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  • Why is T() = T() allowed?

    - by Rimo
    I believe the expression T() creates an rvalue (by the Standard). However, the following code compiles (at least on gcc4.0): class T {}; int main() { T() = T(); } I know technically this is possible because member functions can be invoked on temporaries and the above is just invoking the operator= on the rvalue temporary created from the first T(). But conceptually this is like assigning a new value to an rvalue. Is there a good reason why this is allowed? Edit: The reason I find this odd is it's strictly forbidden on built-in types yet allowed on user-defined types. For example, int(2) = int(3) won't compile because that is an "invalid lvalue in assignment". So I guess the real question is, was this somewhat inconsistent behavior built into the language for a reason? Or is it there for some historical reason? (E.g it would be conceptually more sound to allow only const member functions to be invoked on rvalue expressions, but that cannot be done because that might break some existing code.)

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  • Comparing objects and inheritance

    - by ereOn
    Hi, In my program I have the following class hierarchy: class Base // Base is an abstract class { }; class A : public Base { }; class B : public Base { }; I would like to do the following: foo(const Base& one, const Base& two) { if (one == two) { // Do something } else { // Do something else } } I have issues regarding the operator==() here. Of course comparing an instance A and an instance of B makes no sense but comparing two instances of Base should be possible. (You can't compare a Dog and a Cat however you can compare two Animals) I would like the following results: A == B = false A == A = true or false, depending on the effective value of the two instances B == B = true or false, depending on the effective value of the two instances My question is: is this a good design/idea ? Is this even possible ? What functions should I write/overload ? My apologies if the question is obviously stupid or easy, I have some serious fever right now and my thinking abilities are somewhat limited :/ Thank you.

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  • Correct way to initialize dynamic Array in C++

    - by mef
    Hey guys, I'm currently working on a C++ project, where dynamic arrays often appear. I was wondering, what could be the correct way to initialize a dynamic array using the new-operator? A colleague of mine told me that it's a no-no to use new within the constructor, since a constructor is a construct that shouldn't be prone to errors or shouldn't fail at all, respectively. Now let's consider the following example: We have two classes, a more or less complex class State and a class StateContainer, which should be self-explained. class State { private: unsigned smth; public: State(); State( unsigned s ); }; class StateContainer { private: unsigned long nStates; State *states; public: StateContainer(); StateContainer( unsigned long n ); virtual ~StateContainer(); }; StateContainer::StateContainer() { nStates = SOME_DEFINE_N_STATES; states = new State[nStates]; if ( !states ) { // Error handling } } StateContainer::StateContainer( unsigned long n ) { nStates = n; try { states = new State[nStates] } catch ( std::bad_alloc &e ) { // Error handling } } StateContainer::~StateContainer() { if ( states ) { delete[] states; states = 0; } } Now actually, I have two questions: 1.) Is it ok, to call new within a constructor, or is it better to create an extra init()-Method for the State-Array and why? 2.) Whats the best way to check if new succeeded: if (!ptr) std::cerr << "new failed." or try { /*new*/ } catch (std::bad_alloc) { /*handling*/ } 3.) Ok its three questions ;o) Under the hood, new does some sort of ptr = (Struct *)malloc(N*sizeof(Struct)); And then call the constructor, right?

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  • [javascript] Can I overload an object with a function?

    - by user257493
    Lets say I have an object of functions/values. I'm interested in overloading based on calling behavior. For example, this block of code below demonstrates what I wish to do. var main_thing = { initalized: false, something: "Hallo, welt!", something_else: [123,456,789], load: { sub1 : function() { //Some stuff }, sub2 : function() { //Some stuff }, all : function() { this.load.sub1(); this.load.sub2(); } } init: function () { this.initalized=true; this.something="Hello, world!"; this.something_else = [0,0,0]; this.load(); //I want this to call this.load.all() instead. } } The issue to me is that main_thing.load is assigned to an object, and to call main_thing.load.all() would call the function inside of the object (the () operator). What can I do to set up my code so I could use main_thing.load as an access the object, and main_thing.load() to execute some code? Or at least, similar behavior. Basically, this would be similar to a default constructor in other languages where you don't need to call main_thing.constructor(). If this isn't possible, please explain with a bit of detail.

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  • Why don't languages include implication as a logical operator?

    - by Maciej Piechotka
    It might be a strange question, but why there is no implication as a logical operator in many languages (Java, C, C++, Python Haskell - although as last one have user defined operators its trivial to add it)? I find logical implication much clearer to write (particularly in asserts or assert-like expressions) then negation with or: encrypt(buf, key, mode, iv = null) { assert (mode != ECB --> iv != null); assert (mode == ECB || iv != null); assert (implies(mode != ECB, iv != null)); // User-defined function }

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  • Access to Perl's empty angle "<>" operator from an actual filehandle?

    - by Ryan Thompson
    I like to use the nifty perl feature where reading from the empty angle operator <> magically gives your program UNIX filter semantics, but I'd like to be able to access this feature through an actual filehandle (or IO::Handle object, or similar), so that I can do things like pass it into subroutines and such. Is there any way to do this? This question is particularly hard to google, because searching for "angle operator" and "filehandle" just tells me how to read from filehandles using the angle operator.

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  • What do you call the << operator in Ruby when it's used for appending stuff?

    - by more or less
    In other contexts I know this << is called the bitshift operator. Is there a name for it when it's just used for append operations like you would do in an array or string (not sure what else you can append with it)? I'd like to be able to use an English word to refer to it instead of saying "you know, the operator with the two left arrows that's not really the left bitshift operator".

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  • Who calls the Destructor of the class when operator delete is used in multiple inheritance.

    - by dicaprio-leonard
    This question may sound too silly, however , I don't find concrete answer any where else. With little knowledge on how late binding works and virtual keyword used in inheritance. As in the code sample, when in case of inheritance where a base class pointer pointing to a derived class object created on heap and delete operator is used to deallocate the memory , the destructor of the of the derived and base will be called in order only when the base destructor is declared virtual function. Now my question is : 1) When the destructor of base is not virtual, why the problem of not calling derived dtor occur only when in case of using "delete" operator , why not in the case given below: derived drvd; base *bPtr; bPtr = &drvd; //DTOR called in proper order when goes out of scope. 2) When "delete" operator is used, who is reponsible to call the destructor of the class? The operator delete will have an implementation to call the DTOR ? or complier writes some extra stuff ? If the operator has the implementation then how does it looks like , [I need sample code how this would have been implemented]. 3) If virtual keyword is used in this example, how does operator delete now know which DTOR to call? Fundamentaly i want to know who calls the dtor of the class when delete is used. Sample Code class base { public: base() { cout<<"Base CTOR called"<<endl; } virtual ~base() { cout<<"Base DTOR called"<<endl; } }; class derived:public base { public: derived() { cout<<"Derived CTOR called"<<endl; } ~derived() { cout<<"Derived DTOR called"<<endl; } }; I'm not sure if this is a duplicate, I couldn't find in search. int main() { base *bPtr = new derived(); delete bPtr;// only when you explicitly try to delete an object return 0; }

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  • What version of mongodb was full $text query operator introduced?

    - by Marc Maxson
    Stupid question, right? But the official docs for 'text index' say: http://docs.mongodb.org/manual/core/index-text/ Text Indexes New in version 2.4. To perform queries that access the text index, use the $text query operator. Whereas if you click on the help for searching the index you created with the $text operator, it reads: http://docs.mongodb.org/manual/reference/operator/query/text/#op._S_text $text New in version 2.6. Seems to be 2.4 but still having problems wiht it.

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  • How to write a cctor and op= for a factory class with ptr to abstract member field?

    - by Kache4
    I'm extracting files from zip and rar archives into raw buffers. I created the following to wrap minizip and unrarlib: Archive.hpp #include "ArchiveBase.hpp" #include "ArchiveDerived.hpp" class Archive { public: Archive(string path) { /* logic here to determine type */ switch(type) { case RAR: archive_ = new ArchiveRar(path); break; case ZIP: archive_ = new ArchiveZip(path); break; case UNKNOWN_ARCHIVE: throw; break; } } Archive(Archive& other) { archive_ = // how do I copy an abstract class? } ~Archive() { delete archive_; } void passThrough(ArchiveBase::Data& data) { archive_->passThrough(data); } Archive& operator = (Archive& other) { if (this == &other) return *this; ArchiveBase* newArchive = // can't instantiate.... delete archive_; archive_ = newArchive; return *this; } private: ArchiveBase* archive_; } ArchiveBase.hpp class ArchiveBase { public: // Is there any way to put this struct in Archive instead, // so that outside classes instantiating one could use // Archive::Data instead of ArchiveBase::Data? struct Data { int field; }; virtual void passThrough(Data& data) = 0; /* more methods */ } ArchiveDerived.hpp "Derived" being "Zip" or "Rar" #include "ArchiveBase.hpp" class ArchiveDerived : public ArchiveBase { public: ArchiveDerived(string path); void passThrough(ArchiveBase::Data& data); private: /* fields needed by minizip/unrarlib */ // example zip: unzFile zipFile_; // example rar: RARHANDLE rarFile_; } ArchiveDerived.cpp #include "ArchiveDerived.hpp" ArchiveDerived::ArchiveDerived(string path) { //implement } ArchiveDerived::passThrough(ArchiveBase::Data& data) { //implement } Somebody had suggested I use this design so that I could do: Archive archiveFile(pathToZipOrRar); archiveFile.passThrough(extractParams); // yay polymorphism! How do I write a cctor for Archive? What about op= for Archive? What can I do about "renaming" ArchiveBase::Data to Archive::Data? (Both minizip and unrarlib use such structs for input and output. Data is generic for Zip & Rar and later is used to create the respective library's struct.)

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  • How to approach copying objects with smart pointers as class attributes?

    - by tomislav-maric
    From the boost library documentation I read this: Conceptually, smart pointers are seen as owning the object pointed to, and thus responsible for deletion of the object when it is no longer needed. I have a very simple problem: I want to use RAII for pointer attributes of a class that is Copyable and Assignable. The copy and assignment operations should be deep: every object should have its own copy of the actual data. Also, RTTI needs to be available for the attributes (their type may also be determined at runtime). Should I be searching for an implementation of a Copyable smart pointer (the data are small, so I don't need Copy on Write pointers), or do I delegate the copy operation to the copy constructors of my objects as shown in this answer? Which smart pointer do I choose for simple RAII of a class that is copyable and assignable? (I'm thinking that the unique_ptr with delegated copy/assignment operations to the class copy constructor and assignment operator would make a proper choice, but I am not sure) Here's a pseudocode for the problem using raw pointers, it's just a problem description, not a running C++ code: // Operation interface class ModelOperation { public: virtual void operate = (); }; // Implementation of an operation called Special class SpecialModelOperation : public ModelOperation { private: // Private attributes are present here in a real implementation. public: // Implement operation void operate () {}; }; // All operations conform to ModelOperation interface // These are possible operation names: // class MoreSpecialOperation; // class DifferentOperation; // Concrete model with different operations class MyModel { private: ModelOperation* firstOperation_; ModelOperation* secondOperation_; public: MyModel() : firstOperation_(0), secondOperation_(0) { // Forgetting about run-time type definition from input files here. firstOperation_ = new MoreSpecialOperation(); secondOperation_ = new DifferentOperation(); } void operate() { firstOperation_->operate(); secondOperation_->operate(); } ~MyModel() { delete firstOperation_; firstOperation_ = 0; delete secondOperation_; secondOperation_ = 0; } }; int main() { MyModel modelOne; // Some internal scope { // I want modelTwo to have its own set of copied, not referenced // operations, and at the same time I need RAII to work for it, // as soon as it goes out of scope. MyModel modelTwo (modelOne); } return 0; }

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  • Constructor is being invoked twice

    - by Knowing me knowing you
    In code: LINT a = "12"; LINT b = 3; a = "3";//WHY THIS LINE INVOKES CTOR? std::string t = "1"; //LINT a = t;//Err NO SUITABLE CONV FROM STRING TO LINT. Shouldn't ctor do it? #pragma once #include "LINT_rep.h" class LINT { private: typedef LINT_rep value_type; const value_type* my_data_; template<class T> void init_(const T&); public: LINT(const char* = 0); LINT(const std::string&); LINT(const LINT&); LINT(const long_long&); LINT& operator=(const LINT&); virtual ~LINT(void); LINT operator+()const; //DONE LINT operator+(const LINT&)const;//DONE LINT operator-()const; //DONE LINT operator-(const LINT&)const;//DONE LINT operator*(const LINT&)const;//DONE LINT operator/(const LINT&)const;///WAITS FOR APPROVAL LINT& operator+=(const LINT&);//DONE LINT& operator-=(const LINT&);//DONE LINT& operator*=(const LINT&);//DONE LINT operator/=(const LINT&);///WAITS FOR APPROVAL }; in line number 3 instead of assignment optor ctor is invoked. Why? I'm willing to uppload entire solution on some server otherwise it's hard to put everything in here. I can also upload video file. Another thing is that when I implement this assignment optor I'm getting an error that this optor is already in obj file? What's going on?

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  • Adding a new operator symbol to a custom language in Notepad++?

    - by Wolfpack'08
    I'd like to create a custom operator for a custom language I'm highlighting in Notepad++. Has anyone done this in the past? A special character that often comes up in my language is the yen symbol (?), and I would like to highlight the phrases immediately following and immediately preceding that symbol; unfortunately, I cannot figure a way to add a custom operator, of it is even possible. Also, if anyone has any additional tag recommendations, please edit.

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  • SQL SERVER – Puzzle Involving NULL – Resolve – Error – Operand data type void type is invalid for sum operator

    - by pinaldave
    Today is Monday let us start this week with interesting puzzle. Yesterday I had also posted quick question here: SQL SERVER – T-SQL Scripts to Find Maximum between Two Numbers Run following code: SELECT SUM(data) FROM (SELECT NULL AS DATA) t It will throw following error. Msg 8117, Level 16, State 1, Line 1 Operand data type void type is invalid for sum operator. I can easily fix if I use ISNULL Function as displayed following. SELECT SUM(data) FROM (SELECT ISNULL(NULL,0) AS DATA) t Above script will not throw an error. However, there is one more method how this can be fixed. Can you come up with another method which will not generate error? Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Puzzle, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQLServer, T SQL, Technology

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  • How to list column headers of a SQL Server table using sp_help perhaps?

    - by Hamish Grubijan
    Hi, I have a few tables with 70-80 columns in them. I would like to populate them with somewhat random data, unless I will not be able to do so due to key violation, etc. The first step would be simply to get the list of all headers. There seem to be two ways: A) Run select * from table_of_interest; in MSFT SQL Server Management Studio 2008. Now, right-click the result and click "Copy With headers". However, I get zero rows back, and when I try to copy nothing + headers, I get: TITLE: Microsoft SQL Server Management Studio ------------------------------ Value cannot be null. Parameter name: data (System.Windows.Forms) ------------------------------ BUTTONS: OK ------------------------------ This looks like a bug ... anyhow ... there is another way. B) I can run sp_help table_of_interest;. However, I end up getting too much back. I get 7 different tables back but I am only interested in the second one. The columns of the second table are: Column_name | Type | Computed | Length | Prec | Scale | Nullable | TrimTrailingBlanks | FixedLenNullInSource | Collation I might be interested in just a Column_name and Type, but maybe other columns. So ... since sp_help probably runs a bunch of queries ... how do I get under the hood? How can I run the second query AND filter down the number of columns that I am interested in? Many Thanks!

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  • How do boost operators work?

    - by FredOverflow
    boost::operators automatically defines operators like + based on manual implementations like += which is very useful. To generate those operators for T, one inherits from boost::operators<T> as shown by the boost example: class MyInt : boost::operators<MyInt> I am familiar with the CRTP pattern, but I fail to see how it works here. Specifically, I am not really inheriting any facilities since the operators aren't members. boost::operators seems to be completely empty, but I'm not very good at reading boost source code. Could anyone explain how this works in detail? Is this mechanism well-known and widely used?

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  • how to handle delete by illegal address

    - by Davit Siradeghyan
    Suppose we have a situation like this. How to handle this problem? How to protect code from crashes? I know about and use boost smart pointers. But what to do if we have this situation. struct Test { int a; int b; int c; }; Test global; int main() { Test *p = new Test; p->a = 1; p->b = 2; p->c = 3; p = &global; delete p; return 0; }

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  • Why is this code invalid in C#?

    - by mmattax
    The following code will not compile: string foo = "bar"; Object o = foo == null ? DBNull.Value : foo; I get: Error 1 Type of conditional expression cannot be determined because there is no implicit conversion between 'System.DBNull' and 'string' To fix this, I must do something like this: string foo = "bar"; Object o = foo == null ? DBNull.Value : (Object)foo; This cast seems pointless as this is certainly legal: string foo = "bar"; Object o = foo == null ? "gork" : foo; It seems to me that when the ternary branches are of different types, the compiler will not autobox the values to the type object...but when they are of the same type then the autoboxing is automatic. In my mind the first statement should be legal... Can anyone describe why the compiler does not allow this and why the designers of C# chose to do this? I believe this is legal in Java...Though I have not verified this. Thanks. EDIT: I am asking for an understanding of why Java and C# handle this differently, what is going on underneath the scenes in C# that make this invalid. I know how to use ternary, and am not looking for a "better way" to code the examples. I understand the rules of ternary in C#, but I want to know WHY... EDIT (Jon Skeet): Removed "autoboxing" tag as no boxing is involved in this question.

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  • Bitwise AND, Bitwise Inclusive OR question, in Java

    - by Dave
    I've a few lines of code within a project, that I can't see the value of... buffer[i] = (currentByte & 0x7F) | (currentByte & 0x80); It reads the filebuffer from a file, stored as bytes, and then transfers then to buffer[i] as shown, but I can't understand what the overall purpose is, any ideas? Thanks

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  • What does "<<" mean in C#?

    - by Simon G
    Hi, Basically the questions in the title. I'm looking at the MVC 2 source code: public enum HttpVerbs { Get = 1 << 0, Post = 1 << 1, Put = 1 << 2, Delete = 1 << 3, Head = 1 << 4 } and I'm just curious as to what "<<" does. Thanks

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  • using LIKE with logical operators

    - by ryanthegecko
    i can't seem to figure out how to combine LIKE with an OR or AND: DELETE * FROM persons WHERE FirstName = 'Abe' AND LastName LIKE '%coln'; Looks like it should owrk to me but I get error 1064 (syntax0 Is there a correct way to do this?

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