I would like to use Gtk.Widget's ìnstall_style_property () on a widget I'm writing. In the docs, this method is declared as static, so I am wondering why valac still complains that I am calling it from a static method:
public class MyClass : Gtk.Widget {
public static void init () {
ParamSpecDouble _the_property = new ParamSpecDouble
(
"dummy", "dummy", "dummy,
0, double.MAX, 0,
ParamFlags.READWRITE | ParamFlags.STATIC_STRINGS
);
install_style_property (_the_property);
}
}
void main (string? argv) {
Gtk.init (ref argv);
MyClass.init ();
}
The error message:
test.vala:11.9-11.46: error: Access to instance member `Gtk.Widget.install_style_property' denied
If this does not work, what is the preferred pattern to install custom style properties to a custom widget in Gtk? Personally, I would prefer not to have to call an init () before using my widget, but as adding style properties is done per-class instead of per-instance, putting it into the constructor does not seem right, either.
install_style_property() is not static; it's actually a class method. valadoc.org is showing static for some reason; you'll probably have to report that as a bug (if it hasn't already).
class methods operate on a class itself. GObject classes have shared metadata, and these methods modify that metadata. Such metadata should only be modified when the class is first initialized; therefore, the methods should only be called within that class's GObjectClass.class_init() method. In Vala, this is the static construct method:
public class MyClass : Gtk.Widget {
static construct {
ParamSpecDouble _the_property = new ParamSpecDouble
(
"dummy", "dummy", "dummy,
0, double.MAX, 0,
ParamFlags.READWRITE | ParamFlags.STATIC_STRINGS
);
install_style_property (_the_property);
}
}
Related
I m preparing a class in my flutterfire project and their I want to use some method Which can't change further so that I want to know consept of static keyword in Dart ?
"static" means a member is available on the class itself instead of on instances of the class. That's all it means, and it isn't used for anything else. static modifies members.
Static methods
Static methods (class methods) don’t operate on an instance, and thus don’t have access to this. They do, however, have access to static variables.
void main() {
print(Car.numberOfWheels); //here we use a static variable.
// print(Car.name); // this gives an error we can not access this property without creating an instance of Car class.
print(Car.startCar());//here we use a static method.
Car car = Car();
car.name = 'Honda';
print(car.name);
}
class Car{
static const numberOfWheels =4;
Car({this.name});
String name;
// Static method
static startCar(){
return 'Car is starting';
}
}
static keyword in dart used to declare a variable or method that belongs to just the class not the instants which means the class has only one copy of that variable or method and those static variables(class variables) or static methods(class methods) can not be used by the instances created by the class.
for example if we declare a class as
class Foo {
static String staticVariable = "Class variable";
final String instanceVariable = "Instance variable";
static void staticMethod(){
print('This is static method');
}
void instanceMethod(){
print('instance method');
}
}`
the thing here to remember is static variables are created only once and every instance crated by the class has different instance variables. therefore you can not call static variables form the class instances.
following codes are valid,
Foo.staticVariable;
Foo().instanceVariable;
Foo.staticMethod();
Foo().instanceMethod();
there for following codes will give errors
Foo().staticVariable;
Foo.instanceVariable;
Foo().staticMethod;
Foo.instanceMethod
Use of static variables and methods
you can use static variables when you have constant values or common values that are relevant for the class.
you can read more here - https://dart.dev/guides/language/language-tour#class-variables-and-methods
I have created a library in C# to be used in Q# programs. The library has two scripts, a C# class library called "Class1.cs" and a matching Q# script called "Util.qs", I share the code snippet of each here:
Class1.cs:
using System;
using Microsoft.Quantum.Simulation.Common;
using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
namespace MyLibrary {
class Class1 : QuantumSimulator {
static void Method_1 (string str) { ... }
.
.
.
}
}
Util.qs:
namespace MyLibrary {
operation Op_1 (str : String) : Unit { body intrinsic; }
}
There is another Q# program in a different namespace that uses the namespace "MyLibrary" so after adding reference, in this Q# program I have:
namespace QSharp
{
open Microsoft.Quantum.Canon;
open Microsoft.Quantum.Intrinsic;
open MyLibrary;
operation TestMyLibrary() : Unit {
Op_1("some string");
}
}
When I execute "dotnet run" in the terminal I receive this message:
Unhandled Exception: System.AggregateException: One or more errors
occurred. (Cannot create an instance of MyLibrary.Op_1 because it is
an abstract class.) ---> System.MemberAccessException: Cannot create
an instance of MyLibrary.Op_1 because it is an abstract class.
How can I fix it?
Thanks.
UPDATE:
Following Mariia' answer and also checking Quantum.Kata.Utils, I changed my code as following:
So, I changed Class1 script to:
using System;
using Microsoft.Quantum.Simulation.Common;
using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
namespace MyLibrary {
class Class1 : QuantumSimulator {
private string classString = "";
public Class1() { }
public class Op_1_Impl : Op_1{
string cl_1;
public Op_1_Impl (Class1 c) : base (c) {
cl_1 = c.classString;
}
public override Func<string, QVoid> Body => (__in) => {
return cl1;
};
}
}
Now the error messages are:
error CS0029: Cannot implicitly convert type 'string' to 'Microsoft.Quantum.Simulation.Core.QVoid'
error CS1662: Cannot convert lambda expression to intended delegate type because some of the return types
in the block are not implicitly convertible to the delegate return type
Having checked Quantum.Kata.Utils, I realised I need to create a field and a constructor for Class1 which is a base class and also I should override Func<string, QVoid> as the Op_1 parameter is string type. But I am not sure if each of these steps individually is done properly?
Second Update:
I have changed the previous c# code in first update to the following one:
using System;
using Microsoft.Quantum.Simulation.Common;
using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
namespace MyLibrary {
class Class1 : QuantumSimulator {
public Class1() { }
public class Op_1_Impl : Op_1{
Class1 cl_1;
public Op_1_Impl (Class1 c) : base (c) {
cl_1 = c;
}
public override Func<string, QVoid> Body => (__in) => {
return QVoid.Instance;
};
}
}
Now the error message is the same as the very first one:
Unhandled Exception: System.AggregateException: One or more errors
occurred. (Cannot create an instance of MyLibrary.Op_1 because it is
an abstract class.) ---> System.MemberAccessException: Cannot create
an instance of MyLibrary.Op_1 because it is an abstract class.
And also in this new code shouldn't the constructor public Class1() { } have a parameter? if so what datatype?
In your code, there is nothing connecting the Q# operation Op_1 and the C# code that you intend to implement it in Method_1.
Q# operations are compiled into classes. To define a C# implementation for a Q# operation with the intrinsic body, you have to define a class that implements the abstract class into which your Q# operation gets compiled; so you would have something like public class Op_1_Impl : Op_1.
Getting all the piping right can be a bit tricky (it's a hack, after all!) I would recommend looking at the operation GetOracleCallsCount and its C# implementation to see the exact pieces that have to be in place for it to work.
For the updated question, the signature of your method says that it takes string as an input and returns nothing (QVoid), but the implementation tries to return a string cl_1, so you get a Cannot implicitly convert type 'string' to 'Microsoft.Quantum.Simulation.Core.QVoid'.
To provide a custom C# emulation for your Op_1 Q# operation, you'll need to replace your Class1.cs with something like this:
using System;
using Microsoft.Quantum.Simulation.Core;
namespace MyLibrary
{
public partial class Op_1
{
public class Native : Op_1
{
public Native(IOperationFactory m) : base(m) { }
public override Func<String, QVoid> Body => (str) =>
{
// put your implementation here.
Console.WriteLine(str);
return QVoid.Instance;
};
}
}
}
You can then run the Test1Library using the QuantumSimulator.
That being said, as Mariia said, this is kind of hacky, undocumented functionality that might change in the future, may I ask why you need this?
I work on a project based on Vertx the following line:
vertx.eventBus().send(MyClass.ADDRESS, requestBody, new Handler<Message<Object>>() {
....
}
public class MyClass implements Handler<Message<JsonObject>> {
public static final String ADDRESS = "coupons.api.manager";
...
#Override
public void handle(Message<JsonObject> msg) {
...
}
}
Whereas MyClass.ADDRESS is a static field of type string in the class MyClass, I found out that the line vertx.eventBus(...) creates an object of MyClass and then runs the handle() function.
My question is why? MyClass.ADDRESS is a string, and a static one. How does vertx "know" that it has to create an object from a class that this string is an attribute of?
I looked at the documentation of the send() function: http://vertx.io/docs/apidocs/io/vertx/core/eventbus/EventBus.html#send-java.lang.String-java.lang.Object-io.vertx.core.eventbus.DeliveryOptions-io.vertx.core.Handler-
and it says that the first argument in the function is "the address to send it to". OK. But, who said that the address means instantiating this class?
I made a small research, and yes. Vertx, behind the curtains, connects all the classes that implement Handler> to the string value they have in the attribute ClassName.ADDRESS.
When the statement:
vertx.eventBus().send(MyClass.ADDRESS, requestBody, new Handler<Message<Object>>() {
....
}
is invoked, a new thread is created and runs the handle method in the class MyClass.
If I recall correctly, static linkage means that a variable or function is local to its compilation unit. Meaning that there could be variables or functions with the same name and parameters in other compilation units.
I want this for a class.
Let's say I have multiple compilation units that need to ensure proper deletion at exit. So I use atexit handlers. But every compilation unit should put its own atexit-handler into place.
I did this by making a class like this:
class Init {
private:
static Init* self;
Init() {
std::atexit(Init::cleanup);
}
static void cleanup() {
// Do cleanup
}
};
Init* Init::self = new Init;
But if I have multiple classes called Init in various CUs the linker gets confused. And the compiler won't let me do static class Init {...}.
How do I archive my cleanup (if possible with classes that are called Init)?
You can put your class into an unnamed namespace.
Then, although types don't have linkage, the same effect is borne.
// Everything inside here is unique to this TU
namespace {
class Init { /** whatever **/ };
Init* Init::self = new Init;
}
int main()
{
// "Init" in here will refer to that which you created above
}
I'm messing around with Eclipse(and java in general) for the first time in about a year. among the things I have forgotten is the following:
I have a function (void callvote() that I am hoping will be activated by my main function (that is, automatically, relatively early in the program). I currently have it within the same class (body) as the main function itself.
I try to call it withcallvote(); and get an error, "- Cannot make a static reference to the non-static method callvote() from the type body"
my function callvote is, at the moment, in the space below main and simply says
public void callvote()
{
}
am i committing a horrible sin by putting more functions in the same class as main?
is this a relatively easy fix that I missed somehow?
What does this error mean?
Have I woken Azatoth with this code?
Thanks in advance,
Tormos
Without the static modifier callvote is implicitly an instance method - you need an instance of a class to call it.
You could mark it as static also:
public static void callvote() ...
Or create an instance of the declaring class:
MyClass instance = new MyClass();
instance.callvote();
main() is a static method, meaning you can call it directly from a class whereas non-static members can only be called from an object. In order for you to call the callvote() method you need to first instantiate an object of your class:
public static void main(String [ ] args) {
MyClass myObject = new MyClass();
myObject.callvote();
}
Another way to avoid the error is to make you callvote() method static as well, but it's usually not what you want to do (but it depends on the nature of your class and method).
This post describes some of the dangers with the overuse of static methods: Class with single method -- best approach?
Try this:
public class Main {
public static void main(String[] args) {
new Main().callvote()
}
}
the main() entry point of your java program is static. You cannot call a non static method from a static one.
So you have to instanciate your Class first and call the method after.