaded function to the std::for_each() algorithm. e.g.: void f(char c); void f(int i); std::string s("example"); std::for_each(s.begin(), s.end(), f); I'd expect the compiler to resolve f() by the iterator type. Apparently, it (gcc 4.1.2) doesn't do it. So, how can I specify which f() I want? thanks a lot

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• Delegate Example From C# In Depth Confusion

  - by ChloeRadshaw

  I am looking at this example: List<Product> products = Product. GetSampleProducts() ; products.Sort( (first, second) => first.Name.CompareTo(second. Name) ) ; foreach (Product product in products) { Console. WriteLine(product) ; } What function is actually called in the API when you do that? Does the compiler create a class which implemnents the IComparer interface? I thought delegates were anonymous methods - Here it seems to be an anonymous interface implementation which is casuing confusion

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• Why does a non-constant offsetof expression work?

  - by Chris J. Kiick

  Why does this work: #include <sys/types.h> #include <stdio.h> #include <stddef.h> typedef struct x { int a; int b[128]; } x_t; int function(int i) { size_t a; a = offsetof(x_t, b[i]); return a; } int main(int argc, char **argv) { printf("%d\n", function(atoi(argv[1]))); } If I remember the definition of offsetof correctly, it's a compile time construct. Using 'i' as the array index results in a non-constant expression. I don't understand how the compiler can evaluate the expression at compile time. Why isn't this flagged as an error?

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• Can I reproduce Scala's behavior for == ?

  - by JPP

  In Programming in Scala, I can read that the == operator behaves as if it was defined like this: final def == (that: Any): Boolean = if (null eq this) {null eq that} else {this equals that} But there must actually be compiler magic to avoid null pointer exceptions, right? Is there any way for me to replicate this behavior with pure Scala; i.e., have an operator/method return one thing if the receiver is null and another one if it isn't? What I mean is an actual implementation of null eq this. I suppose I can write a "pimp" and then define the method on the wrapper class, but is there a more direct way to do this?

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• How to explain to a developer that adding extra if - else if conditions is not a good way to "improv

  - by Lilit

  Recently I've bumped into the following C++ code: if (a) { f(); } else if (b) { f(); } else if (c) { f(); } Where a, b and c are all different conditions, and they are not very short. I tried to change the code to: if (a || b || c) { f(); } But the author opposed saying that my change will decrease readability of the code. I had two arguments: 1) You should not increase readability by replacing one branching statement with three (though I really doubt that it's possible to make code more readable by using else if instead of ||). 2) It's not the fastest code, and no compiler will optimize this. But my arguments did not convince him. What would you tell a programmer writing such a code? Do you think complex condition is an excuse for using else if instead of OR?

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• Same memory space being allocated again & again

  - by shadyabhi

  In each loop iteration, variable j is declared again and again. Then why is its address remaining same? Shouldn't it be given some random address each time? Is this compiler dependent? #include<stdio.h> #include<malloc.h> int main() { int i=3; while (i--) { int j; printf("%p\n", &j); } return 0; } Testrun:- shadyabhi@shadyabhi-desktop:~/c$ gcc test.c shadyabhi@shadyabhi-desktop:~/c$ ./a.out 0x7fffc0b8e138 0x7fffc0b8e138 0x7fffc0b8e138 shadyabhi@shadyabhi-desktop:~/c$

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• C++ standard: dereferencing NULL pointer to get a reference?

  - by shoosh

  I'm wondering about what the C++ standard says about code like this: int* ptr = NULL; int& ref = *ptr; int* ptr2 = &ref; In practice the result is that ptr2 is NULL but I'm wondering, is this just an implementation detail or is this well defined in the standard? Under different circumstances a dereferencing of a NULL pointer should result in a crash but here I'm dereferencing it to get a reference which is implemented by the compiler as a pointer so there's really no actual dereferencing of NULL.

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• Is there a TextWriter interface to the System.Diagnostics.Debug class?

  - by John Källén

  I'm often frustrated by the System.Diagnostics.Debug.Write/WriteLine methods. I would like to use the Write/WriteLine methods familiar from the TextWriter class, so I often write Debug.WriteLine("# entries {0} for connection {1}", countOfEntries, connection); which causes a compiler error. I end up writing Debug.WriteLine(string.Format("# entries {0} for connection {1}", countOfEntries, connection)); which is really awkward. Does the CLR have a class deriving from TextWriter that "wraps" System.Debug, or should I roll my own?

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• c++ - FIFO implementation

  - by Narek

  While implementing a FIFO I have used the following structure: struct Node { T info_; Node* link_; Node(T info, Node* link=0): info_(info), link_(link) {} }; I think this a well known trick for lots of STL containers (for example for List). Is this a good practice? What it means for compiler when you say that Node has a member with a type of it's pointer? Is this a kind of infinite loop? And finally, if this is a bad practice, how I could implement a better FIFO.

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• Inter-project dependencies

  - by Mike Hordecki

  Hello! I'm doing some Delphi (2010) work this summer, and I've stumbled upon this problem: My project consists of reusable backend library and a bunch of GUIs that tap into its interface. In this circumstances I've decided to make the backend and GUIs separate projects within single project group (I hope my train of thought is correct). The problem is, how can I include units from the backend in a GUI project? I've tried to modify Project Options > Directories and Conditionals but compiler still complains about being unable to find proper .dcu's. Any ideas? Your help will be appreciated.

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• Can a method return an NSRange?

  - by Dan Donaldson

  I have a method that returns an NSRange. When I call this method from outside the class I get a compile error. NSRange tmpRange; tmpRange = [phrase rangeInString:searchString forString:theLetter goingForward:YES]; return tmpRange.location == -1; in the .h file: #import <Foundation/Foundation.h> @interface Phrase : NSObject { } - (NSRange) rangeInString:(NSString *) tgt forString:(NSString *) find goingForward:(BOOL) fwd; @end This method is called within the Phrase object by other methods without problems. The compiler says 'incompatible types in assignment'. Can anyone explain this to me? I assume it has to do with returning an NSRange/struct type value generated outside the object, but I don't know why it works in one place and not the other.

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• Using an int as the numerical representation of a string in C#

  - by bluewall21

  I'm trying to use an integer as the numerical representation of a string, for example, storing "ABCD" as 0x41424344. However, when it comes to output, I've got to convert the integer back into 4 ASCII characters. Right now, I'm using bit shifts and masking, as follows: int value = 0x41424344; string s = new string ( new char [] { (char)(value >> 24), (char)(value >> 16 & 0xFF), (char)(value >> 8 & 0xFF), (char)(value & 0xFF) }); Is there a cleaner way to do this? I've tried various casts, but the compiler, as expected, complained about it.

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• Usability for notification messages, colors

  - by metrobalderas

  In each app I develop, I like to add three types of messages: Green/blue for success messages Yellow for warnings Red for errors And perhaps, a neutral one for information, which is gray or blue if the success one is green. The success one is used for when an item is created or updated, the yellow one is when there's something wrong, but not we-are-going-to-die wrong and the red one is when something is blocked or we are going to die. However, there's one thing I can't figure out, when I delete an object, what kind of notification should I use? I think the success one is not because it is not expected, altough the deletion was successful, the user tends not to read the message, just to see the color. The red one might be, but it can be misunderstood (I tried to delete it but there was an error), the warning and the information one might be good choices, but I'm not really sure. Also, when you ask for confirmation about deleting something, the 'cancel' button should be green or red? I'm just curious how you guys handle this. Thanks.

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• Misunderstanding function pointer - passing it as an argument

  - by Stef

  I want to pass a member function of class A to class B via a function pointer as argument. Please advise whether this road is leading somewhere and help me fill the pothole. #include <iostream> using namespace std; class A{ public: int dosomeA(int x){ cout<< "doing some A to "<<x <<endl; return(0); } }; class B{ public: B(int (*ptr)(int)){ptr(0);}; }; int main() { A a; int (*APtr)(int)=&A::dosomeA; B b(APtr); return 0; } This brilliant piece of code leaves me with the compiler error: cannot convert int (A::*)(int)' toint (*)(int)' in initialization Firstly I want it to compile. Secondly I don't want dosomeA to be STATIC.

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• C++ rvalue temporaries in template

  - by aaa

  hello. Can you please explain me the difference between mechanism of the following: int function(); template<class T> void function2(T&); void main() { function2(function()); // compiler error, instantiated as int & const int& v = function(); function2(v); // okay, instantiated as const int& } is my reasoning correct with respect to instantiation? why is not first instantiated as const T&? Thank you

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• F# match char values

  - by rwallace

  I'm trying to match an integer expression against character literals, and the compiler complains about type mismatch. let rec read file includepath = let ch = ref 0 let token = ref 0 use stream = File.OpenText file let readch() = ch := stream.Read() let lex() = match !ch with | '!' -> readch() | _ -> token := !ch ch has to be an int because that's what stream.Read returns in order to use -1 as end of file marker. If I replace '!' with int '!' it still doesn't work. What's the best way to do this?

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• a macro question for c language (#define)

  - by Daniel

  I am reading source code of hoard memory allocator, and in the file of gnuwrapper.cpp, there are the following code #define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x) What's the meaning of CUSTOM_PREFIX(malloc)(x)? is CUSTOM_PREFIX a function? But as a function it didn't defined anywhere. If it's variable, then how can we use variable like var(malloc)(x)? more code: #ifndef __GNUC__ #error "This file requires the GNU compiler." #endif #include <string.h> #include <stdlib.h> #include <stdio.h> #include <malloc.h> #ifndef CUSTOM_PREFIX ==> here looks like it's a variable, so if it doesn't define, then define here. #define CUSTOM_PREFIX #endif #define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x) ===> what's the meaning of this? #define CUSTOM_FREE(x) CUSTOM_PREFIX(free)(x) #define CUSTOM_REALLOC(x,y) CUSTOM_PREFIX(realloc)(x,y) #define CUSTOM_MEMALIGN(x,y) CUSTOM_PREFIX(memalign)(x,y)

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• [VB.NET] Same name methods in different modules cause ambiguity.

  - by smwikipedia

  I have 2 modules. Each contains a Sub with the same name. See below: Module moduleA Public Sub f(ByVal arg1 As myType) Console.WriteLine("module A") End Sub End Module Module moduleB Public Sub f(ByVal arg1 As myType, ByVal arg2 As Boolean) Console.WriteLine("module B") End Sub End Module But the compiler complains that there's ambiguity between moduleA and moduleB. How could this be? I have totally different signatures. However, if I put the 2 methods into the same module, there's no ambiguity at all. Could someone tell me why? Many thanks.

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• Cast A primitive type pointer to A structure pointer - Alignment and Padding?

  - by Seçkin Savasçi

  Just 20 minutes age when I answered a question, I come up with an interesting scenario that I'm not sure of the behavior: Let me have an integer array of size n, pointed by intPtr; int* intPtr; and let me also have a struct like this: typedef struct { int val1; int val2; //and less or more integer declarations goes on like this(not any other type) }intStruct; My question is if I do a cast intStruct* structPtr = (intStruct*) intPtr; Am I sure to get every element correctly if I traverse the elements of the struct? Is there any possibility of miss-alignment(possible because of padding) in any architecture/compiler?

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• hook to save action in eclipse plugin

  - by 4485670

  I want to create a Google Closure Compiler plugin for eclipse. I already have a popup menu entry to compile a Javascript file to its minified version. But it would be more than helpful if every time you save a *.js that minified version would be generated automatically. I read/heard about natures and builders, extension points and IResourceChangeListener. But I did not manage to figure out what I should use and especially how to get it to work. Is there a working example of a plugin that does "the same kind of thing" so I can work from that or a tutorial to write such? With the answer below I searched for projects that use the IResourceChangeListener and came up with this code: manifest: http://codepaste.net/3yahwe plugin.xml: http://codepaste.net/qek3rw activator: http://codepaste.net/s7xowm DummyStartup: http://codepaste.net/rkub82 MinifiedJavascriptUpdater: http://codepaste.net/koweuh There in the MinifiedJavascriptUpdater.java which holds the code for the IResourceChangeListener the "resourceChanged" function is never reached.

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• Why does Generic class signature requires specifying new() if type T needs instantiation ?

  - by this. __curious_geek

  I'm writing a Generic class as following. public class Foo<T> : where T : Bar, new() { public void MethodInFoo() { T _t = new T(); } } As you can see the object(_t) of type T is instantiated at run-time. To support instantiation of generic type T, language forces me to put new() in the class signature. I'd agree to this if Bar is an abstract class but why does it need to be so if Bar standard non-abstract class with public parameter-less constructor. compiler prompts following message if new() is not found. Cannot create an instance of the variable type 'T' because it does not have the new() constraint

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• Why can't I project ToString() in VB?

  - by Martinho Fernandes

  If you try to compile the query below in Visual Basic .NET, it fails. From x In {1, 2} Select x.ToString() The error given by the compiler is: Range variable name cannot match the name of a member of the 'Object' class. There is nothing wrong with the equivalent C# query, though: from x in new[]{1, 2} select x.ToString() This does not happen with the ToString overload that takes a format (it is a member of Int32, not Object). It does happen with other members of Object, as long as they don't take an argument: with GetType and GetHashCode it fails; with Equals(object) it compiles. Why is this restriction in place, and what alternatives can I use?

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• Perl Linux::Inotify2 - can't respond to events anymore

  - by alcy

  I am getting some really weird behavior when using Linux::Inotify2 module for watching a directory for any newly created files. I had made a test script to see how it worked, and once that was done, I went on to incorporating its usage in the other scripts, in which it didn't work. Then, when I tried my earlier test script again to find some information, strangely that stopped working as well. It hasn't worked since then. There were no package/distro upgrades during that time. The problem is that it has stopped responding to events. Here's the test script: #!/usr/bin/perl use strict; use warnings; use Linux::Inotify2; my $inotify = new Linux::Inotify2 or die "unable to create new inotify object: $!"; my $dir = "/my/dir"; $inotify->watch($dir, IN_CREATE, sub { my $e = shift; print $e->fullname; }) or die " Can't watch $!"; 1 while $inotify->poll; A strace on the running script kills the script. Otherwise when strace is used when starting the script, then it does seem to read the new events, but there's no response to those events. Any suggestions for debugging this further ?

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