I wish to have the sub-classes of a super-class "registered" by an arbitrary name - whenever I declare a sub-class I wish to also have it entered into the super-class.sub Map.
Is there any way to accomplish this outside of main()?
// base class
class Mineral{
final String formula;
static Map<String,Mineral> sub = {}
Mineral( this.formula );
}
// sub class - declare and register
class Mica extends Mineral{
Mica( String formula ) : super( formula );
}
Mineral.sub['mica'] = Mica; // oops!
when I run this, I get
Error: line 10 pos 1: unexpected token 'Mineral' Mineral.sub['mica'] = Mica;
assuming that executable code is not allowed outside main().
cannot put within the super-class since other sub-classes may declared later, outside the library.
Dart has no way to run code as part of a library being loaded.
Executable code can only be put inside methods, or in field initializers, and static field initializers are lazy so they won't execute any code until you try to read them.
This is done to ensure quick startup - a Dart program doesn't have to execute any code before starting the main library's "main" method.
So, no, there is no way to initialize something that isn't constant before main is called.
Either
Mineral.sub['mica'] = new Mica();
or
static Map<String,Type> sub = {};
When you assign Mica you assign the Type Mica. new Mica() is an instance of Mica that is of the kind Mineral and can be assigned to the map you declared.
edit
Maybe you want to initialize the sub map:
static Map<String,Mineral> sub = {'mica': new Mica()};
hint: the semicolon is missing in this line in your question.
Related
I am writing some functions for a C extension module for python and need to import a module I wrote directly in python for access to a custom python type. I use PyImport_ImportModule() in the body of my C function, then PyObject_GetAttrString() on the module to get the custom python type. This executes every time the C function is called and seems like it's not very efficient and may not be best practice. I'm looking for a way to have access to the python custom type as a PyObject* or PyTypeObject* in my source code for efficiency and I may need the type in more than one C function also.
Right now the function looks something like
static PyObject* foo(PyObject* self, PyObject* args)
{
PyObject* myPythonModule = PyImport_ImportModule("my.python.module");
if (!myPythonModule)
return NULL;
PyObject* myPythonType = PyObject_GetAttrString(myPythonModule, "MyPythonType");
if (!myPythonType) {
Py_DECREF(myPythonModule);
return NULL;
}
/* more code to create and return a MyPythonType instance */
}
To avoid retrieving myPythonType every function call I tried adding a global variable to hold the object at the top of my C file
static PyObject* myPythonType;
and initialized it in the module init function similar to the old function body
PyMODINIT_FUNC
PyInit_mymodule(void)
{
/* more initializing here */
PyObject* myPythonModule = PyImport_ImportModule("my.python.module");
if (!myPythonModule) {
/* clean-up code here */
return NULL;
}
// set the static global variable here
myPythonType = PyObject_GetAttrString(myPythonModule, "MyPythonType");
Py_DECREF(myPythonModule);
if (!myPythonType) {
/* clean-up code here */
return NULL;
/* finish initializing module */
}
which worked, however I am unsure how to Py_DECREF the global variable whenever the module is finished being used. Is there a way to do that or even a better way to solve this whole problem I am overlooking?
First, just calling import each time probably isn't as bad as you think - Python does internally keep a list of imported modules, so the second time you call it on the same module the cost is much lower. So this might be an acceptable solution.
Second, the global variable approach should work, but you're right that it doesn't get cleaned up. This is rarely a problem because modules are rarely unloaded (and most extension modules don't really support it), but it isn't great. It also won't work with isolated sub-interpreters (which isn't much of a concern now, but may become more more popular in future).
The most robust way to do it needs multi-phase initialization of your module. To quickly summarise what you should do:
You should define a module state struct containing this type of information,
Your module spec should contain the size of the module state struct,
You need to initialize this struct within the Py_mod_exec slot.
You need to create an m_free function (and ideally the other GC functions) to correctly decref your state during de-initialization.
Within a global module function, self will be your module object, and so you can get the state with PyModule_GetState(self)
I'm using the Dart annotation #protected because I want to limit a member to only be called from a single other class, like so
class Foo {
#projected
void doSomethingSpecial() {}
}
class Bar {
final foo = Foo();
consumeDoSomethingSpecial() {
foo.doSomethingSpecial();
}
}
Understandably, foo.doSomethingSpecial() triggers the warning
The member 'doSomethingSpecial' can only be used within instance members of subclasses...
In the C++ world I would annotate Bar as a friend of Foo to permit this call but I'm not seeing the equivalent annotation in Dart?
I did see that I can suppress a warning by adding
// ignore: the_appropriate_lint_rule
above the line with the warning but I'm not seeing a lint rule that applies to using the #protected annotation?
"friend" in Dart is essentially implemented by ensuring that the identifiers are "library-local" (begin with underscore), and all friends are in the same library. A library in Dart is typically just a single file; however, using part/part-of, it can span multiple files.
Any reference to an underscore-prefixed identifier is in-scope for the same library, but out of scope anywhere else, even if you import that file.
I have a reusable header which is consumed by multiple components. I introduced a member function and a member variable to the outer class and called it from one component. The implementation of the function was done in the .cpp of the reusable header. It was built successfully but while performing the workflow and opening the component, it could not get loaded and threw an error. The other components consuming the reusable header also couldn't get loaded.
To resolve it... In the header file, I introduced an inner class and created the member variable and member function to the inner class. I created the object of the inner class as a member of the outer class. Now everything is working fine. All the components using the header are properly loading.
Can anyone please tell what could have gone wrong when the member variable and function was introduced for the outer class and how did it get resolved by using the inner class?
I just know that by introducing the member variable to the outer class could have increased the dll size which might not be getting consumed properly. But, how did it get rectified by using inner class.
Is the memory allocation playing a role here or, is it due to some other reason?
Below is the sample code from the reusable header that (1) didn't work and (2) did work:
(1)
class AFX_EXT_CLASS Class1
{
public:
Class1() {var = 8;};
void Func1(int local);
int var;
};
Note: Func1() gets called from multiple components other than the one it has been implemented into.
(2)
class AFX_EXT_CLASS Class1
{
public:
Class1();
class InnerClass
{
public:
InnerClass(): var(8) {}
void Func1(int local, Class1::InnerClass* objInnerClass)
{
objInnerClass->var = local;
}
int var;
};
public:
Class1::InnerClass objMainInnerClass;
//This object is used to call Func1() from the components using the reusable header.
};
I'm learning to use the new Dart extension methods.
I'm doing this:
extension StringInsersion on StringBuffer {
void insertCharCodeAtStart(int codeUnit) {
final end = this.toString();
this.clear();
this.writeCharCode(codeUnit);
this.write(end);
}
int codeUnitAt(int index) {
return this.toString().codeUnitAt(index);
}
}
So that I can do something like this:
myStringBuffer.insertCharCodeAtStart(0x0020);
int value = myStringBuffer.codeUnitAt(2);
However, I get the following lint warning:
Don't access members with this unless avoiding shadowing.
Should I be doing something different?
The warning you received means the following:
There is no need to reference the current instance using keyword this. Everything will work without reference to the current instance because the static extension method itself acts as an instance method of extensible type.
Simply put, just remove the reference to the current instance from your code.
From this:
final end = this.toString();
To this:
final end = toString();
It's a style thing, based on Dart's guide. There are examples in https://dart-lang.github.io/linter/lints/unnecessary_this.html.
You can find more about style in https://dart.dev/guides/language/effective-dart/style.
I turn off this rule globally by changing "analysis_options.yaml"
include: package:flutter_lints/flutter.yaml
linter:
rules:
unnecessary_this: false
Should I get the following error:
class.dart:11:11: Error: The getter '_privateID' isn't defined for the class 'Y'.
- 'Y' is from 'class.dart'.
Try correcting the name to the name of an existing getter, or defining a getter or field named '_privateID'.
From the following code?
mixin.dart:
class Mixin {
static int _nextID = 0;
int publicID = _nextID++; // I only need one of these lines
int _privateID = _nextID++; // but this variable is inaccessible
}
class.dart:
import 'mixin.dart';
class X with Mixin {
void run() {
print(publicID); // no error here
}
}
class Y with Mixin {
void run() {
print(_privateID); // Error: _privateID not defined
}
}
void main() {
Y().run();
}
Or is this a bug? If it's not a bug, I'd like to understand why this behavior is reasonable.
When I instead define the mixin in the same file as the above classes, I get no error.
(Dart SDK 2.4.1.)
It is not a bug.
The private field is inherited, but you cannot access it because its name is private to a different library.
Dart's notion of "privacy" is library private names.
The name _privateID in the mixin.dart library introduces a library private name. This name is special in that it can only be written inside the same library.
If someone writes _privateID in a different library, it is a different name, one unique to that library instead.
It is as if private names includes the library URI of the library it is written in, so what you really declare is a name _privateID#mixin.dart.
When you try to read that field in class.dart, you write ._privateID, but because it is in a different library, what you really write is ._privateID#class.dart, a completely different name, and the classs does not have any declarations with that name.
So, if one class needs to access a private member of another class (or mixin, or anything), then the two needs to be declared in the same library, because otherwise they cannot even write the name of that variable.
That is why the code works if you write the mixin in the same library.
If you want to move the mixin to a separate file, but not necessarily a separate library, you can use a part file.