I'm trying to use an autoBuild macro on my Component class so that every class extending Component get an incrementing ID (0, 1, 2, 3...), the problem is that it seems haxe caches some files, so when recompiling some classes won't get rebuilt, so let's say some already built classes have ID 0, 1 and 2. Now I create some new classes and they will also get ID 0, 1 and 2 because the already built classes arent processed so they don't increment the ID. So I end up with some classes with the same ID in the generated code.
The code I use with the autoBuild macro is this:
#if (macro)
public static function build():Array<Field>
{
var pos = Context.currentPos();
var c = macro : {
override function get_index()
{
return $v{componentCounter};
}
public static inline var INDEX = $v{componentCounter};
};
componentCounter++;
switch (c)
{
case TAnonymous(fields):
return Context.getBuildFields().concat(fields);
default:
throw 'unreachable';
}
}
#end
public static var componentCounter = 0;
Is there any way to make something like this work?
Related
I'm really not too sure of how to say this but hopefully through showing the code someone might be able to help.
I'm trying to access the variable data in the semibreve which is an instance of the MainMusicNotesClass and then when i create a new class have it inherit the previous instance data and add on the new fields.
class MainMusicNoteValues {
String noteName;
double noteValue;
AssetImage? noteApperanace;
MainMusicNoteValues(this.noteName, this.noteValue, this.noteApperanace);
}
class MainNotes {
static final semibreve = MainMusicNoteValues(
"Semibreve", 4, const AssetImage('/assets/NoteValues/semibreve.png'));
}
class NVLiteracyNote extends MainMusicNoteValues {
int fastestTime;
int correct;
int incorrect;
NVLiteracyNote([this.correct = 0, this.incorrect = 0, this.fastestTime = 0])
: super('', 0.0, null);
//I understand that the super is the parent constructor and will determine the noteName, NoteValue and Appearance
}
Now I want to have the new variable (with the same name) in another class inherit the previous MainNotes.semibreve data and add on the new fields.
class NVNotes{
static final semibreve = NVLiteracyNote();
}
I'm thinking of how to take the MainNotes.semibreve data and pass in the noteName,noteValue and Appearance into the new NVNotes.semibreve.
Essentially at the end NVNotes.semibreve Should have the data:
noteName: 'Semibreve'
noteValue: 4.0
Appearance: const AssetImage('/assets/NoteValues/semibreve.png')
correct: 0
incorrect: 0
fastestTime: 0
I understand that this could be a terrible explanation of what i am trying to achieve but i would love any help.
I want to initialize a variable class for my Base class and modify it only in some children classes.
The initialization of this class variable is in the header:
class Base{
public:
Base();
int a = 1;
The header of my derived class is:
class ChildA : public Base{
public:
ChildA ();
int a = 2;
}
Problem
I tried to run this:
Base classe*;
classe = new ChildA();
std::cout << classe->a << std::endl;
The problem is that, instead of printing 2 as I expected, it prints 1 (value initialized in my parent class). For other derived classes, I want classe->a to still return 1.
How can I solve this?
What you are seeing are the results of upcasting - having a base class point to a derived class object.
I highly recommend you read more on the topic, but a simplified picture is that the base class "carries" the whole object (base and derived content), but can access only it's original, own content. This is why you see a value from the base class rather than the derived class.
Lippman's C++ Primer has a very comprehensive explanation of upcasting, downcasting, object slicing, and other inheritance-related concepts. This includes implementing virtual functions which give you the functionality to invoke derived class functions through an interface that's common with base.
|----------------|-----------------|
| a (for Base) | a (for ChildA) |
|----------------|-----------------|
\________________/
^
|_ classe
Sorry if my drawing is not that good! As you can see in the above picture, when you create an object of type ChildA, this object contains a part for keeping the data members of the base class (i.e. Base) and another part for the data members of derived class (i.e. ChildA). Considering the fact that you have a pointer to the base class (i.e. classe), this pointer only allows you to access the base member variables and methods. So calling classe->a, returns the Base::a for you, that is 1.
If you change the type of classe pointer into ChildA, in that case calling classe->a will return 2, because it refers to the a inside the derived class, i.e. ChildA::a.
For other derived classes, I want classe->a to still return 1. How can
I solve this?
If you need in some derived classes to access different as, it is better to have a virtual function in the base class and override the base function if needed:
class Base {
public:
Base() {}
virtual int getA() { return a; }
private:
int a = 1;
};
class ChildA : public Base
{
public:
ChildA() {}
int a = 2;
};
class ChildB : public Base
{
public:
ChildB() {}
int getA() { return a; }
private:
int a = 2;
};
Now by calling getA() on a pointer of Base or ChildA, you will have 1, but calling getA on an object of type ChildB will return 2.
This question already has answers here:
Should I initialize variable within constructor or outside constructor [duplicate]
(11 answers)
Closed 6 years ago.
What is the difference between below 2 ways of assigning values for variables of a class.
Class A{
Private Variable v = someValue;
}
vs
Class A{
private Variable v;
//constructor
public A(){
this.v = someValue;
}
}
Can someone please explain?
There is no real difference from a code execution point of view.
As a previous answer says, I prefer declaring the variable outside of the constructor; for example:
public class A {
private int aValue = 100;
}
Instead of
public class A {
private int aValue;
public A() {
this.aValue = 100;
}
}
The reason being that if you have multiple constructors, you do not have to keep writing this.aValue = 100; and you are unable to "forget" to initialize the variable in a constructor.
As others have said however, there are times when it is better to initialize the variable in the constructor.
If it will change based on values passed to it via the constructor, obviously initialize it there.
If the variable you are initializing may throw an error and you need to use try / catch - it is clearly better to initialize it in the constructor
If you are working on a team that uses a specific coding standard and they require you to initialize your variables in the constructor, you should do so.
Given freedom and none of the above, I still declare it at the top - makes it much easier to find all of your variables in one place (in my experience).
See this duplicate answer: https://stackoverflow.com/a/3919225/1274820
What is the difference between below 2 ways of assigning values for
variables of a class.
Generally nothing, but ...
class constructor is an entry point when creating a new instance, so all assignments should be done there for readability and maintainability.
When you want create a new instance you start reading a source code at the constructor. Here is an example. All informations about new instance are in one proper place.
public class C {
private int aValue;
private int bValue;
private int cValue;
private int dValue;
public C(int a, int b) {
this.aValue = a;
this.bValue = b;
this.cValue = a * b;
this.dValue = 1000;
}
}
If you look at the MSIL of this class:
namespace Demo
{
public class MyClass
{
private string str = "hello world";
private int b;
public MyClass(int b)
{
this.b = b;
}
}
}
.method public hidebysig specialname rtspecialname
instance void .ctor(int32 b) cil managed
{
// Code size 25 (0x19)
.maxstack 8
IL_0000: ldarg.0
IL_0001: ldstr "hello world"
IL_0006: stfld string Demo.MyClass::str <---- RIGHT HERE
IL_000b: ldarg.0
IL_000c: call instance void [mscorlib]System.Object::.ctor()
IL_0011: ldarg.0
IL_0012: ldarg.1
IL_0013: stfld int32 Demo.MyClass::b
IL_0018: ret
} // end of method MyClass::.ctor
You can see that the constructor is "injected" with the assignment of this.str = "hello world".
So once your code is compiled, there is no difference what so ever. Yet, there are quite a few good reasons why you should not do it (user1274820's answer has some them)
I'd like to write my own macro for creating property like objects in Haxe.
This question is not so much about properties but more about writing macros.
(probably NME has already a macro for that).
having this class in haxe
class Foo {
#:property var bar:String;
}
I like this to be expanded into
class Foo {
private var bar:String;
public function setBar(_val:String):void {
this.bar = _val;
}
public function getBar():String {
return this.bar;
}
}
I read the corresponding docs but honestly I find them very confusing.
thanks
You might want to take a look at how tinkerbell resolves the same issue: https://github.com/back2dos/tinkerbell/wiki/tink_lang#wiki-accessors
This Type Builder example (pasted below for reference, but there's better description at the link) found in the Haxe Manual is a nice, simple example of adding a function to a Class.
Adding a property would be much the same. I added a trace(field) loop to help get a feel for how they're defined:
Main.hx
#:build(TypeBuildingMacro.build("myFunc"))
class Main {
static public function main() {
trace(Main.myFunc); // my default
}
}
TypeBuildingMacro.hx
import haxe.macro.Context;
import haxe.macro.Expr;
class TypeBuildingMacro {
macro static public function build(fieldName:String):Array<Field> {
var fields = Context.getBuildFields();
for (field in fields) { trace(field); }
var newField = {
name: fieldName,
doc: null,
meta: [],
access: [AStatic, APublic],
kind: FVar(macro : String, macro "my default"),
pos: Context.currentPos()
};
fields.push(newField);
return fields;
}
}
Note that Main.hx must invoke the macro with the #:build metadata, so the compiler knows to run the macro (which adds the function) before processing the Main class itself.
I am having confusion with the following code:
class A
{
int x;
static void F(B b) {
b.x = 1; /* Ok,
I want to know how is this ok, in a static block how a non static
instance variables are called because as I know that static block
gets memory at compile time before execution of a single command
while non static at run time and static method accessing a non static
variable which is not created yet please elaborate me on this
*/
}
}
class B: A
{
static void F(B b) {
b.x = 1; // Error, x not accessible
}
}
Nothing gets memory at compile time. Static fields are indeed placed in the static block of memory when the type gets initialized. Call stacks for static methods are allocated at run time exactly like in case of instance methods.
Now, why static methods don't have access to the instance fields. Consider this:
class A {
public int Value;
static int GetValue() {
return Value;
}
}
There you have a class with an instance field and a static method. Now, somewhere else you try this:
var a1 = new A();
a1.Value = 5;
var a2 = new A();
a2.Value = 10;
int result = A.GetValue();
Now, if compiler allowed this, what value would the result get? 5 or 10 or something else? This just doesn't make sense, because static methods are declared for class as a whole and aren't aware of instances of this class. So in the code of static method you don't know how many (if any) instances of this class exist and can't access their instance fields.
Hope this makes a little sense.
Either you changed the code in question a bit or I didn't read very carefully. Seems like it's completely different problem right now. The variable x is indeed not accessible for the class B because of its level of protection (default in C# is private). Class A can modify X because it's declared in class A and visible to its method. Class B can't do it (you must make x protected or public for that).