In Objective-C I often use the pattern of using a static void* as an identification tag. At times these tags are only used within that function/method, hence it's convenient to place the variable inside the function.
For example:
MyObscureObject* GetSomeObscureProperty(id obj) {
static void* const ObscurePropertyTag = &ObscurePropertyTag;
MyObscureObject* propValue = objc_getAssociatedObject(id,ObscurePropertyTag);
if(!propValue) {
propValue = ... // lazy-instantiate property
objc_setAssociatedObject(obj,ObscurePropertyTag,propValue, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
return propValue;
}
The question is, how to write the ObscurePropertyTag private-constant-pointer-to-itself in Swift? (Preferrably 2.1 but future already-announced versions should be okay)
I've looked around and it seems that I have to put this ObscurePropertyTag as a member variable and there doesn't seem to be a way around it.
Unlike (Objective-)C, you cannot take the address of an
uninitialized variable in Swift. Therefore creating a self-referencing
pointer is a two-step process:
Swift 2:
var ptr : UnsafePointer<Void> = nil
withUnsafeMutablePointer(&ptr) { $0.memory = UnsafePointer($0) }
Swift 3:
var ptr = UnsafeRawPointer(bitPattern: 1)!
ptr = withUnsafePointer(to: &ptr) { UnsafeRawPointer($0) }
For your purpose, is it easier to use the address of a global variable with &, see for
example
Is there a way to set associated objects in Swift?.
If you want to restrict the scope of the "tag" to the function itself
then you can use a static variable inside a local struct. Example:
func obscureProperty(obj : AnyObject) -> MyObscureObject {
struct Tag {
static var ObscurePropertyTag : Int = 0
}
if let propValue = objc_getAssociatedObject(obj, &Tag.ObscurePropertyTag) as? MyObscureObject {
return propValue
}
let propValue = ... // lazy instantiate property value
objc_setAssociatedObject(obj, &Tag.ObscurePropertyTag,propValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
return propValue
}
Try this:
var GetSomeObscureProperty: MyObscureObject = nil
withUnsafePointer(& GetSomeObscureProperty) {
GetSomeObscureProperty = MyObscureObject($0)
}
In short
let GetSomeObscureProperty = UnsafePointer<()>()
Related
I have something similar to the following code snippets. I am simplifying the code here for attempted brevity.
First, a subclass of QAbstractListModel with the following data() implementation, and Q_INVOKABLE get_thing() method, which returns a pointer to a QObject subclass, QML_thing:
QVariant data(QModelIndex& index, int role) {
const auto& thing = m_data.at(index.row()); // shared pointer to QML_thing
switch(role)
{
case Qt::DisplayRole:
return thing->name(); // this works
case WholeThingRole:
return QVariant::fromValue(QML_thing*>(thing.get());
}
}
QML_thing* getThing(int index) const
{
const thing = m_data.at(index); // shared pointer
return thing.get();
}
Next, I have a Repeater in a QML file that has this code:
Repeater {
id: repeater
model: thingModel
ThingDelegate {
thing: wholeThing // This calls the role, but ends up being null
}
onItemAdded {
item.thing = model.getThing(index) // this works, but 'breaks' the binding
}
}
My question is: why doesn't the thing: binding in QML work, but the thing = version does?
I have a native C++ method, from external unchangable DLL, that expects a std::vector as a argument with 2 native objects. (for example sake, it's an image library returning width/height of a 2 images)
I would like the native objects in the std::vector to be referenced in my own wrapped MyManagedImageObj.
Somehow the std::vector seems to copy values and has no way to add pointers (correct?); so after calling the NativeMethod; I need to copy the properties (width/height) back again to MyManagedImageObj.
I thought about first declaring the std::vec and getting the pointer of the results and put that in MyManagedImageObj as a pointer. But if I understand it correctly std::vector will clean that native memory up once out of scope. (my c++ experience is 1 week; c# long time)
Is there a better way to do this without reassigning the properties one by one?
Ej
The code looks like this:
//create managed object that wraps also native pointer.
MyManagedImageObj^ obj1 = gcnew MyManagedImageObj();
MyManagedImageObj^ obj2 = gcnew MyManagedImageObj();
//keep list of result
List<MyManagedImageObj^>^ resultList;
resultList->Add(obj1);
resultList->Add(obj2);
//call to native method. Dereference pointers of native wrapped objects... not working?
std::vector<DLLNativeImageObj> nativeImageVec { *obj1->GetInstance(), *obj2->GetInstance() };
bool result = otherNativePtr->NativeMethod(nativeImageVec);
//we still need copy it over results now to 'our' managed objects.
int i = 0;
for (DLLNativeImageObj c : nativeImageVec)
{
resultList[i]->ImageHeight = c.imageHeight;
resultList[i]->ImageWidth = c.imageWidth;
++i;
}
The MyManagedImageObj class looks like this:
//MyManagedImageObj.h
public ref class MyManagedImageObj
{
protected:
DLLNativeImageObj* m_Instance;
public:
MyManagedImageObj(DLLNativeImageObj* instance)
: m_Instance(instance)
{
};
~MyManagedImageObj() //destructor will be called whenever we do 'delete'
{
if (m_Instance != nullptr)
{
delete m_Instance;
}
}
!MyManagedImageObj() //finalizer, called by the garbage collector when it's destroys the wrapper object. So safety check to dispose unmanaged item.
{
if (m_Instance != nullptr)
{
delete m_Instance;
}
}
DLLNativeImageObj* GetInstance() //return the pointer to the unmanaged object
{
return m_Instance;
}
property uint32_t ImageWidth
{
public:
uint32_t get()
{
return m_Instance->imageWidth;
}
public:
void set(uint32_t value)
{
m_Instance->imageWidth = value;
}
}
property uint32_t ImageHeight
{
public:
uint32_t get()
{
return m_Instance->imageHeight;
}
public:
void set(uint32_t value)
{
m_Instance->imageHeight = value;
}
}
};
//MyManagedImageObj.cpp
MyManagedImageObj::MyManagedImageObj(new DLLNativeImageObj())
{
// new keyword important: it returns a pointer to the location and does not get deleted when out of scope. Manual delete is required.
}
I have project which I am compiling with /clr. I have a class like below..
ref class A
{
public:
void CheckValue(void * test);
typedef ref struct val
{
std::string *x;
}val_t;
};
in my implementation I ahve to use something like below..
void A::CheckValue(void *test)
{
a::val_t^ allVal = (a::val_t^)test;
}
in my main I have used like..
int main()
{
A^ obj = gcnew A();
a::val_t valObj = new std::string("Test");
obj->CheckValue((void*)valObj);
}
I am getting type cast error and two places -
obj->CheckValue((void*)valObj);
and at
obj->CheckValue((void*)valObj);
error C2440: 'type cast' : cannot convert from 'void*' to 'A::val_t ^'
This snippet is just to show behavior at my end and I ahve to use it this way only. Earlier I was running it using non /clr so it compiled fine.
Now question I have how can I make this type casting work in C++/CLI type project?
Replace void * with Object^. You can also write a generic version of CheckValue but then there is not much point of having a weak-typed parameter when you have the type in the generic parameter.
A reference handle represents an object on the managed heap. Unlike a native pointer, CLR could move the object around during a function call, so the behavior of a pointer and a reference handle is different, and a type cast would fail. You can also pin the object being referenced using pin_ptr if you really need a void* so CLR would not be moving the object during the function call.
Here is how I would get around the limitation you are seeing, just remove the struct from the managed object, since it contains native pointer types.
struct val_t
{
char* x;
};
ref class A
{
public:
void CheckValue(void* test);
};
void A::CheckValue(void* test)
{
val_t* allVal = (val_t*)test;
}
int main()
{
A^ obj = gcnew A();
val_t valObj;
valObj.x = "Test";
obj->CheckValue((void*)&valObj);
}
Now, if you absolutely need the struct to be managed, here is how to do it:
ref class A
{
public:
void CheckValue(void * test);
value struct val_t
{
char* x;
};
};
void A::CheckValue(void *test)
{
a::val_t* allVal = (a::val_t*)test;
}
int main()
{
A^ obj = gcnew A();
a::val_t valObj;
valObj.x = "Test";
pin_ptr<a::val_t> valPin = &valObj;
obj->CheckValue((void*)valPin);
}
Dart specification states:
Reified type information reflects the types of objects at runtime and may always be queried by dynamic typechecking constructs (the
analogs of instanceOf, casts, typecase etc. in other languages).
Sounds great, but there is no instanceof-like operator. So how do we perform runtime type-checking in Dart? Is it possible at all?
The instanceof-operator is called is in Dart. The spec isn't exactly friendly to a casual reader, so the best description right now seems to be http://www.dartlang.org/articles/optional-types/.
Here's an example:
class Foo { }
main() {
var foo = new Foo();
if (foo is Foo) {
print("it's a foo!");
}
}
Dart Object type has a runtimeType instance member (source is from dart-sdk v1.14, don't know if it was available earlier)
class Object {
//...
external Type get runtimeType;
}
Usage:
Object o = 'foo';
assert(o.runtimeType == String);
As others have mentioned, Dart's is operator is the equivalent of Javascript's instanceof operator. However, I haven't found a direct analogue of the typeof operator in Dart.
Thankfully the dart:mirrors reflection API has recently been added to the SDK, and is now available for download in the latest Editor+SDK package. Here's a short demo:
import 'dart:mirrors';
getTypeName(dynamic obj) {
return reflect(obj).type.reflectedType.toString();
}
void main() {
var val = "\"Dart is dynamically typed (with optional type annotations.)\"";
if (val is String) {
print("The value is a String, but I needed "
"to check with an explicit condition.");
}
var typeName = getTypeName(val);
print("\nThe mirrored type of the value is $typeName.");
}
There are two operators for type testing: E is T tests for E an instance of type T while E is! T tests for E not an instance of type T.
Note that E is Object is always true, and null is T is always false unless T===Object.
Exact type matching is done via runtimeType property. Checking if an instance or any of its parent types (in the inheritance chain) is of the given type is done via is operator:
class xxx {}
class yyy extends xxx {}
void main() {
var y = yyy();
print(y is xxx);
print(y.runtimeType == xxx);
}
Returns:
true
false
Simply use .runtimeType on the property like below,
print(unknownDataTypeProperty.runtimeType)
Just to clarify a bit the difference between is and runtimeType. As someone said already (and this was tested with Dart V2+) the following code:
class Foo {
#override
Type get runtimeType => String;
}
main() {
var foo = Foo();
if (foo is Foo) {
print("it's a foo!");
}
print("type is ${foo.runtimeType}");
}
will output:
it's a foo!
type is String
Which is wrong.
Now, I can't see the reason why one should do such a thing...
T is The type
print( T.runtimeType)
if(value is int ) Returns true if the type of the value is int,
else if(value is! int )
To check the type of a variable use runtimeType
void main() {
int a = 10;
print(a.runtimeType);
}
to check whether the type of a variable is the same as your expected use is or runtimeType
void main() {
int a = 10;
print(a.runtimeType == int); // true
//or
print(a is int); // true
}
print("enter your phone number:\n");
var phone number = stdin.readLineSync();
if(phone number.runtimeType is int == true) // checks if the values input are integers
{
print('you have successfully input your phone number!');
}
else{
print('only numbers are allowed');
}
I'm somewhat new to object oriented programming in Javascript and I'm trying to build a handler object and library for a list of items I get back from an API call. Ideally, I'd like the library functions to be members of the handler class. I'm having trouble getting my class method to work however. I defined as part of the class bcObject the method getModifiedDateTime, but when I try to echo the result of the objects call to this method, I get this error:
Error on line 44 position 26: Expected ';'
this.getModifiedDateTime: function(epochtime) {
which leads me to believe that I simply have a syntax issue with my method definition but I can't figure out where.
response(
{
"items":
[
{"id":711,"name":"Shuttle","lastModifiedDate":"1268426336727"},
{"id":754,"name":"Formula1","lastModifiedDate":"1270121717721"}
],
"extraListItemsAttr1":"blah",
"extraListItemsAttr2":"blah2"
});
function response(MyObject) {
bcObject = new bcObject(MyObject);
thing = bcObject.getModifiedDateTime(bcObject.videoItem[0].lastModifiedDate);
SOSE.Echo(thing);
}
function bcObject(listObject) {
// define class members
this.responseList = {};
this.videoCount = 0;
this.videoItem = [];
this.responseListError = "";
// instantiate members
this.responseList = listObject;
this.videoCount = listObject.items.length;
// populate videoItem array
for (i=0;i<this.videoCount;i++) {
this.videoItem[i] = listObject.items[i];
}
this.getModifiedDateTime: function(epochtime) {
var dateStringOutput = "";
var myDate = new Date(epochtime);
dateStringOutput = myDate.toLocaleString();
return dateStringOutput;
};
}
You use = to assign values in JS, not ::
this.getModifiedDateTime = function(epochtime) {
You should use the = operator for methods defined as you did there (this.<methodName> = function (...) {).
The colon notation is used when declaring object literals.