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• Is there any way to manipulate variables passed in to a child class constructor before passing it of

  - by Matt

  Hi, Is there any way to delay calling a superclass constructor so you can manipulate the variables first? Eg. public class ParentClass { private int someVar; public ParentClass(int someVar) { this.someVar = someVar; } } public class ChildClass : ParentClass { public ChildClass(int someVar) : base(someVar) { someVar = someVar + 1 } } I want to be able to send the new value for someVar (someVar + 1) to the base class constructor rather than the one passed in to the ChildClass constructor. Is there any way to do this? Thanks, Matt

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• Is it possible to defer member initialization to the constructor body?

  - by Kjir

  I have a class with an object as a member which doesn't have a default constructor. I'd like to initialize this member in the constructor, but it seems that in C++ I can't do that. Here is the class: #include <boost/asio.hpp> #include <boost/array.hpp> using boost::asio::ip::udp; template<class T> class udp_sock { public: udp_sock(std::string host, unsigned short port); private: boost::asio::io_service _io_service; udp::socket _sock; boost::array<T,256> _buf; }; template<class T> udp_sock<T>::udp_sock(std::string host = "localhost", unsigned short port = 50000) { udp::resolver res(_io_service); udp::resolver::query query(udp::v4(), host, "spec"); udp::endpoint ep = *res.resolve(query); ep.port(port); _sock(_io_service, ep); } The compiler tells me basically that it can't find a default constructor for udp::socket and by my research I understood that C++ implicitly initializes every member before calling the constructor. Is there any way to do it the way I wanted to do it, or is it too "Java-oriented" and not feasible in C++? I worked around the problem by defining my constructor like this: template<class T> udp_sock<T>::udp_sock(std::string host = "localhost", unsigned short port = 50000) : _sock(_io_service) { udp::resolver res(_io_service); udp::resolver::query query(udp::v4(), host, "spec"); udp::endpoint ep = *res.resolve(query); ep.port(port); _sock.bind(ep); } So my question is more out of curiosity and to better understand OOP in C++

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• Is it OK to write a constructor which does nothing?

  - by Roman

  To use methods of a class I need to instantiate a class. At the moment the class has not constructor (so I want to write it). But than I have realized that the constructor should do nothing (I do need to specify values of fields). In this context I have a question if it is OK to write constructor which does nothing. For example: public Point() { }

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• Why can't I create an abstract constructor on an abstract C# class?

  - by Anthony D

  I am creating an abstract class. I want each of my derived classes to be forced to implement a specific signature of constructor. As such, I did what I would have done has I wanted to force them to implement a method, I made an abstract one. public abstract class A { abstract A(int a, int b); } However I get a message saying the abstract modifier is invalid on this item. My goal was to force some code like this. public class B : A { public B(int a, int b) : base(a, b) { //Some other awesome code. } } This is all C# .NET code. Can anyone help me out? Update 1 I wanted to add some things. What I ended up with was this. private A() { } protected A(int a, int b) { //Code } That does what some folks are saying, default is private, and the class needs to implement a constructor. However that doesn't FORCE a constructor with the signature A(int a, int b). public abstract class A { protected abstract A(int a, int b) { } } Update 2 I should be clear, to work around this I made my default constructor private, and my other constructor protected. I am not really looking for a way to make my code work. I took care of that. I am looking to understand why C# does not let you do this.

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• Implicit constructor available for all types derived from Base excepted the current type?

  - by Vincent

  The following code sum up my problem : template<class Parameter> class Base {}; template<class Parameter1, class Parameter2, class Parameter> class Derived1 : public Base<Parameter> { }; template<class Parameter1, class Parameter2, class Parameter> class Derived2 : public Base<Parameter> { public : // Copy constructor Derived2(const Derived2& x); // An EXPLICIT constructor that does a special conversion for a Derived2 // with other template parameters template<class OtherParameter1, class OtherParameter2, class OtherParameter> explicit Derived2( const Derived2<OtherParameter1, OtherParameter2, OtherParameter>& x ); // Now the problem : I want an IMPLICIT constructor that will work for every // type derived from Base EXCEPT // Derived2<OtherParameter1, OtherParameter2, OtherParameter> template<class Type, class = typename std::enable_if</* SOMETHING */>::type> Derived2(const Type& x); }; How to restrict an implicit constructor to all classes derived from the parent class excepted the current class whatever its template parameters, considering that I already have an explicit constructor as in the example code ? EDIT : For the implicit constructor from Base, I can obviously write : template<class OtherParameter> Derived2(const Base<OtherParameter>& x); But in that case, do I have the guaranty that the compiler will not use this constructor as an implicit constructor for Derived2<OtherParameter1, OtherParameter2, OtherParameter> ? EDIT2: Here I have a test : (LWS here : http://liveworkspace.org/code/cd423fb44fb4c97bc3b843732d837abc) #include <iostream> template<typename Type> class Base {}; template<typename Type> class Other : public Base<Type> {}; template<typename Type> class Derived : public Base<Type> { public: Derived() {std::cout<<"empty"<<std::endl;} Derived(const Derived<Type>& x) {std::cout<<"copy"<<std::endl;} template<typename OtherType> explicit Derived(const Derived<OtherType>& x) {std::cout<<"explicit"<<std::endl;} template<typename OtherType> Derived(const Base<OtherType>& x) {std::cout<<"implicit"<<std::endl;} }; int main() { Other<int> other0; Other<double> other1; std::cout<<"1 = "; Derived<int> dint1; // <- empty std::cout<<"2 = "; Derived<int> dint2; // <- empty std::cout<<"3 = "; Derived<double> ddouble; // <- empty std::cout<<"4 = "; Derived<double> ddouble1(ddouble); // <- copy std::cout<<"5 = "; Derived<double> ddouble2(dint1); // <- explicit std::cout<<"6 = "; ddouble = other0; // <- implicit std::cout<<"7 = "; ddouble = other1; // <- implicit std::cout<<"8 = "; ddouble = ddouble2; // <- nothing (normal : default assignment) std::cout<<"\n9 = "; ddouble = Derived<double>(dint1); // <- explicit std::cout<<"10 = "; ddouble = dint2; // <- implicit : WHY ?!?! return 0; } The last line worry me. Is it ok with the C++ standard ? Is it a bug of g++ ?

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• In Java, can a final field be initialized from a constructor helper?

  - by csj

  I have a final non-static member: private final HashMap<String,String> myMap; I would like to initialize it using a method called by the constructor. Since myMap is final, my "helper" method is unable to initialize it directly. Of course I have options: I could implement the myMap initialization code directly in the constructor. MyConstructor (String someThingNecessary) { myMap = new HashMap<String,String>(); myMap.put("blah","blahblah"); // etc... // other initialization stuff unrelated to myMap } I could have my helper method build the HashMap, return it to the constructor, and have the constructor then assign the object to myMap. MyConstructor (String someThingNecessary) { myMap = InitializeMyMap(someThingNecessary); // other initialization stuff unrelated to myMap } private HashMap<String,String> InitializeMyMap(String someThingNecessary) { HashMap<String,String> initializedMap = new HashMap<String,String>(); initializedMap.put("blah","blahblah"); // etc... return initializedMap; } Method #2 is fine, however, I'm wondering if there's some way I could allow the helper method to directly manipulate myMap. Perhaps a modifier that indicates it can only be called by the constructor? MyConstructor (String someThingNecessary) { InitializeMyMap(someThingNecessary); // other initialization stuff unrelated to myMap } // helper doesn't work since it can't modify a final member private void InitializeMyMap(String someThingNecessary) { myMap = new HashMap<String,String>(); myMap.put("blah","blahblah"); // etc... }

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• Calling a constructor to reinitialize variables doesn't seem to work?

  - by Matt

  I wanted to run 1,000 iterations of a program, so set a counter for 1000 in main. I needed to reinitialize various variables after each iteration, and since the class constructor had all the initializations already written out - I decided to call that after each iteration, with the result of each iteration being stored in a variable in main. However, when I called the constructor, it had no effect...it took me a while to figure out - but it didn't reinitialize anything! I created a function exactly like the constructor - so the object would have its own version. When I called that, it reinitialized everything as I expected. int main() { Class MyClass() int counter = 0; while ( counter < 1000 ) { stuff happens } Class(); // This is how I tried to call the constructor initially. // After doing some reading here, I tried: // Class::Class(); // - but that didn't work either /* Later I used... MyClass.function_like_my_constructor; // this worked perfectly */ } ...Could someone try to explain why what I did was wrong, or didn't work, or was silly or what have you? I mean - mentally, I just figured - crap, I can call this constructor and have all this stuff reinitialized. Are constructors (ideally) ONLY called when an object is created?

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• Is polymorphism another term for overloading?

  - by yogogo

  Is polymorphism another term for overloading?

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• Overloading *(iterator + n) and *(n + iterator) in a C++ iterator class?

  - by exscape

  (Note: I'm writing this project for learning only; comments about it being redundant are... uh, redundant. ;) I'm trying to implement a random access iterator, but I've found very little literature on the subject, so I'm going by trial and error combined with Wikpedias list of operator overload prototypes. It's worked well enough so far, but I've hit a snag. Code such as exscape::string::iterator i = string_instance.begin(); std::cout << *i << std::endl; works, and prints the first character of the string. However, *(i + 1) doesn't work, and neither does *(1 + i). My full implementation would obviously be a bit too much, but here's the gist of it: namespace exscape { class string { friend class iterator; ... public: class iterator : public std::iterator<std::random_access_iterator_tag, char> { ... char &operator*(void) { return *p; // After some bounds checking } char *operator->(void) { return p; } char &operator[](const int offset) { return *(p + offset); // After some bounds checking } iterator &operator+=(const int offset) { p += offset; return *this; } const iterator operator+(const int offset) { iterator out (*this); out += offset; return out; } }; }; } int main() { exscape::string s = "ABCDEF"; exscape::string::iterator i = s.begin(); std::cout << *(i + 2) << std::endl; } The above fails with (line 632 is, of course, the *(i + 2) line): string.cpp: In function ‘int main()’: string.cpp:632: error: no match for ‘operator*’ in ‘*exscape::string::iterator::operator+(int)(2)’ string.cpp:105: note: candidates are: char& exscape::string::iterator::operator*() *(2 + i) fails with: string.cpp: In function ‘int main()’: string.cpp:632: error: no match for ‘operator+’ in ‘2 + i’ string.cpp:434: note: candidates are: exscape::string exscape::operator+(const char*, const exscape::string&) My guess is that I need to do some more overloading, but I'm not sure what operator I'm missing.

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• How do I go about overloading C++ operators to allow for chaining?

  - by fneep

  I, like so many programmers before me, am tearing my hair out writing the right-of-passage-matrix-class-in-C++. I have never done very serious operator overloading and this is causing issues. Essentially, by stepping through This is what I call to cause the problems. cMatrix Kev = CT::cMatrix::GetUnitMatrix(4, true); Kev *= 4.0f; cMatrix Baz = Kev; Kev = Kev+Baz; //HERE! What seems to be happening according to the debugger is that Kev and Baz are added but then the value is lost and when it comes to reassigning to Kev, the memory is just its default dodgy values. How do I overload my operators to allow for this statement? My (stripped down) code is below. //header class cMatrix { private: float* _internal; UInt32 _r; UInt32 _c; bool _zeroindexed; //fast, assumes zero index, no safety checks float cMatrix::_getelement(UInt32 r, UInt32 c) { return _internal[(r*this->_c)+c]; } void cMatrix::_setelement(UInt32 r, UInt32 c, float Value) { _internal[(r*this->_c)+c] = Value; } public: cMatrix(UInt32 r, UInt32 c, bool IsZeroIndexed); cMatrix( cMatrix& m); ~cMatrix(void); //operators cMatrix& operator + (cMatrix m); cMatrix& operator += (cMatrix m); cMatrix& operator = (const cMatrix &m); }; //stripped source file cMatrix::cMatrix(cMatrix& m) { _r = m._r; _c = m._c; _zeroindexed = m._zeroindexed; _internal = new float[_r*_c]; UInt32 size = GetElementCount(); for (UInt32 i = 0; i < size; i++) { _internal[i] = m._internal[i]; } } cMatrix::~cMatrix(void) { delete[] _internal; } cMatrix& cMatrix::operator+(cMatrix m) { return cMatrix(*this) += m; } cMatrix& cMatrix::operator*(float f) { return cMatrix(*this) *= f; } cMatrix& cMatrix::operator*=(float f) { UInt32 size = GetElementCount(); for (UInt32 i = 0; i < size; i++) { _internal[i] *= f; } return *this; } cMatrix& cMatrix::operator+=(cMatrix m) { if (_c != m._c || _r != m._r) { throw new cCTException("Cannot add two matrix classes of different sizes."); } if (!(_zeroindexed && m._zeroindexed)) { throw new cCTException("Zero-Indexed mismatch."); } for (UInt32 row = 0; row < _r; row++) { for (UInt32 column = 0; column < _c; column++) { float Current = _getelement(row, column) + m._getelement(row, column); _setelement(row, column, Current); } } return *this; } cMatrix& cMatrix::operator=(const cMatrix &m) { if (this != &m) { _r = m._r; _c = m._c; _zeroindexed = m._zeroindexed; delete[] _internal; _internal = new float[_r*_c]; UInt32 size = GetElementCount(); for (UInt32 i = 0; i < size; i++) { _internal[i] = m._internal[i]; } } return *this; }

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• Dependency injection: what belongs in the constructor?

  - by Adam Backstrom

  I'm evaluating my current PHP practices in an effort to write more testable code. Generally speaking, I'm fishing for opinions on what types of actions belong in the constructor. Should I limit things to dependency injection? If I do have some data to populate, should that happen via a factory rather than as constructor arguments? (Here, I'm thinking about my User class that takes a user ID and populates user data from the database during construction, which obviously needs to change in some way.) I've heard it said that "initialization" methods are bad, but I'm sure that depends on what exactly is being done during initialization. At the risk of getting too specific, I'll also piggyback a more detailed example onto my question. For a previous project, I built a FormField class (which handled field value setting, validation, and output as HTML) and a Model class to contain these fields and do a bit of magic to ease working with fields. FormField had some prebuilt subclasses, e.g. FormText (<input type="text">) and FormSelect (<select>). Model would be subclassed so that a specific implementation (say, a Widget) had its own fields, such as a name and date of manufacture: class Widget extends Model { public function __construct( $data = null ) { $this->name = new FormField('length=20&label=Name:'); $this->manufactured = new FormDate; parent::__construct( $data ); // set above fields using incoming array } } Now, this does violate some rules that I have read, such as "avoid new in the constructor," but to my eyes this does not seem untestable. These are properties of the object, not some black box data generator reading from an external source. Unit tests would progressively build up to any test of Widget-specific functionality, so I could be confident that the underlying FormFields were working correctly during the Widget test. In theory I could provide the Model with a FieldFactory() which could supply custom field objects, but I don't believe I would gain anything from this approach. Is this a poor assumption?

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• Purpose of Explicit Default Constructors

  - by Dennis Zickefoose

  I recently noticed a class in C++0x that calls for an explicit default constructor. However, I'm failing to come up with a scenario in which a default constructor can be called implicitly. It seems like a rather pointless specifier. I thought maybe it would disallow Class c; in favor of Class c = Class(); but that does not appear to be the case. Some relevant quotes from the C++0x FCD, since it is easier for me to navigate [similar text exists in C++03, if not in the same places] 12.3.1.3 [class.conv.ctor] A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value initialization (8.5). It goes on to provide an example of an explicit default constructor, but it simply mimics the example I provided above. 8.5.6 [decl.init] To default-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); 8.5.7 [decl.init] To value-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9) with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor); In both cases, the standard calls for the default constructor to be called. But that is what would happen if the default constructor were non-explicit. For completeness sake: 8.5.11 [decl.init] If no initializer is specified for an object, the object is default-initialized; From what I can tell, this just leaves conversion from no data. Which doesn't make sense. The best I can come up with would be the following: void function(Class c); int main() { function(); //implicitly convert from no parameter to a single parameter } But obviously that isn't the way C++ handles default arguments. What else is there that would make explicit Class(); behave differently from Class();? The specific example that generated this question was std::function [20.8.14.2 func.wrap.func]. It requires several converting constructors, none of which are marked explicit, but the default constructor is.

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• Overloading interface buttons, what are the best practices?

  - by XMLforDummies

  Imagine you'll have always a button labeled "Continue" in the same position in your app's GUI. Would you rather make a single button instance that takes different actions depending on the current state? private State currentState = State.Step1; private ContinueButton_Click() { switch(currentState) { case State.Step1: DoThis(); currentState = State.Step2; break; case State.Step2: DoThat(); break; } } Or would you rather have something like this? public Form() { this.ContinueStep2Button.Visible = false; } private ContinueStep1Button_Click() { DoThis(); this.ContinueStep1Button.Visible = false; this.ContinueStep2Button.Visible = true; } private ContinueStep2Button_Click() { DoThat(); }

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• Information overloading

  - by sonyram

  Want to know the amount of information we are dealing  in everyday life, read on http://blogs.msdn.com/stevecla01/archive/2010/03/15/a-yottabyte-you-say.aspx  

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• Why use object.prototype.constructor in OOP javascript?

  - by Matt

  I've recently started reading up on OOP javascript and one thing that authors seem to skip over is when an object A has been declared and suddenly I see "A.prototype.constructor =A; For example, var A = function(){}; // This is the constructor of "A" A.prototype.constructor = A; A.prototype.value = 1; A.prototype.test = function() { alert(this.value); } var a = new A(); // create an instance of A alert(a.value); // => 1 So I run the command in firebug "var A = function(){};" and then "A.Constructor" Which reveals it's a function. I understand this. I run the code "A.prototype.constructor = A;" and I thought this changes the A constructor from Function to A. The constructor property of A has been changed right? Instead when I run "A.constructor" it gives me function () still. What's the point? I also see A.constructor.prototype.constructor.prototype.. what is going on?

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• Subterranean IL: Constructor constraints

  - by Simon Cooper

  The constructor generic constraint is a slightly wierd one. The ECMA specification simply states that it: constrains [the type] to being a concrete reference type (i.e., not abstract) that has a public constructor taking no arguments (the default constructor), or to being a value type. There seems to be no reference within the spec to how you actually create an instance of a generic type with such a constraint. In non-generic methods, the normal way of creating an instance of a class is quite different to initializing an instance of a value type. For a reference type, you use newobj: newobj instance void IncrementableClass::.ctor() and for value types, you need to use initobj: .locals init ( valuetype IncrementableStruct s1 ) ldloca 0 initobj IncrementableStruct But, for a generic method, we need a consistent method that would work equally well for reference or value types. Activator.CreateInstance<T> To solve this problem the CLR designers could have chosen to create something similar to the constrained. prefix; if T is a value type, call initobj, and if it is a reference type, call newobj instance void !!0::.ctor(). However, this solution is much more heavyweight than constrained callvirt. The newobj call is encoded in the assembly using a simple reference to a row in a metadata table. This encoding is no longer valid for a call to !!0::.ctor(), as different constructor methods occupy different rows in the metadata tables. Furthermore, constructors aren't virtual, so we would have to somehow do a dynamic lookup to the correct method at runtime without using a MethodTable, something which is completely new to the CLR. Trying to do this in IL results in the following verification error: newobj instance void !!0::.ctor() [IL]: Error: Unable to resolve token. This is where Activator.CreateInstance<T> comes in. We can call this method to return us a new T, and make the whole issue Somebody Else's Problem. CreateInstance does all the dynamic method lookup for us, and returns us a new instance of the correct reference or value type (strangely enough, Activator.CreateInstance<T> does not itself have a .ctor constraint on its generic parameter): .method private static !!0 CreateInstance<.ctor T>() { call !!0 [mscorlib]System.Activator::CreateInstance<!!0>() ret } Going further: compiler enhancements Although this method works perfectly well for solving the problem, the C# compiler goes one step further. If you decompile the C# version of the CreateInstance method above: private static T CreateInstance() where T : new() { return new T(); } what you actually get is this (edited slightly for space & clarity): .method private static !!T CreateInstance<.ctor T>() { .locals init ( [0] !!T CS$0$0000, [1] !!T CS$0$0001 ) DetectValueType: ldloca.s 0 initobj !!T ldloc.0 box !!T brfalse.s CreateInstance CreateValueType: ldloca.s 1 initobj !!T ldloc.1 ret CreateInstance: call !!0 [mscorlib]System.Activator::CreateInstance<T>() ret } What on earth is going on here? Looking closer, it's actually quite a clever performance optimization around value types. So, lets dissect this code to see what it does. The CreateValueType and CreateInstance sections should be fairly self-explanatory; using initobj for value types, and Activator.CreateInstance for reference types. How does the DetectValueType section work? First, the stack transition for value types: ldloca.s 0 // &[!!T(uninitialized)] initobj !!T // ldloc.0 // !!T box !!T // O[!!T] brfalse.s // branch not taken When the brfalse.s is hit, the top stack entry is a non-null reference to a boxed !!T, so execution continues to to the CreateValueType section. What about when !!T is a reference type? Remember, the 'default' value of an object reference (type O) is zero, or null. ldloca.s 0 // &[!!T(null)] initobj !!T // ldloc.0 // null box !!T // null brfalse.s // branch taken Because box on a reference type is a no-op, the top of the stack at the brfalse.s is null, and so the branch to CreateInstance is taken. For reference types, Activator.CreateInstance is called which does the full dynamic lookup using reflection. For value types, a simple initobj is called, which is far faster, and also eliminates the unboxing that Activator.CreateInstance has to perform for value types. However, this is strictly a performance optimization; Activator.CreateInstance<T> works for value types as well as reference types. Next... That concludes the initial premise of the Subterranean IL series; to cover the details of generic methods and generic code in IL. I've got a few other ideas about where to go next; however, if anyone has any itching questions, suggestions, or things you've always wondered about IL, do let me know.

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• C++ - Constructor or Initialize Method to Startup

  - by Bob Fincheimer

  I want to determine when to do non-trivial initialization of a class. I see two times to do initialization: constructor and other method. I want to figure out when to use each. Choice 1: Constructor does initialization MyClass::MyClass(Data const& data) : m_data() { // does non-trivial initialization here } MyClass::~MyClass() { // cleans up here } Choice 2: Defer initialization to an initialize method MyClass::MyClass() : m_data() {} MyClass::Initialize(Data const& data) { // does non-trivial initialization here } MyClass::~MyClass() { // cleans up here } So to try and remove any subjectivity I want to figure out which is better in a couple of situations: Class that encapsulates a resource (window/font/some sort of handle) Class that composites resources to do something (a control/domain object) Data structure classes (tree/list/etc.) [Anything else you can think of] Things to analyze: Performance Ease of use by other developers How error-prone/opportunities for bugs [Anything else you can think of]

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• Create singleton with parameters in constructor

  This article describes how you can create singletons which uses a constructor with one or more parameters.

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• C#: Struct Constructor: "fields must be fully assigned before control is returned to the caller."

  - by Rosarch

  Here is a struct I am trying to write: public struct AttackTraits { public AttackTraits(double probability, int damage, float distance) { Probability = probability; Distance = distance; Damage = damage; } private double probability; public double Probability { get { return probability; } set { if (value > 1 || value < 0) { throw new ArgumentOutOfRangeException("Probability values must be in the range [0, 1]"); } probability = value; } } public int Damage { get; set; } public float Distance { get; set; } } This results in the following compilation errors: The 'this' object cannot be used before all of its fields are assigned to Field 'AttackTraits.probability' must be fully assigned before control is returned to the caller Backing field for automatically implemented property 'AttackTraits.Damage' must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer. Backing field for automatically implemented property 'AttackTraits.Distance' must be fully assigned before control is returned to the caller. Consider calling the default constructor from a constructor initializer. What am I doing wrong?

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• Why cant we create Object if constructor is in private section?

  - by Abhi

  Dear all I want to know why cant we create object if the constructor is in private section. I know that if i make a method static i can call that method using <classname> :: <methodname(...)>; But why cant we create object is my doubt... I also know if my method is not static then also i can call function by the following... class A { A(); public: void fun1(); void fun2(); void fun3(); }; int main() { A *obj =(A*)malloc(sizeof(A)); //Here we can't use new A() because constructor is in private //but we can use malloc with it, but it will not call the constructor //and hence it is harmful because object may not be in usable state. obj->fun1(); obj->fun2(); obj->fun3(); } So only doubt is why cant we create object when constructor is in private section? Thanks in advance

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• How is the C++ synthesized move constructor affected by volatile and virtual members?

  - by user1827766

  Look at the following code: struct node { node(); //node(const node&); //#1 //node(node&&); //#2 virtual //#3 ~node (); node* volatile //#4 next; }; main() { node m(node()); //#5 node n=node(); //#6 } When compiled with gcc-4.6.1 it produces the following error: g++ -g --std=c++0x -c -o node.o node.cc node.cc: In constructor node::node(node&&): node.cc:3:8: error: expression node::next has side-effects node.cc: In function int main(): node.cc:18:14: note: synthesized method node::node(node&&) first required here As I understand the compiler fails to create default move or copy constructor on line #6, if I uncomment either line #1 or #2 it compiles fine, that is clear. The code compiles fine without c++0x option, so the error is related to default move constructor. However, what in the node class prevents default move constructor to be created? If I comment any of the lines #3 or #4 (i.e. make the destructor non-virtual or make data member non-volatile) it compiles again, so is it the combination of these two makes it not to compile? Another puzzle, line #5 does not cause an compilation error, what is different from line #6? Is it all specific for gcc? or gcc-4.6.1?

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• Why isn't the static constructor of the parent class called when invoking a method on a nested class

  - by Ryan Ische

  Given the following code, why isn't the static constructor of "Outer" called after the first line of "Main"? namespace StaticTester { class Program { static void Main( string[] args ) { Outer.Inner.Go(); Console.WriteLine(); Outer.Go(); Console.ReadLine(); } } public static partial class Outer { static Outer() { Console.Write( "In Outer's static constructor\n" ); } public static void Go() { Console.Write( "Outer Go\n" ); } public static class Inner { static Inner() { Console.Write( "In Inner's static constructor\n" ); } public static void Go() { Console.Write( "Inner Go\n" ); } } } }

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• What is the difference between a constructor and a procedure in Delphi records?

  - by HMcG

  Is there difference in behavior between a constructor call and a procedure call in Delphi records? I have a D2010 code sample I want to convert to D2009 (which I am using). The sample uses a parameterless constructor, which is not permitted in Delphi 2009. If I substitute a simple parameterless procedure call, is there any functional difference for records? I.E. TVector = record private FImpl: IVector; public constructor Create; // not allowed in D2009 end; becomes TVector = record private FImpl: IVector; public procedure Create; // so change to procedure end; As far as I can see this should work, but I may be missing something.

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• How are Flash library symbols constructed? Why are width/height already available in constructor?

  - by Triynko

  Suppose I draw a square on the stage, convert it to a symbol, export it for ActionScript with a classname of "MySquare" (and of course a base class of MovieClip). How is it that in the MySquare constructor, the width and height of this MovieClip are already available? In fact, any named instances in the clip are also available. I'm confused about how the Flash player seems to be able to pre-construct my movie clip by populating its properties and child clips before my constructor for the class ever runs. I thought that it would have to first construct the clip, calling my constructor code, and then construct and add any children, but obviously the player is doing something special for clips designed in the Flash authoring environment.

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• Overloading operator>> to a char buffer in C++ - can I tell the stream length?

  - by exscape

  I'm on a custom C++ crash course. I've known the basics for many years, but I'm currently trying to refresh my memory and learn more. To that end, as my second task (after writing a stack class based on linked lists), I'm writing my own string class. It's gone pretty smoothly until now; I want to overload operator that I can do stuff like cin my_string;. The problem is that I don't know how to read the istream properly (or perhaps the problem is that I don't know streams...). I tried a while (!stream.eof()) loop that .read()s 128 bytes at a time, but as one might expect, it stops only on EOF. I want it to read to a newline, like you get with cin to a std::string. My string class has an alloc(size_t new_size) function that (re)allocates memory, and an append(const char *) function that does that part, but I obviously need to know the amount of memory to allocate before I can write to the buffer. Any advice on how to implement this? I tried getting the istream length with seekg() and tellg(), to no avail (it returns -1), and as I said looping until EOF (doesn't stop reading at a newline) reading one chunk at a time.

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