I have just started out in Vala, and I tried to make a simple program that asks two inputs:
An int that specifies a cycle degree; and
A char that contains I / R for either an iterative or recursive process.
Just before compiling, I got this error:
test0.vala:8.5-8.16: error: Access to instance member `test0.test_exec' denied
test_exec(q);
^^^^^^^^^^^ //the entire statement
Compilation failed: 1 error(s), 0 warning(s)
The pastebin for the very simple program is located here.
Here's a snippet:
public static void main(string[] args)
{
stdout.printf("Greetings! How many cycles would you like? INPUT: ");
int q=0;
stdin.scanf("%d", out q);
test_exec(q);
}
public void test_exec(int q)
{
//method code here
}
Can you please enlighten me about what to do, and some tips? Thanks.
You defined test_exec as an instance (non-static) method. Unlike a static method, an instance method needs to be called on an instance of the given class. However you're trying to call it without such an instance and thus get an error.
So you either need to create an instance of the test0 class and call test_exec on that (though that would make little sense since test_exec does not depend on or change any state of the object - as a matter of fact the test0 class does not have any state) or make test_exec as well as the other methods called by test_exec static.
Related
I have the following Dart code:
void main() {
provide(1, 'A');
}
void provide<A>(A one, A two) {
print('one $one two $two');
}
In java the call to provide will give a compile time error as the two parameters should be both of type A. With java as soon as you pass an argument to a typed parameter that defines the type.
My understanding is that with Dart if I don't follow 'provide' with a type then the type is dynamic.
To get the above code to work correctly in dart I have to write:
void main() {
provide<int>(1, 'A');
}
void provide<A>(A one, A two) {
print('one $one two $two');
}
This will now give a compile type error as 'A' is not an int.
This however is error prone as the user is likely to forget to add the type when callng provide.
Is there anyway I can make calls to the provide method type safe without having to write provide<int>(....
I've trivialised the example, the aim is to make a call to a method such as :
void provide(Token<T> token, T value);
If T is not the correct type then the user should get a compile error.
To lone travelers stumbling upon this: see comments for the answer.
...
Writing a Java wrapper for a native library. A device generates data samples and stores them as structs. Two native ways of accessing them: either you request one with a getSample(&sampleStruct) or you set a callback function. Now, here is what does work:
The polling method does fill the JNA Structure
The callback function is called after being set
In fact, I am currently getting the sample right from the callback function
The problem: trying to do anything with the callback argument, which should be a struct, causes an "invalid memory access". Declaring the argument as the Structure does this, so I declared it as a Pointer. Trying a Pointer.getInt(0) causes invalid memory access. So then I declared the argument as an int, and an int is delivered; in fact, it looks very much like the first field of the struct I am trying to get! So does it mean that the struct was at that address but disappeared before Java had time to access it?
This is what I am doing now:
public class SampleCallback implements Callback{
SampleStruct sample;
public int callback(Pointer refToSample) throws IOException{
lib.INSTANCE.GetSample(sample); // works no problem
adapter.handleSample(sample);
return 1;
} ...
But neither of these does:
public int callback(SampleStruct sample) throws IOException{
adapter.handleSample(sample);
return 1;
}
...
public int callback(Pointer refToSample) throws IOException{
SampleStruct sample = new SampleStruct();
sample.timestamp = refToSample.getInt(0);
...
adapter.handleSample(sample);
return 1;
}
Also, this does in fact deliver the timestamp,
public int callback(int timestamp) throws IOException{
System.out.println("It is " + timestamp + "o'clock");
return 1;
}
but I would really prefer the whole struct.
This is clearly not going to be a popular topic and I do have a working solution, so the description is not exactly full. Will copy anything else that might be helpful if requested. Gratitude prematurely extended.
I'm adapting the Clang tool-template (as described here) to search for a particular method call in my code. In order to later rewrite that call, I would like to get the type of the parameters the method was called with, as well as the type of the object the method was called on.
I managed to find a matcher that calls back the following:
class AddListenerPrinter : public MatchFinder::MatchCallback
{
public :
virtual void run(const MatchFinder::MatchResult &Result) {
if (const auto *FS = Result.Nodes.getNodeAs<clang::MemberExpr>("ListeningBound"))
{
FS->dump();
}
}
};
which prints out:
MemberExpr 0x7fb05b07b948 '<bound member function type>' .addListener 0x7fb05b077670
`-MemberExpr 0x7fb05b07b918 'class MyCore' lvalue ->mCore 0x7fb05b078e30
`-CXXThisExpr 0x7fb05b07b900 'class MyComponent *' this
Now I can't find any way to retrieve the type of the object the method was called on (here class MyCore) or the type of the method argument (here class MyComponent).
How can I do this?
I found the answer by browsing the code of the existing matchers.
Using matcher = memberCallExpr( callee(methodDecl(hasName("addListener"))) )
I was able to retrieve a CXXMemberCallExpr node. Then getting the type of the object the method was called on:
// FS is the CXXMemberCallExpr
// Prints out the type of x in x.method()
llvm::outs() << FS->getRecordDecl()->getName();
and the method parameters are accessible through FS->getArg(n).
Bottom line is: Find the CXX object that contains what you're looking for first (e.g. which class has methods to access function arguments?), then find the matcher that will return the same type of object in ASTMatchers.h.
Hoping this can help anybody else with the same problem.
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.
Say I have an instance of a class Foo, and I want to grab a list of all of its methods that are annotated a certain way. I want to have a reference to the method itself, so I'm not looking to use reflection to invoke the method each time, just to grab a reference to it the first time.
In other words, I want to do the reflection equivalent of this:
class Foo {
a() {print("a");}
}
void main() {
var f = new Foo();
var x = f.a; // Need reflective way of doing this
x(); // prints "a"
}
I have tried using InstanceMirror#getField, but methods are not considered fields so that didn't work. Any ideas?
As far as I understand reflection in Dart, there's no way to get the actual method as you wish to. (I'll very happily delete this answer if someone comes along and shows how to do that.)
The best I can come up with to ameliorate some of what you probably don't like about using reflection to invoke the method is this:
import 'dart:mirrors';
class Foo {
a() {print("a");}
}
void main() {
var f = new Foo();
final fMirror = reflect(f);
final aSym = new Symbol('a');
final x = () => fMirror.invoke(aSym, []);
x(); // prints "a"
}
Again, I know that's not quite what you're looking for, but I believe it's as close as you can get.
Side note: getField invokes the getter and returns the result -- it's actually fine if the getter is implemented as a method. It doesn't work for you here, but for a different reason than you thought.
What you're trying to get would be described as the "closurized" version of the method. That is, you want to get the method as a function, where the receiver is implicit in the function invocation. There isn't a way to get that from the mirror. You could get a methodMirror as
reflect(foo).type.methods[const Symbol("a")]
but you can't invoke the result.