I have a function which returns QFuture object as a result of a QtConcurrent::run computation. However, there could be some conditions before running the concurrent method where I need to manually return a value-holding future from my function.
QFuture<bool> foo(const QString &bar)
{
if (bar.isEmpty()) {
return QFuture<bool>(false); // This does not work.
// Here I need to return from the function, but I don't know how to do it.
}
return QtConcurrent::run([=]() -> bool {
// Asynchronous computations...
});
}
How to manually create the QFuture object?
Or (more globally) how to properly return from such method?
When there's no data to return, things are easy - this should be the first thing to try anyway in modern C++:
return {};
Or, if you're targeting some obsolete platform (<Qt 5.6):
return QFuture<bool>();
That way you get an invalid future. There's no way to directly create a future that carries preset data, you'd have to use QFutureInterface for that:
// https://github.com/KubaO/stackoverflown/tree/master/questions/qfuture-immediate-50772976
#include <QtConcurrent>
template <typename T> QFuture<T> finishedFuture(const T &val) {
QFutureInterface<T> fi;
fi.reportFinished(&val);
return QFuture<T>(&fi);
}
QFuture<bool> foo(bool val, bool valid) {
if (!valid)
return {};
return finishedFuture(val);
}
int main() {
Q_ASSERT(foo(true, true));
Q_ASSERT(!foo(false, true));
Q_ASSERT(foo(false, false).isCanceled());
Q_ASSERT(foo(true, false).isCanceled());
}
Related
I'd like to make wrapper to implement simple data binding pattern -- while some data have been modified all registered handlers are got notified. I have started with this (for js target):
class Main {
public static function main() {
var target = new Some();
var binding = new Bindable(target);
binding.one = 5;
// binding.two = 0.12; // intentionally unset field
binding.three = []; // wrong type
binding.four = 'str'; // no such field in wrapped class
trace(binding.one, binding.two, binding.three, binding.four, binding.five);
// outputs: 5, null, [], str, null
trace(target.one, target.two, target.three);
// outputs: 5, null, []
}
}
class Some {
public var one:Int;
public var two:Float;
public var three:Bool;
public function new() {}
}
abstract Bindable<TClass>(TClass) {
public inline function new(source) { this = source; }
#:op(a.b) public function setField<T>(name:String, value:T) {
Reflect.setField(this, name, value);
// TODO notify handlers
return value;
}
#:op(a.b) public function getField<T>(name:String):T {
return cast Reflect.field(this, name);
}
}
So I have some frustrating issues: interface of wrapped object doesn't expose to wrapper, so there's no auto completion or strict type checking, some necessary attributes can be easily omitted or even misspelled.
Is it possible to fix my solution or should I better move to the macros?
I almost suggested here to open an issue regarding this problem. Because some time ago, there was a #:followWithAbstracts meta available for abstracts, which could be (or maybe was?) used to forward fields and call #:op(a.b) at the same time. But that's not really necessary, Haxe is powerful enough already.
abstract Binding<TClass>(TClass) {
public function new(source:TClass) { this = source; }
#:op(a.b) public function setField<T>(name:String, value:T) {
Reflect.setField(this, name, value);
// TODO notify handlers
trace("set: $name -> $value");
return value;
}
#:op(a.b) public function getField<T>(name:String):T {
trace("get: $name");
return cast Reflect.field(this, name);
}
}
#:forward
#:multiType
abstract Bindable<TClass>(TClass) {
public function new(source:TClass);
#:to function to(t:TClass) return new Binding(t);
}
We use here multiType abstract to forward fields, but resolved type is actually regular abstract. In effect, you have completion working and #:op(a.b) called at the same time.
You need #:forward meta on your abstract. However, this will not make auto-completion working unless you remove #:op(A.B) because it shadows forwarded fields.
EDIT: it seems that shadowing happened first time I added #:forward to your abstract, afterwards auto-completion worked just fine.
Flow's dynamic code example indicates that Flow can figure out runtime type-checking:
function foo(x) {
if (typeof x === 'string') {
return x.length; // flow is smart enough to see this is safe
} else {
return x;
}
}
var res = foo('Hello') + foo(42);
But in real life, typeof isn't good enough. I usually use lodash's type-checking functions (_.isFunction, _.isString etc), which handle a lot of edge cases.
The problem is, if we change the example to use lodash for the runtime type-checking, Flow no longer understands it:
function foo(x) {
if (_.isString(x)) {
return x.length; // warning: `length` property not found in Number
} else {
return x;
}
}
var res = foo('Hello') + foo(42);
I tried using iflow-lodash but it doesn't seem to make a difference here.
What's the best solution to make Flow understand code that uses lodash for runtime type-checking? I'm new to Flow btw.
This would depend on having predicate types in your lodash libdefs.
Predicate types have recently been added to Flow. Although they are still in an experimental state so I would recommend being careful about their usage for anything serious for now.
function isString(x): boolean %checks { // << declare that the method is a refinement
return typeof x === 'string';
}
function method(x: string | number): number {
if (isString(x)) { // << valid refinement
return x.charCodeAt(0); // << no errors
} else {
return x;
}
}
[try it out]
Note: This answer may quickly fall out of date in one of the next releases as this is a brand new feature. Check out Flow's changelog for the latest information.
The solution for now if possible is to use the built-in refinements.
function method(x: string | number): number {
if (typeof x === "string") { // << Inline the check
return x.charCodeAt(0);
} else {
return x;
}
}
The most obvious solution for this specific case is:
if (_.isString(x) && typeof x === 'string') {
In general, you might be able to overcome Flow errors with creative error suppression, like this:
if (_.isString(x)) {
// #ManuallyTyped
var xStr: string = x;
return xStr.length;
} else { ... }
Make sure to define // #ManuallyTyped as a custom suppress_comment in your flow config file for this to work. You might need an ugly regex for that, see flow docs.
It's been a while since I've last done this, but if I recall correctly Flow will assume that your xStr is a string, while the rest of type checking will work just fine.
I'm not sure of the best approach for handling scoping of "this" in TypeScript.
Here's an example of a common pattern in the code I am converting over to TypeScript:
class DemonstrateScopingProblems {
private status = "blah";
public run() {
alert(this.status);
}
}
var thisTest = new DemonstrateScopingProblems();
// works as expected, displays "blah":
thisTest.run();
// doesn't work; this is scoped to be the document so this.status is undefined:
$(document).ready(thisTest.run);
Now, I could change the call to...
$(document).ready(thisTest.run.bind(thisTest));
...which does work. But it's kinda horrible. It means that code can all compile and work fine in some circumstances, but if we forget to bind the scope it will break.
I would like a way to do it within the class, so that when using the class we don't need to worry about what "this" is scoped to.
Any suggestions?
Update
Another approach that works is using the fat arrow:
class DemonstrateScopingProblems {
private status = "blah";
public run = () => {
alert(this.status);
}
}
Is that a valid approach?
You have a few options here, each with its own trade-offs. Unfortunately there is no obvious best solution and it will really depend on the application.
Automatic Class Binding
As shown in your question:
class DemonstrateScopingProblems {
private status = "blah";
public run = () => {
alert(this.status);
}
}
Good/bad: This creates an additional closure per method per instance of your class. If this method is usually only used in regular method calls, this is overkill. However, if it's used a lot in callback positions, it's more efficient for the class instance to capture the this context instead of each call site creating a new closure upon invoke.
Good: Impossible for external callers to forget to handle this context
Good: Typesafe in TypeScript
Good: No extra work if the function has parameters
Bad: Derived classes can't call base class methods written this way using super.
Bad: The exact semantics of which methods are "pre-bound" and which aren't create an additional non-typesafe contract between your class and its consumers.
Function.bind
Also as shown:
$(document).ready(thisTest.run.bind(thisTest));
Good/bad: Opposite memory/performance trade-off compared to the first method
Good: No extra work if the function has parameters
Bad: In TypeScript, this currently has no type safety
Bad: Only available in ECMAScript 5, if that matters to you
Bad: You have to type the instance name twice
Fat arrow
In TypeScript (shown here with some dummy parameters for explanatory reasons):
$(document).ready((n, m) => thisTest.run(n, m));
Good/bad: Opposite memory/performance trade-off compared to the first method
Good: In TypeScript, this has 100% type safety
Good: Works in ECMAScript 3
Good: You only have to type the instance name once
Bad: You'll have to type the parameters twice
Bad: Doesn't work with variadic parameters
Another solution that requires some initial setup but pays off with its invincibly light, literally one-word syntax is using Method Decorators to JIT-bind methods through getters.
I've created a repo on GitHub to showcase an implementation of this idea (it's a bit lengthy to fit into an answer with its 40 lines of code, including comments), that you would use as simply as:
class DemonstrateScopingProblems {
private status = "blah";
#bound public run() {
alert(this.status);
}
}
I haven't seen this mentioned anywhere yet, but it works flawlessly. Also, there is no notable downside to this approach: the implementation of this decorator -- including some type-checking for runtime type-safety -- is trivial and straightforward, and comes with essentially zero overhead after the initial method call.
The essential part is defining the following getter on the class prototype, which is executed immediately before the first call:
get: function () {
// Create bound override on object instance. This will hide the original method on the prototype, and instead yield a bound version from the
// instance itself. The original method will no longer be accessible. Inside a getter, 'this' will refer to the instance.
var instance = this;
Object.defineProperty(instance, propKey.toString(), {
value: function () {
// This is effectively a lightweight bind() that skips many (here unnecessary) checks found in native implementations.
return originalMethod.apply(instance, arguments);
}
});
// The first invocation (per instance) will return the bound method from here. Subsequent calls will never reach this point, due to the way
// JavaScript runtimes look up properties on objects; the bound method, defined on the instance, will effectively hide it.
return instance[propKey];
}
Full source
The idea can be also taken one step further, by doing this in a class decorator instead, iterating over methods and defining the above property descriptor for each of them in one pass.
Necromancing.
There's an obvious simple solution that doesn't require arrow-functions (arrow-functions are 30% slower), or JIT-methods through getters.
That solution is to bind the this-context in the constructor.
class DemonstrateScopingProblems
{
constructor()
{
this.run = this.run.bind(this);
}
private status = "blah";
public run() {
alert(this.status);
}
}
You can write an autobind method to automatically bind all functions in the constructor of the class:
class DemonstrateScopingProblems
{
constructor()
{
this.autoBind(this);
}
[...]
}
export function autoBind(self)
{
for (const key of Object.getOwnPropertyNames(self.constructor.prototype))
{
const val = self[key];
if (key !== 'constructor' && typeof val === 'function')
{
// console.log(key);
self[key] = val.bind(self);
} // End if (key !== 'constructor' && typeof val === 'function')
} // Next key
return self;
} // End Function autoBind
Note that if you don't put the autobind-function into the same class as a member function, it's just autoBind(this); and not this.autoBind(this);
And also, the above autoBind function is dumbed down, to show the principle.
If you want this to work reliably, you need to test if the function is a getter/setter of a property as well, because otherwise - boom - if your class contains properties, that is.
Like this:
export function autoBind(self)
{
for (const key of Object.getOwnPropertyNames(self.constructor.prototype))
{
if (key !== 'constructor')
{
// console.log(key);
let desc = Object.getOwnPropertyDescriptor(self.constructor.prototype, key);
if (desc != null)
{
if (!desc.configurable) {
console.log("AUTOBIND-WARNING: Property \"" + key + "\" not configurable ! (" + self.constructor.name + ")");
continue;
}
let g = desc.get != null;
let s = desc.set != null;
if (g || s)
{
var newGetter = null;
var newSetter = null;
if (g)
newGetter = desc.get.bind(self);
if (s)
newSetter = desc.set.bind(self);
if (newGetter != null && newSetter == null) {
Object.defineProperty(self, key, {
get: newGetter,
enumerable: desc.enumerable,
configurable: desc.configurable
});
}
else if (newSetter != null && newGetter == null) {
Object.defineProperty(self, key, {
set: newSetter,
enumerable: desc.enumerable,
configurable: desc.configurable
});
}
else {
Object.defineProperty(self, key, {
get: newGetter,
set: newSetter,
enumerable: desc.enumerable,
configurable: desc.configurable
});
}
continue; // if it's a property, it can't be a function
} // End if (g || s)
} // End if (desc != null)
if (typeof (self[key]) === 'function')
{
let val = self[key];
self[key] = val.bind(self);
} // End if (typeof (self[key]) === 'function')
} // End if (key !== 'constructor')
} // Next key
return self;
} // End Function autoBind
In your code, have you tried just changing the last line as follows?
$(document).ready(() => thisTest.run());
I have in an Object an QVector of Coordinates (my type) that I want to transfer to an other Vector ( I validate and than want to use ist ).
Header
bool getVector(QVector<Coordinates> &getCoordinates );
C File
static QVector<Coordinates> current;
int getVector( QVector<Coordinates> &getCoordinates)
{
.... stuff ...
getCoordinates = current;
.... stuff ....
return 0;
}
And I use it like
....
QVector<Coordinates> currentCoordinates;
getVector(currentCoordinates);
currentCoordinates.X // CRASH
The debugger goes to this line where an Live Crash happens
inline QVector(const QVector<T> &v) : d(v.d) { d->ref.ref(); if (!d->sharable) detach_helper(); }
So my how can I fix this? As I can use this to get all the other Variables with this methode.
A likely cause of your problem is that current has not been constructed before getVector is called. Initialization of static objects in C++ is a thorny area, and a frequent source of bugs - for more information, see this question, and the static initialization order fiasco FAQ entry.
A simple solution to this problem is to provide access to current via a function, i.e. replace
static QVector<Coordinates> current;
with
static QVector<Coordinates>& getCurrent()
{
static QVector<Coordinates> current;
return current;
}
Note, however, that the function as written above is not thread-safe. If multiple threads may call getCurrent, then it should be protected with a QMutex.
For gareth and the Forum :
the header:
typedef QVector<Coordinates> VesselCoordinates;
bool (*getVessel)(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates );
later i bind tis function pointer to an static function ( cause this part of my Program will be one day convertet to c)
cpp file lower layer:
static struct {
Charakter currentPlayerVessel;
VesselCoordinates possibility;
}data;
static bool getVessel(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates );
// funktion to bind the funktion pointer to this static funktion so it can be called outside the File
static bool serverNamespace::getVessel(Charakter forCharakter, Vessel& getVessel,VesselCoordinates &getCoordinates )
{
bool retValue= false;
if ( forCharakter == data.currentPlayerVessel){
// TODO abfragen ob die Adresse regestriert ist!
if ((true == minSize()) and ((true == shipsInRow())or (true == shipsInLine())))
{
retValue = true;
Vessel test = (Vessel)data.possibility.size();
getVessel = test;
getCoordinates = data.possibility;
}
}
return retValue;
}
And then i can use this in the upper layer cpp file to get the information i need:
// in an Funktion :
VesselCoordinates currentCoordinates;
currentCoordinates.clear();
Vessel currentVessel;
if (true == basicFleet->getVessel(currentCharakter,currentVessel, currentCoordinates ))
// doing stuff to it
so its worik fine but your idea worked just as fine. Maybe you can see why my idea is also working.
Thank you
elektor
I'm rusty with delegates and closures in JavaScript, and think I came across a situation where I'd like to try to use one or both.
I have a web app that behaves a lot like a forms app, with fields hitting a server to change data on every onBlur or onChange (depending on the form element). I use ASP.NET 3.5's Web Services and jQuery to do most of the work.
What you need to know for the example:
isBlocking() is a simple mechanism to form some functions to be synchronous (like a mutex)
isDirty(el) checks to make sure the value of the element actually changed before wasting a call to the server
Agent() returns a singleton instance of the WebService proxy class
getApplicationState() passes a base-64 encoded string to the web service. This string represents the state of the application -- the value of the element and the state are passed to a service that does some calculations. The onSuccess function of the web service call returns the new state, which the client processes and updates the entire screen.
waitForCallback() sets a flag that isBlocking() checks for the mutex
Here's an example of one of about 50 very similar functions:
function Field1_Changed(el) {
if (isBlocking()) return false;
if (isDirty(el)) {
Agent().Field1_Changed($j(el).val(), getApplicationState());
waitForCallback();
}
}
The big problem is that the Agent().Field_X_Changed methods can accept a different number of parameters, but it's usually just the value and the state. So, writing these functions gets repetitive. I have done this so far to try out using delegates:
function Field_Changed(el, updateFunction, checkForDirty) {
if (isBlocking()) return false;
var isDirty = true; // assume true
if (checkForDirty === true) {
isDirty = IsDirty(el);
}
if (isDirty) {
updateFunction(el);
waitForCallback();
}
}
function Field1_Changed(el) {
Field_Changed(el, function(el) {
Agent().Field1_Changed($j(el).val(), getTransactionState());
}, true);
}
This is ok, but sometimes I could have many parameters:
...
Agent().Field2_Changed($j(el).val(), index, count, getApplicationState());
....
What I'd ultimately like to do is make one-linen calls, something like this (notice no getTransactionState() calls -- I would like that automated somehow):
// Typical case: 1 value parameter
function Field1_Changed(el) {
Field_Changed(el, delegate(Agent().Field1_Changed, $j(el).val()), true);
}
// Rare case: multiple value parameters
function Field2_Changed(el, index, count) {
Field_Changed(el, delegate(Agent().Field1_Changed, $j(el).val(), index, count), true);
}
function Field_Changed(el, theDelegate, checkIsDirty) {
???
}
function delegate(method) {
/* create the change delegate */
???
}
Ok, my first question is: Is this all worth it? Is this harder to read but easier to maintain or the other way around? This is a pretty good undertaking, so I may end up putting a bounty on this one, but I'd appreciate any help you could offer. Thanks!
UPDATE
So, I've accepted an answer based on the fact that it pointed me in the right direction. I thought I'd come back and post my solution so that others who may just be starting out with delegates have something to model from. I'm also posting it to see if anybody wants to try an optimize it or make suggestions. Here's the common Field_Changed() method I came up with, with checkForDirty and omitState being optional parameters:
function Field_Changed(el, args, delegate, checkForDirty, omitState) {
if (isBlocking()) return false;
if (!$j.isArray(args) || args.length == 0) {
alert('The "args" parameter in Field_Changed() must be an array.');
return false;
}
checkForDirty = checkForDirty || true; // assume true if not passed
var isDirty = true; // assume true for updates that don't require this check
if (checkForDirty === true) {
isDirty = fieldIsDirty(el);
}
if (isDirty) {
omitState = omitState || false; // assume false if not passed
if (!omitState) {
var state = getTransactionState();
args.push(state);
}
delegate.apply(this, args);
waitForCallback();
}
}
It handles everything I need it to (check for dirty, applying the application state when I need it to, and forcing synchronous webservice calls. I use it like this:
function TransactionAmount_Changed(el) {
Field_Changed(el, [cleanDigits($j(el).val())], Agent().TransactionAmount_Changed, true);
}
cleanDigits strips out junk characters the user may have tried to type in. So, thanks to everyone, and happy coding!
OK, few things:
Delegates are extremely simple in javascript since functions are first class members.
Function.apply lets you call a function with an array of arguments.
So you can write it this way
function Field_Changed(delegate, args)
{
if (isBlocking()) return false;
if (isDirty(args[0])) { //args[0] is el
delegate.apply(this, args);
waitForCallback();
}
}
And call it as:
Field_Changed(Agent().Field2_Changed, [el, getApplicationState(), whatever...]);
I have been using the following utility function that I wrote a long time ago:
/**
* #classDescription This class contains different utility functions
*/
function Utils()
{}
/**
* This method returns a delegate function closure that will call
* targetMethod on targetObject with specified arguments and with
* arguments specified by the caller of this delegate
*
* #param {Object} targetObj - the object to call the method on
* #param {Object} targetMethod - the method to call on the object
* #param {Object} [arg1] - optional argument 1
* #param {Object} [arg2] - optional argument 2
* #param {Object} [arg3] - optional argument 3
*/
Utils.createDelegate = function( targetObj, targetMethod, arg1, arg2, arg3 )
{
// Create an array containing the arguments
var initArgs = new Array();
// Skip the first two arguments as they are the target object and method
for( var i = 2; i < arguments.length; ++i )
{
initArgs.push( arguments[i] );
}
// Return the closure
return function()
{
// Add the initial arguments of the delegate
var args = initArgs.slice(0);
// Add the actual arguments specified by the call to this list
for( var i = 0; i < arguments.length; ++i )
{
args.push( arguments[i] );
}
return targetMethod.apply( targetObj, args );
};
}
So, in your example, I would replace
function Field1_Changed(el) {
Field_Changed(el, delegate(Agent().Field1_Changed, $j(el).val()), true);
}
With something along the lines
function Field1_Changed(el) {
Field_Changed(el, Utils.createDelegate(Agent(), Agent().Field1_Changed, $j(el).val()), true);
}
Then, inside of Agent().FieldX_Changed I would manually call getApplicationState() (and encapsulate that logic into a generic method to process field changes that all of the Agent().FieldX_Changed methods would internally call).
Closures and delegates in JavaScript:
http://www.terrainformatica.com/2006/08/delegates-in-javascript/
http://www.terrainformatica.com/2006/08/delegates-in-javascript-now-with-parameters/