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• difference between -> and . for member selection operator.

  - by TimothyTech

  in this book i have I'm learning pointers, and i just got done with the chapter about OOP (spits on ground) anyways its telling me i can use a member selection operator like this ( - ). it sayd that is is like the "." except points to objects rather than member objects. whats the difference, it looks like it is used the same way...

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• what is the official name of C++'s arrow (->) operator?

  - by rmeador

  I think the subject says it all. I always call it the "arrow operator", but I'm sure it has an official name. I quickly skimmed the C++ standard and didn't see it mentioned by name.

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• lambda vs. operator.attrGetter('xxx') as sort key in Python

  - by Paul McGuire

  I am looking at some code that has a lot of sort calls using comparison functions, and it seems like it should be using key functions. If you were to change seq.sort(lambda x,y: cmp(x.xxx, y.xxx)), which is preferable: seq.sort(key=operator.attrgetter('xxx')) or: seq.sort(key=lambda a:a.xxx) I would also be interested in comments on the merits of making changes to existing code that works.

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• How can I "store" an operator inside a variable in Perl?

  - by Ricky

  I'm looking for a way to do this in Perl: $a = "60"; $b = "< 80"; if ( $a $b ) { then .... } Here, $b "holds" an operator... can I do this? Maybe some other way?

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• lambda vs. operator.attrgetter('xxx') as sort key function in Python

  - by Paul McGuire

  I am looking at some code that has a lot of sort calls using comparison functions, and it seems like it should be using key functions. If you were to change seq.sort(lambda x,y: cmp(x.xxx, y.xxx)), which is preferable: seq.sort(key=operator.attrgetter('xxx')) or: seq.sort(key=lambda a:a.xxx) I would also be interested in comments on the merits of making changes to existing code that works.

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• Input string is not in correct format ternary operator using in grid view .

  - by A.Goutam

  i am using this ternary operator for the display the value but it always says that Input string is not in correct format . <asp:TextBox ID="txtPerOfBase" runat="server" Style="text-align: right;" Text='<%# decimal.Parse(Eval("CommissionableAmountBase").ToString()) == 0 ? Eval("CommissionablePercentBase","{0:N2}"): Eval("CommissionableAmountBase","{0:N2)")%>' Width="80px"></asp:TextBox>

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• C++ template function specialization using TCHAR on Visual Studio 2005

  - by Eli

  I'm writing a logging class that uses a templatized operator<< function. I'm specializing the template function on wide-character string so that I can do some wide-to-narrow translation before writing the log message. I can't get TCHAR to work properly - it doesn't use the specialization. Ideas? Here's the pertinent code: // Log.h header class Log { public: template <typename T> Log& operator<<( const T& x ); template <typename T> Log& operator<<( const T* x ); template <typename T> Log& operator<<( const T*& x ); ... } template <typename T> Log& Log::operator<<( const T& input ) { printf("ref"); } template <typename T> Log& Log::operator<<( const T* input ) { printf("ptr"); } template <> Log& Log::operator<<( const std::wstring& input ); template <> Log& Log::operator<<( const wchar_t* input ); And the source file // Log.cpp template <> Log& Log::operator<<( const std::wstring& input ) { printf("wstring ref"); } template <> Log& Log::operator<<( const wchar_t* input ) { printf("wchar_t ptr"); } template <> Log& Log::operator<<( const TCHAR*& input ) { printf("tchar ptr ref"); } Now, I use the following test program to exercise these functions // main.cpp - test program int main() { Log log; log << "test 1"; log << L"test 2"; std::string test3( "test3" ); log << test3; std::wstring test4( L"test4" ); log << test4; TCHAR* test5 = L"test5"; log << test4; } Running the above tests reveals the following: // Test results ptr wchar_t ptr ref wstring ref ref Unfortunately, that's not quite right. I'd really like the last one to be "TCHAR", so that I can convert it. According to Visual Studio's debugger, the when I step in to the function being called in test 5, the type is wchar_t*& - but it's not calling the appropriate specialization. Ideas? I'm not sure if it's pertinent or not, but this is on a Windows CE 5.0 device.

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• Combining two operators in Evil-mode Emacs

  - by Dyslexic Tangent

  In vim I've remapped > and < when in visual mode to >gv and <gv respectively, like so: vnoremap > >gv vnoremap < <gv Since my target for this question are folks experienced with emacs and not vim, what > and < do is indent/dedent visually selected text. What gv does is reselect the previously selected text. These maps cause > and < to indent/dedent and then reselect the previously selected text. I'm trying out emacs with evil-mode and I'd like to do the same, but I'm having some difficulty figuring out how, exactly, to accomplish the automatic reselection. It looks like I need to somehow call evil-shift-right and evil-visual-restore sequentially, but I don't know how to create a map that will do both, so I tried creating my own function which would call both sequentially and map that instead, but it didn't work, possibly due to the fact that both of them are defined, not as functions with defun but instead as operators with evil-define-operator. I tried creating my own operators: (evil-define-operator shift-left-reselect (beg end) (evil-shift-left beg end) (evil-visual-restore)) (evil-define-operator shift-right-reselect (beg end) (evil-shift-right beg end) (evil-visual-restore)) but that doesn't restore visual as expected. A stab in the dark gave me this: (evil-define-operator shift-left-reselect (beg end) (evil-shift-left beg end) ('evil-visual-restore)) (evil-define-operator shift-right-reselect (beg end) (evil-shift-right beg end) ('evil-visual-restore)) but that selects one additional line whenever it is supposed to reselect. For now I've been using the following, which only has the problem where it reselects an additional line in the < operator. (evil-define-operator shift-right-reselect (beg end) (evil-shift-right beg end) (evil-visual-make-selection beg end)) (evil-define-operator shift-left-reselect (beg end) (evil-shift-left beg end) (evil-visual-make-selection beg end)) and I've mapped them: (define-key evil-visual-state-map ">" 'shift-right-reselect) (define-key evil-visual-state-map "<" 'shift-left-reselect) any help / pointers / tips would be greatly appreciated. Thanks in advance.

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• LINQ To SQL exception: Local sequence cannot be used in LINQ to SQL implementation of query operator

  - by pcampbell

  Consider this LINQ To SQL query. It's intention is to take a string[] of search terms and apply the terms to a bunch of different fields on the SQL table: string[] searchTerms = new string[] {"hello","world","foo"}; List<Cust> = db.Custs.Where(c => searchTerms.Any(st => st.Equals(c.Email)) || searchTerms.Any(st => st.Equals(c.FirstName)) || searchTerms.Any(st => st.Equals(c.LastName)) || searchTerms.Any(st => st.Equals(c.City)) || searchTerms.Any(st => st.Equals(c.Postal)) || searchTerms.Any(st => st.Equals(c.Phone)) || searchTerms.Any(st => c.AddressLine1.Contains(st)) ) .ToList(); An exception is raised: Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator Question: Why is this exception raised, and how can the query be rewritten to avoid this exception?

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• Why does the minus operator give different result than the TIMESTAMPDIFF() function in mysql?

  - by f3r3nc

  Since TIMESTAMP in mysql is stored as a 32bit value representing the time interval from 1970-jan-1 0:00:00 in seconds, I assumed that using minus (-) operator on TIMESTAMP values would give the difference of these values in seconds. Actually not: +---------------------------------------------------------------------+ | TIMESTAMP("2010-04-02 10:30:00") - TIMESTAMP("2010-04-02 10:29:59") | +---------------------------------------------------------------------+ | 41.000000 | +---------------------------------------------------------------------+ 1 row in set (0.05 sec) mysql> select timestampdiff(SECOND,TIMESTAMP("2010-04-02 10:30:00"),TIMESTAMP("2010-04-02 10:29:59")); +-----------------------------------------------------------------------------------------+ | timestampdiff(SECOND,TIMESTAMP("2010-04-02 10:30:00"),TIMESTAMP("2010-04-02 10:29:59")) | +-----------------------------------------------------------------------------------------+ | -1 | +-----------------------------------------------------------------------------------------+ mysql> select TIMESTAMP("2010-04-02 10:30:00") - TIMESTAMP("2010-04-02 10:30:01") ; +---------------------------------------------------------------------+ | TIMESTAMP("2010-04-02 10:30:00") - TIMESTAMP("2010-04-02 10:30:01") | +---------------------------------------------------------------------+ | -1.000000 | +---------------------------------------------------------------------+ +---------------------------------------------------------------------+ | TIMESTAMP("2010-04-02 10:30:00") - TIMESTAMP("2010-04-02 10:31:00") | +---------------------------------------------------------------------+ | -100.000000 | +---------------------------------------------------------------------+ It seems like one minute difference is 100 instead of 60. Why is this?

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• Equality and Assigment Operators

  - by Jeremy Smith

  I have a assembly compiled in VB.NET that contains two operators: Public Shared Operator =(quarterA As CalendarQuarter, quarterB As CalendarQuarter) As Boolean Return quarterA.StartDate = quarterB.StartDate AndAlso quarterA.EndDate = quarterB.EndDate AndAlso quarterA.Quarter = quarterB.Quarter End Operator Public Shared Operator <>(quarterA As CalendarQuarter, quarterB As CalendarQuarter) As Boolean Return Not (quarterA = quarterB) End Operator However, when using the assembly in C# to perform equality checks if (qtr != null) I receive the error: Cannot implicity convert type 'object' to 'bool' My original intent with the = operator was only for assignment purposes in VB, so I may be way off base (I don't use custom operators too often). What do I need to do to make the operator behave with both equality and assignment operations?

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• No whitespace between a cast and a namespace operator?

  - by Pod

  Hello. Could anyone please explain the following line of code, found on http://docs.openttd.org/ai__cargo_8cpp_source.html return (AICargo::TownEffect)::CargoSpec::Get(cargo_type)->town_effect; If this line was: return (AICargo::TownEffect) ::CargoSpec::Get(cargo_type)->town_effect; (note the space between TownEffect) and the ::) then I would understand it fine. However there is no whitespace in that document*, which would mean (AICargo::TownEffect) is the left operand of the :: operator. How does this code work/compile? Or are the two things equivilent due to some obscure C++ rule? *It's the same in the cpp file as well.

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• How is the ">" operator implemented (on 32 bit integers)?

  - by Ron Klein

  Let's say that the environment is x86. How do compilers compile the "" operator on 32 bit integers. Logically, I mean. Without any knowledge of Assembly. Let's say that the high level language code is: int32 x, y; x = 123; y = 456; bool z; z = x > y; What does the compiler do for evaluating the expression x > y? Does it perform something like (assuming that x and y are positive integers): w = sign_of(x - y); if (w == 0) // expression is 'false' else if (w == 1) // expression is 'true' else // expression is 'false' Is there any reference for such information?

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• Simple Scala syntax - trying to define "==" operator - what am I missing?

  - by Alex R

  While experimenting with some stuff on the REPL, I got to a point where I needed something like this: scala class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } Just a simple class with an "==" operator. Why doesn't it work??? Here's the result: :10: error: type mismatch; found : A required: ?{val x: ?} Note that implicit conversions are not applicable because they are ambiguous: both method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A] and method any2Ensuring in object Predef of type [A](x: A)Ensuring[A] are possible conversion functions from A to ?{val x: ?} class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } ^ This is scala 2.8 RC1. Thanks

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• netbeans autocompletion when using singleton to retrieve object instead of new operator?

  - by fayer

  when i use the 'new' operator to instantiate a class, netbeans has no problem to autocomplete the members of the object. $instance = new Singleton(); $instance-> // shows test() method but when i use a singleton to retrieve an object it cannot autocomplete the members in the object retrieved. the getInstance code looks like this: public function test() { echo "hello"; } public static function getInstance() { if ( ! is_object(self::$_instance)) { self::$_instance = new self(); self::$_instance->initialize(); } return self::$_instance; } so i use: $instance = Singleton::getInstance(); $instance-> // no autocompletion! does anyone have the same problem? how do i work around it? thanks!

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• C# Bug or Brain Teaser? Cast working only with Coalesce (??) Operator

  - by Alex

  This is very strange, maybe someone can explain what's happening, or this is a bug (though I tend to think that this is probably just something intricate about C#). The following code throws the error "Cannot implicitly convert type 'uint?' to 'uint'.": public void Test(UInt32? p) { UInt32 x = p; } However, this code works without error: public void Test(UInt32? p) { UInt32 x = p ?? 1; } Huh? Why does this work? Why would the coalesce operator cause implicit conversion of UInt32? (nullable) to UInt32 (non-nullable), while the first error message says that there is no implicit conversion between those types?

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• C# string equality operator returns false, but I'm pretty sure it should be true... What?!

  - by Daniel Schaffer

  I'm trying to write a unit test for a piece of code that generates a large amount of text. I've run into an issue where the "expected" and "actual" strings appear to be equal, but Assert.AreEqual throws, and both the equality operator and Equals() return false. The result of GetHashCode() is different for both values as well. However, putting both strings into text files and comparing with DiffMerge tells me they're the same. Additionally, using Encoding.ASCII.GetBytes() on both values and then using SequenceEquals to compare the resulting byte arrays returns true. The values are 34KB each, so I'll hold off putting them here for now. Any ideas? I'm completely stumped.

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• How does C++ free the memory when a constructor throws an exception and a custom new is used

  - by Joshua

  I see the following constructs: new X will free the memory if X constructor throws. operator new() can be overloaded. The canonical definition of an operator new overload is void *operator new(heap h) and the corrisponding operator delete. The most common operator new overload is pacement new, which is void *operator new(void *p) { return p; } You almost always cannot call delete on the pointer given to placement new. This leads to a single question. How is memory cleaned up when X constructor throws and an overloaded new is used?

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• Template class + virtual function = must implement?

  - by sold

  This code: template <typename T> struct A { T t; void DoSomething() { t.SomeFunction(); } }; struct B { }; A<B> a; is easily compiled without any complaints, as long as I never call a.DoSomething(). However, if I define DoSomething as a virtual function, I will get a compile error saying that B doesn't declare SomeFunction. I can somewhat see why it happens (DoSomething should now have an entry in the vtable), but I can't help feeling that it's not really obligated. Plus it sucks. Is there any way to overcome this? EDIT 2: Okay. I hope this time it makes sence: Let's say I am doing intrusive ref count, so all entities must inherit from base class Object. How can I suuport primitive types too? I can define: template <typename T> class Primitive : public Object { T value; public: Primitive(const T &value=T()); operator T() const; Primitive<T> &operator =(const T &value); Primitive<T> &operator +=(const T &value); Primitive<T> &operator %=(const T &value); // And so on... }; so I can use Primitive<int>, Primitive<char>... But how about Primitive<float>? It seems like a problem, because floats don't have a %= operator. But actually, it isn't, since I'll never call operator %= on Primitive<float>. That's one of the deliberate features of templates. If, for some reason, I would define operator %= as virtual. Or, if i'll pre-export Primitive<float> from a dll to avoid link errors, the compiler will complain even if I never call operator %= on a Primitive<float>. If it would just have fill in a dummy value for operator %= in Primitive<float>'s vtable (that raises an exception?), everything would have been fine.

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• How to call operator<< on "this" in a descendant of std::stringstream?

  - by romkyns

  class mystream : public std::stringstream { public: void write_something() { this << "something"; } }; This results in the following two compile errors on VC++10: error C2297: '<<' : illegal, right operand has type 'const char [10]' error C2296: '<<' : illegal, left operand has type 'mystream *const ' Judging from the second one, this is because what this points at can't be changed, but the << operator does (or at least is declared as if it does). Correct? Is there some other way I can still use the << and >> operators on this?

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• C++ volatile required when spinning on boost::shared_ptr operator bool()?

  - by JaredC

  I have two threads referencing the same boost::shared_ptr: boost::shared_ptr<Widget> shared; On thread is spinning, waiting for the other thread to reset the boost::shared_ptr: while(shared) boost::thread::yield(); And at some point the other thread will call: shared.reset(); My question is whether or not I need to declare the shared pointer as volatile to prevent the compiler from optimizing the call to shared.operator bool() out of the loop and never detecting the change? I know that if I were simply looping on a variable, waiting for it to reach 0 I would need volatile, but I'm not sure if boost::shared_ptr is implemented in such a way that it is not necessary here.

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• Shortcut to get all private field names combined with && operator as a generated line of code?

  - by burak ozdogan

  Hi, Is it possible to use reflection when we write a code-template? I was just thinking if it is possible to write a piece of code-template that will return me a list of all private field in the class separated by && operator in Visual Stuudio? Let's say, I will enter "getAllPrivates&&" (or press a shortcut key) in the VS editor and it will return me something like this: private bool _privateFiel1 = false; private bool _privateFiel2 = true; private bool _privateFiel3 = false; private bool _privateFiel4 = false; private bool _privateFiel5 = true; // Here I press the shortcut or call the code-template and get his: _privateFiel1 && _privateFiel2 && _privateFiel3 && _privateFiel4 && _privateFiel5

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• How to implement copy operator for such C++ structure?

  - by Kabumbus

  So having struct ResultStructure { ResultStructure(const ResultStructure& other) { // copy code in here ? using memcpy ? how??? } ResultStructure& operator=(const ResultStructure& other) { if (this != &other) { // copy code in here ? } return *this } int length; char* ptr; }; How to implement copy? (sorry - I am C++ nube)

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• Is it possible to apply NOT(!) operator while DataBinding a property of a control ?

  - by Ashish Ashu

  I have Button control and I have to set the IsEanbled property based on the bool variable. bool IsBoolVariable I want if IsBoolVariable property is true then I want to set the IsEnabled property to false AND when IsBoolVariable property is false then I want to set the IsEnabled property to true. Is it possible to apply not(!) operator while defining the binding as given below <Button IsEnabled = "{Binding Path = **!**IsBoolVariable}" /> I can very well do that by applying convertors , but that is the only way to do that?

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• How to define a ternary operator in Scala which preserves leading tokens?

  - by Alex R

  I'm writing a code generator which produces Scala output. I need to emulate a ternary operator in such a way that the tokens leading up to '?' remain intact. e.g. convert the expression c ? p : q to c something. The simple if(c) p else q fails my criteria, as it requires putting if( before c. My first attempt (still using c/p/q as above) is c match { case(true) = p; case _ = q } another option I found was: class ternary(val g: Boolean = Any) { def |: (b:Boolean) = g(b) } implicit def autoTernary (g: Boolean = Any): ternary = new ternary(g) which allows me to write: c |: { b: Boolean = if(b) p else q } I like the overall look of the second option, but is there a way to make it less verbose? Thanks

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