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  • Should a C++ constructor do real work?

    - by Wade Williams
    I'm strugging with some advice I have in the back of my mind but for which I can't remember the reasoning. I seem to remember at some point reading some advice (can't remember the source) that C++ constructors should not do real work. Rather, they should initialize variables only. The advice when on to explain that real work should be done in some sort of init() method, to be called separately after the instance was created. The situation is I have a class that represents a hardware device. It makes logical sense to me for the constructor to call the routines that query the device in order to build up the instance variables that describe the device. In other words, once new instantiates the object, the developer receives an object which is ready to be used, no separate call to object-init() required. Is there a good reason why constructors shouldn't do real work? Obviously it could slow allocation time, but that wouldn't be any different if calling a separate method immediately after allocation. Just trying to figure out what gotchas I not currently considering that might have lead to such advice.

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  • Is it bad practise to initialise fields outside of an explicit constructor

    - by MrTortoise
    So its monday and we are arguing about coding practises. The examples here are a litttle too simple, but the real deal has several constructors. In order to initialise the simple values (eg dates to their min value) I have moved the code out of the constructors and into the field definitions. public class ConstructorExample { string _string = "John"; } public class ConstructorExample2 { string _string; public ConstructorExample2() { _string = "John"; } } How should it be done by the book. I tend to be very case by case and so am maybe a little lax abotu this kind of thing. However i feel that accams razor tells me to move the initialisation out of multiple constructors. Of course I could always move this shared initialisation into a private method. The question is essentially ... is initialising fields where they are defined as opposed to the constructor bad in any way? The argument I am facing is one of error handling, but i do not feel it is relevant as there are no possible exceptions that won't be picked up at compile time.

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  • calling constructor of the class in the destructor of the same class

    - by dicaprio
    Experts !! I know this question is one of the lousy one , but still I dared to open my mind , hoping I would learn from all. I was trying some examples as part of my routine and did this horrible thing, I called the constructor of the class from destructor of the same class. I don't really know if this is ever required in real programming , I cant think of any real time scenarios where we really need to call functions/CTOR in our destructor. Usually , destructor is meant for cleaning up. If my understanding is correct, why the compiler doesn't complain ? Is this because it is valid for some good reasons ? If so what are they ? I tried on Sun Forte, g++ and VC++ compiler and none of them complain about it. using namespace std; class test{ public: test(){ cout<<"CTOR"<<endl; } ~test() {cout<<"DTOR"<<endl; test(); }};

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  • Why do we need private variables?

    - by rak
    Why do we need private variables in classes in the context of programming? Every book on programming I've read says this is a private variable, this is how you define it but stops there. The wording of these explanations always seemed to me like we really have a crisis of trust in our profession. The explanations always sounded like other programmers are out to mess up our code. Yet, there are many programming languages that do not have private variables. What do private variables help prevent? How do you decide if a particular of properties should be private or not? If by default every field SHOULD be private then why are there public data members in a class? Under what circumstances should a variable be made public?

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  • Tuple struct constructor complains about private fields

    - by Grubermensch
    I am working on a basic shell interpreter to familiarize myself with Rust. While working on the table for storing suspended jobs in the shell, I have gotten stuck at the following compiler error message: tsh.rs:8:18: 8:31 error: cannot invoke tuple struct constructor with private fields tsh.rs:8 let mut jobs = job::JobsList(vec![]); ^~~~~~~~~~~~~ It's unclear to me what is being seen as private here. As you can see below, both of the structs are tagged with pub in my module file. So, what's the secret sauce? tsh.rs use std::io; mod job; fn main() { // Initialize jobs list let mut jobs = job::JobsList(vec![]); loop { /*** Shell runtime loop ***/ } } job.rs use std::fmt; pub struct Job { jid: int, pid: int, cmd: String } impl fmt::Show for Job { /*** Formatter ***/ } pub struct JobsList(Vec<Job>); impl fmt::Show for JobsList { /*** Formatter ***/ }

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  • J2ME private folder(only accessible to my midlet)

    - by Shankar
    I have two midlets, one will download some files form server everyday and the other uses these files. If i download the files to a normal folder the mobile user may delete the folder or files manually. So i need a private folder which is hidden and only accessible for my midlets. I heard about private folders which symbian platform provides for each application which are not accessible to users. I need such a folder for my j2me app. How to create such folder?? Shankar

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  • How to Access a Private Variable?

    - by SoulBeaver
    This question isn't meant to sound as blatantly insulting as it probably is right now. This is a homework assignment, and the spec sheet is scarce and poorly designed to say the least. We have a function: double refuel( int liter, GasStation *gs ) { // TODO: Access private variable MaxFuel of gs and decrement. } Sound simple enough? It should be, but the class GasStation comes with no function that accesses the private variable MaxFuel. So how can I access it anyway using the function refuel? I'm not considering creating a function setFuel( int liter ) because the teacher always complains rather energetically if I change his specification. So... I guess I have to do some sort of hack around it, but I'm not sure how to go about this without explicitely changing the only function in GasStation and giving it a parameter so that I can call it here. Any hints perhaps?

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  • A pragmatic view on private vs public

    - by Denis Gorbachev
    Hello everybody! I've always wondered on the topic of public, protected and private properties. My memory can easily recall times when I had to hack somebody's code, and having the hacked-upon class variables declared as private was always upsetting. Also, there were (more) times I've written a class myself, and had never recognized any potential gain of privatizing the property. I should note here that using public vars is not in my habit: I adhere to the principles of OOP by utilizing getters and setters. So, what's the whole point in these restrictions?

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  • delegating into private parts

    - by FredOverflow
    Sometimes, C++'s notion of privacy just baffles me :-) class Foo { struct Bar; Bar* p; public: Bar* operator->() const { return p; } }; struct Foo::Bar { void baz() { std::cout << "inside baz\n"; } }; int main() { Foo::Bar b; // error: 'struct Foo::Bar' is private within this context Foo f; f->baz(); // fine } Since Foo::Bar is private, I cannot declare b in main. Yet I can call methods from Foo::Bar just fine. Why the hell is this allowed? Was that an accident or by design?

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  • .NET Properties - Use Private Set or ReadOnly Property?

    - by tgxiii
    In what situation should I use a Private Set on a property versus making it a ReadOnly property? Take into consideration the two very simplistic examples below. First example: Public Class Person Private _name As String Public Property Name As String Get Return _name End Get Private Set(ByVal value As String) _name = value End Set End Property Public Sub WorkOnName() Dim txtInfo As TextInfo = _ Threading.Thread.CurrentThread.CurrentCulture.TextInfo Me.Name = txtInfo.ToTitleCase(Me.Name) End Sub End Class // ---------- public class Person { private string _name; public string Name { get { return _name; } private set { _name = value; } } public void WorkOnName() { TextInfo txtInfo = System.Threading.Thread.CurrentThread.CurrentCulture.TextInfo; this.Name = txtInfo.ToTitleCase(this.Name); } } Second example: Public Class AnotherPerson Private _name As String Public ReadOnly Property Name As String Get Return _name End Get End Property Public Sub WorkOnName() Dim txtInfo As TextInfo = _ Threading.Thread.CurrentThread.CurrentCulture.TextInfo _name = txtInfo.ToTitleCase(_name) End Sub End Class // --------------- public class AnotherPerson { private string _name; public string Name { get { return _name; } } public void WorkOnName() { TextInfo txtInfo = System.Threading.Thread.CurrentThread.CurrentCulture.TextInfo; _name = txtInfo.ToTitleCase(_name); } } They both yield the same results. Is this a situation where there's no right and wrong, and it's just a matter of preference?

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  • What is constructor injection?

    - by TheSilverBullet
    I have been looking at the terms constructor injection and dependency injection while going through articles on (Service locator) design patterns. When I googled about constructor injection, I got unclear results, which prompted me to check in here. What is constructor injection? Is this a specific type of dependency injection? A canonical example would be a great help! Edit Revisiting this questions after a gap of a week, I can see how lost I was... Just in case anyone else pops in here, I will update the question body with a little learning of mine. Please do feel free to comment/correct. Constructor injection and property injection are two types of Dependency Injection.

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  • private class calling a method from its outer class

    - by oxinabox.ucc.asn.au
    Ok, so I have a class for a "Advanced Data Structure" (in this case a kinda tree) SO I implimented a Iterator as a private class with in it. So the iterator needs to implement a remove function to remove the last retuirned element. now my ADT already impliments a remove function, and in this case there is very little (thinking about it, i think nothing) to be gain by implimenting a different remove function for the iterator. so how do I go about calling the remove from my ADT sketch of my struture: public class ADT { ... private class ADT_Iterator impliments java.util.Itorator{ ... public void remove(){ //where I want to call the ADT's remove function from } ... public void remove( Object paramFoo ) { ... } ... } So just calling remove(FooInstance) won't work (will it?) and this.remove(FooInstance) is the same thing. what do i call? (and changign the name of the ADT's remove function is not an option, as that AD T has to meet an Interace wich I am note at liberty to change) I could make both of them call a removeHelper functon, I guess...

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  • class modifier issues in C# with "private" classes

    - by devoured elysium
    I had a class that had lots of methods: public class MyClass { public bool checkConditions() { return checkCondition1() && checkCondition2() && checkCondition3(); } ...conditions methods public void DoProcess() { FirstPartOfProcess(); SecondPartOfProcess(); ThirdPartOfProcess(); } ...process methods } I identified two "vital" work areas, and decided to extract those methods to classes of its own: public class MyClass { private readonly MyClassConditions _conditions = new ...; private readonly MyClassProcessExecution = new ...; public bool checkConditions() { return _conditions.checkConditions(); } public void DoProcess() { _process.DoProcess(); } } In Java, I'd define MyClassConditions and MyClassProcessExecution as package protected, but I can't do that in C#. How would you go about doing this in C#? Setting both classes as inner classes of MyClass? I have 2 options: I either define them inside MyClass, having everything in the same file, which looks confusing and ugly, or I can define MyClass as a partial class, having one file for MyClass, other for MyClassConditions and other for MyClassProcessExecution. Defining them as internal? I don't really like that much of the internal modifier, as I don't find these classes add any value at all for the rest of my program/assembly, and I'd like to hide them if possible. It's not like they're gonna be useful/reusable in any other part of the program. Keep them as public? I can't see why, but I've let this option here. Any other? Name it! Thanks

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  • C++: Retriving values of static const variables at a constructor of a static variable

    - by gilbertc
    I understand that the code below would result segmentation fault because at the cstr of A, B::SYMBOL was not initialized yet. But why? In reality, A is an object that serves as a map that maps the SYMBOLs of classes like B to their respective IDs. C holds this map(A) static-ly such that it can provide the mapping as a class function. The primary function of A is to serve as a map for C that initializes itself at startup. How should I be able to do that without segmentation fault, provided that I can still use B::ID and B::SYMBOL in the code (no #define pls)? Thanks! Gil. class A { public: A() { std::cout<<B::ID<<std::endl; std::cout<<B::SYMBOL<<std::endl; } }; class B { public: static const int ID; static const std::string SYMBOL; } const int B::ID = 1; const std::string B::SYMBOL = "B"; class C { public: static A s_A; }; A C::s_A; int main(int c, char** p) { }

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  • add methods in subclasses within the super class constructor

    - by deamon
    I want to add methods (more specifically: method aliases) automatically to Python subclasses. If the subclass defines a method named 'get' I want to add a method alias 'GET' to the dictionary of the subclass. To not repeat myself I'd like to define this modifation routine in the base class. But if I check in the base class init method, there is no such method, since it is defined in the subclass. It will become more clear with some source code: class Base: def __init__(self): if hasattr(self, "get"): setattr(self, "GET", self.get) class Sub(Base): def get(): pass print(dir(Sub)) Output: ['__doc__', '__init__', '__module__', 'get'] It should also contain 'GET'. Is there a way to do it within the base class?

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  • Passing constructor arguments when using StructureMap

    - by Mosh
    Hello, I'm using StructureMap for my DI. Imagine I have a class that takes 1 argument like: public class ProductProvider : IProductProvider { public ProductProvider(string connectionString) { .... } } I need to specify the "connectionString at run-time when I get an instance of IProductProvider. I have configured StructureMap as follows: ForRequestedType<IProductProvider>.TheDefault.Is.OfConcreteType<ProductProvider>(). WithCtorArgument("connectionString"); However, I don't want to call EqualTo("something...") method here as I need some facility to dynamically specify this value at run-time. My question is: how can I get an instance of IProductProvider by using ObjectFactory? Currently, I have something like: ObjectFactory.GetInstance<IProductProvider>(); But as you know, this doesn't work... Any advice would be greatly appreciated.

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  • Copy constructor demo (crashing... case 2)

    - by AKN
    Please have a glance at this program: class CopyCon { public: char *name; CopyCon() { name = new char[20]; name = "Hai";//_tcscpy(name,"Hai"); } CopyCon(const CopyCon &objCopyCon) { name = new char[_tcslen(objCopyCon.name)+1]; _tcscpy(name,objCopyCon.name); } ~CopyCon() { if( name != NULL ) { delete[] name; name = NULL; } } }; int main() { CopyCon obj1; CopyCon obj2(obj1); cout<<obj1.name<<endl; cout<<obj2.name<<endl; } This program crashes on execution. Error: "Expression: _BLOCK_TYPE_IS_VALID(pHead-nBlockUse)" If I assign "Hai" to name using aasignment operator, its crashing. Where as when I use string func _tcscpy to assign "Hai" to name, its working perfectly. Can some one explain why so?

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