I want to access default constructor from a class, (after I create another constructors with any parameters), without declared him!
How can I access default constructor without declared him?
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Often times I have an instance field that needs to be initialized in a constructor. For example, it might be needed to be calculated based on other instance fields, hence I can't initialize it inline (where declared) or with a constructor initializer list.
But it either has to be nullable, or declared late, if I need to initialize it in constructor. The rationale behind late keyword is that programmer states "I'll initialize this before using, trust me", when it can not be determined by the compiler that initialization will take place before first usage. BUT: this "programmer guarantee" seems A) terrible and B) unnecessary in case of constructors, because it can be determined by compiler whether the field was initialized in a constructor (and constructor itself is obviously guaranteed to execute before any other instance methods).
Obvious downside to using late fields in such scenarios is that nothing enforces them compile-time to be actually initialized during construction (or anywhere, for that matter). Plus, every time the late field is read, a runtime check is inserted to make sure it has been assigned a value - I don't need that when I initialize in constructors.
Therefore, it seems that, technically it should be possible to have non-nullable non-late fields that are initialized within a constructor body (and if they are not - compiler can throw an error).
So what is the rationale of requiring constructor-initialized fields to be either nullable, or declared as late? Is there a technical reason why this limitation is imposed, or is it just a design oversight by the Dart team?
Dart executes constructor bodies inside-out, from base class to derived class. This allows virtual dispatch to occur in the constructor body. The fact that virtual dispatch can occur in the constructor body means that the compiler cannot statically determine what code will be executed by the constructor body, and therefore it cannot deduce what the constructor body might ultimately initialize.
That the constructor body can execute arbitrary code (including callbacks) that might initialize members or that might depend on initialized members makes it even more complicated.
Furthermore, allowing members to be not initialized when the constructor body runs would be error-prone and a source for confusion. For example, with:
class SomeClass {
int member;
SomeClass() {
updateMember(0);
}
void updateMember(int value) {
print(value); // Oops.
member = value;
}
}
With Dart's current design, all instance methods (and their overrides) can be guaranteed that members are initialized when the method is called. If members were allowed to be uninitialized when the constructor body is executed, that would not longer be true, and all instance methods then would need to consider if they might be invoked from the constructor (or from a base class constructor), possibly indirectly from other method calls, and whether accessed members might not be initialized yet.
(I'll grant that that previous point isn't terribly strong since it currently can still happen that a member is initialized to an object that the constructor body must mutate, but typically instance methods receiving an empty List, Map, etc. is less of a problem than receiving uninitialized members. The above situation also could happen with late members, but that's the baggage that comes with choosing to use late.)
Null-safety disallows the possibility of accessing uninitialized non-late variables, but your proposal would make that possible.
Also, because there is a distinction between initializing members via an initializer list and via a constructor body, people are encouraged to use initializer lists when possible.
The point that you are ignoring here is that when you want to pass a variable to a constructor you will definitely initialize it, otherwise you wouldn't be able to use that widget because you have to pass the variables needed to its constructor. so this late or nullable keywords can be used for the values that you are trying to pass to a widget and not in the widget itself that you are passing them to, but before it.
I'm reading through the Swift documentation about type methods and type properties, and I cannot for the life of me figure out why it says this particular thing (in bold):
Within the body of a type method, the implicit self property refers to
the type itself, rather than an instance of that type. For
structures and enumerations, this means that you can use self to
disambiguate between type properties and type method parameters, just
as you do for instance properties and instance method parameters.
More generally, any unqualified method and property names that you use
within the body of a type method will refer to other type-level
methods and properties. A type method can call another type method
with the other method’s name, without needing to prefix it with the
type name. Similarly, type methods on structures and enumerations
can access type properties by using the type property’s name without a
type name prefix.
So, why is this pointing out structures and enumerations being able to do these things when, as far as I know, you can do these things with any kind of type methods/parameters (i.e. classes as well)? It makes me think I'm missing something.
The page in the documentation I'm looking at is here: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Methods.html
Apparently I wasn't the only one with this question. I'm not sure how I didn't see this post before: https://softwareengineering.stackexchange.com/questions/276962/static-properties-and-implicit-self-property-in-structures-and-enumerations-vs
My superclass initializes member variables through reflection via annotation-based injection. How do I prevent a subclass from overriding the value of a member variable set by the superclass? The problem is that the primary constructor of the superclass initializes the subclass's member variable and then the subclass's primary constructor (which is called subsequently) writes over that value when the member variable's declaration is executed.
The answer I discovered is to initialize the subclass's member variable with an underscore. This is actually explained quite nicely in the language specification in section 4.2 which talks about giving variable's initialized with an underscore a getter function that returns its current value.
I have a C# class called MyCustomRuleTemplate which is inherited from Ektron.Cms.Content.Targeting.Rules.RuleTemplate class. In that I have added a constructor such as below
public MyCustomRuleTemplate(): base("someKey")
{
//Some code here
}
Its working fine without any error. If I given it as
public MyCustomRuleTemplate()
{
//Some code here
}
Im getting error like 'Ektron.Cms.Content.Targeting.Rules.RuleTemplate' does not contain a constructor that takes 0 arguments.
Can anybody help me to know why it is?
The reason you are seeing "does not contain a constructor that takes 0 arguments" when instantiating your class object using the second constructor is because when you call your constructor, c# tries to call the constructor on the base class as well, which in this case takes a parameter.
See this post on msdn:
http://msdn.microsoft.com/en-us/library/ms173115%28v=vs.80%29.aspx
Key parts:
"In this example, the constructor for the base class is called before the block for the constructor is executed. The base keyword can be used with or without parameters. Any parameters to the constructor can be used as parameters to base, or as part of an expression. For more information, see base.
In a derived class, if a base-class constructor is not called explicitly using the base keyword, then the default constructor, if there is one, is called implicitly."
And: "If a base class does not offer a default constructor, the derived class must make an explicit call to a base constructor using base."
I've created an ADO.NET model called EF and added a DbContext generator, which populates my /Model folder with an EF.tt and .cs files, one for each entity.
in general the system creates classes with parameterless constructors... for some reason I can't fathom I have an entity that's missing this constructore. It is not an abstract class, has not base type and has public access. I have tons of other such classes but they all have parameterless constructors. I've googled and looked around VS trying to figure what's special about this one, and how I can make it generate the constructor, but find no answer.
I can always create this in a partial definition but I'd rather figure it out. Also, if I right-mouse click over the EF.tt I see a choice in the menu called "Run Custom Tool" but when I select it nothing seems to happen. How does one regenerate the .cs files?
p.s. yes, I have cleaned and rebuilt the solution in case it just got messed up but still problem
In C# (are you using C#?):
When you define no constructor in your class, a parameterless constructor will be created by compiler by default
When you define parameterless constructor (and maybe some others) the parameterless constructor will also be present as you defined it
When you define more than zero constructors, but no parameterless one, the compiler does not create a parameterless constructor for you. In this case it's your responsibility to define it (in partial class or not).
Default constructor exists by default, it is not generated. If class doesn't have any explicitly defined constructor it always have default parameterless constructor. If you specify any constructor elsewhere (partial class) default parameterless constructor doesn't exist any more and you have to create it yourselves if you want to use it (EF always wants to use it).