I encode an Int64 (on the graphics card) with unsafeAddressOf of some instances (that are properly stored in and retained by some array).
I then would like to get back to my instances from my Int64. I manage to get an UnsafePointer<MyClass> correctly initialized from my Int64.
Then, I do:
let x = UnsafePointer<MyClass>.memory.
But using x crashes.
I understand it is unsafe and tricky considering ARC. But still, is there any way to achieve this in Swift, or is it helpless?
Thanks.
What you are trying to do is extremely unsafe, as you yourself said, and there are probably safer approaches to solving whatever problem you are working on, but here is a complete example that does it:
func getPointer<T:AnyObject>(obj : T) -> Int64
{
return unsafeBitCast(unsafeAddressOf(obj), Int64.self)
}
func recoverObject<T:AnyObject>(ptr : Int64) -> T
{
return Unmanaged<T>.fromOpaque(COpaquePointer(bitPattern: Int(ptr))).takeUnretainedValue()
}
class C {
var cID = 0
func foo() {
print("Instance of C, id = \(cID)")
}
}
class D {
var dID = 0
func foo() {
print("Instance of D, id = \(dID)")
}
}
let c : C = C()
let d : D = D()
c.cID = 123;
d.dID = 321;
let cPtr : Int64 = getPointer(c)
let dPtr : Int64 = getPointer(d)
c.cID *= 10
let c1 : C = recoverObject(cPtr)
let d1 : D = recoverObject(dPtr)
c1.foo()
d1.foo()
The output is:
Instance of C, id = 1230
Instance of D, id = 321
Thanks to user3441734 for some helpful hints! Note the use of generics. The functions for getting a "pointer" and de-referencing it should work with any class (won't work with structs, though).
As far as safer ways of doing this, the answer would depend on the context of what you are doing. Why can't the array of instances be accessed wherever you need the instances instead of obtaining them via Int64s? If the array is actually maintained by (Objective-)C code in a 3rd party library, then there are additional problems that may arise due to memory alignment etc. It may be possible to write C code that would return instances to Swift code in a safer way.
Please let me know about more details if the answer doesn't work for some reason.
class C {
func foo() {
print("C")
}
}
var c = C()
let addr = unsafeAddressOf(c)
dump(addr)
/*
▿ UnsafePointer(0x7F86E942B230)
- pointerValue: 140217415807536
*/
let pC = withUnsafePointer(&c) { (p) -> UnsafePointer<C> in
return p
}
dump(pC)
/*
▿ UnsafePointer(0x10DE5B390)
- pointerValue: 4528124816
*/
dump(pC.memory)
/*
- C #0
*/
let opaquePointer = COpaquePointer(addr)
let unmanagedC = Unmanaged<C>.fromOpaque(opaquePointer)
dump(unmanagedC)
/*
▿ Swift.Unmanaged<C>
- _value: C #0
*/
// I still dont have an idea how to use it ...
ha, i got it!
unmanagedC.takeUnretainedValue().foo() // prints C !
Related
While refactoring my F# code, I found a record with a field of type bool ref:
type MyType =
{
Enabled : bool ref
// other, irrelevant fields here
}
I decided to try changing it to a mutable field instead
// Refactored version
type MyType =
{
mutable Enabled : bool
// other fields unchanged
}
Also, I applied all the changes required to make the code compile (i.e. changing := to <-, removing incr and decr functions, etc).
I noticed that after the changes some of the unit tests started to fail.
As the code is pretty large, I can't really see what exactly changed.
Is there a significant difference in implementation of the two that could change the behavior of my program?
Yes, there is a difference. Refs are first-class values, while mutable variables are a language construct.
Or, from a different perspective, you might say that ref cells are passed by reference, while mutable variables are passed by value.
Consider this:
type T = { mutable x : int }
type U = { y : int ref }
let t = { x = 5 }
let u = { y = ref 5 }
let mutable xx = t.x
xx <- 10
printfn "%d" t.x // Prints 5
let mutable yy = u.y
yy := 10
printfn "%d" !u.y // Prints 10
This happens because xx is a completely new mutable variable, unrelated to t.x, so that mutating xx has no effect on x.
But yy is a reference to the exact same ref cell as u.y, so that pushing a new value into that cell while referring to it via yy has the same effect as if referring to it via u.y.
If you "copy" a ref, the copy ends up pointing to the same ref, but if you copy a mutable variable, only its value gets copied.
You have the difference not because one is first-value, passed by reference/value or other things. It's because a ref is just a container (class) on its own.
The difference is more obvious when you implement a ref by yourself. You could do it like this:
type Reference<'a> = {
mutable Value: 'a
}
Now look at both definitions.
type MyTypeA = {
mutable Enabled: bool
}
type MyTypeB = {
Enabled: Reference<bool>
}
MyTypeA has a Enabled field that can be directly changed or with other word is mutable.
On the other-side you have MyTypeB that is theoretically immutable but has a Enabled that reference to a mutable class.
The Enabled from MyTypeB just reference to an object that is mutable like the millions of other classes in .NET. From the above type definitions, you can create objects like these.
let t = { MyTypeA.Enabled = true }
let u = { MyTypeB.Enabled = { Value = true }}
Creating the types makes it more obvious, that the first is a mutable field, and the second contains an object with a mutable field.
You find the implementation of ref in FSharp.Core/prim-types.fs it looks like this:
[<DebuggerDisplay("{contents}")>]
[<StructuralEquality; StructuralComparison>]
[<CompiledName("FSharpRef`1")>]
type Ref<'T> =
{
[<DebuggerBrowsable(DebuggerBrowsableState.Never)>]
mutable contents: 'T }
member x.Value
with get() = x.contents
and set v = x.contents <- v
and 'T ref = Ref<'T>
The ref keyword in F# is just the built-in way to create such a pre-defined mutable Reference object, instead that you create your own type for this. And it has some benefits that it works well whenever you need to pass byref, in or out values in .NET. So you should use ref. But you also can use a mutable for this. For example, both code examples do the same.
With a reference
let parsed =
let result = ref 0
match System.Int32.TryParse("1234", result) with
| true -> result.Value
| false -> result.Value
With a mutable
let parsed =
let mutable result = 0
match System.Int32.TryParse("1234", &result) with
| true -> result
| false -> result
In both examples you get a 1234 as an int parsed. But the first example will create a FSharpRef and pass it to Int32.TryParse while the second example creates a field or variable and passes it with out to Int32.TryParse
I was reading some Rust code and I came across this line
if let Some(path) = env::args().nth(1) {
Inside of this function
fn main() {
if let Some(path) = env::args().nth(1) {
// Try reading the file provided by the path.
let mut file = File::open(path).expect("Failed reading file.");
let mut content = String::new();
file.read_to_string(&mut content);
perform_conversion(content.as_str()).expect("Conversion failed.");
} else {
println!(
"provide a path to a .cue file to be converted into a MusicBrainz compatible tracklist."
)
}
}
The line seems to be assigning the env argument to the variable path but I can't work out what the Some() around it is doing.
I took a look at the documentation for Option and I understand how it works when used on the right hand side of = but on the left hand side I am a little confused.
Am I right in thinking this line is equivalent to
if let path = Some(env::args().nth(1)) {
From the reference :
An if let expression is semantically similar to an if expression but
in place of a condition expression it expects the keyword let followed
by a refutable pattern, an = and an expression. If the value of the
expression on the right hand side of the = matches the pattern, the
corresponding block will execute, otherwise flow proceeds to the
following else block if it exists. Like if expressions, if let
expressions have a value determined by the block that is evaluated.
In here the important part is refutability. What it means refutable pattern in here it can be in different forms. For example :
enum Test {
First(String, i32, usize),
Second(i32, usize),
Third(i32),
}
You can check the x's value for a value for 3 different pattern like :
fn main() {
let x = Test::Second(14, 55);
if let Test::First(a, b, c) = x {}
if let Test::Second(a, b) = x {} //This block will be executed
if let Test::Third(a) = x {}
}
This is called refutability. But consider your code like this:
enum Test {
Second(i32, usize),
}
fn main() {
let x = Test::Second(14, 55);
if let Test::Second(a, b) = x {}
}
This code will not compile because x's pattern is obvious, it has single pattern.
You can get more information from the reference of refutability.
Also you are not right thinking for this:
if let path = Some(env::args().nth(1)) {
Compiler will throw error like irrefutable if-let pattern because as the reference says: "keyword let followed by a refutable pattern". In here there is no refutable pattern after "let". Actually this code tries to create a variable named path which is an Option and this make no sense because there is no "If" needed,
Instead Rust expects from you to write like this:
let path = Some(env::args().nth(1)); // This will be seem like Some(Some(value))
The other answers go into a lot of detail, which might be more than you need to know.
Essentially, this:
if let Some(path) = env::args().nth(1) {
// Do something with path
} else {
// otherwise do something else
}
is identical to this:
match env::args().nth(1) {
Some(path) => { /* Do something with path */ }
_ => { /* otherwise do something else */ }
}
This can be done in python / javascript:
# Python:
a = ""
b = "test"
c = a or b # test
// javascript
a = "";
b = "test";
c = a || b; // test
Can the same be accomplished in go without doing some conditional block? The only way I'm able to do this so far is with conditional blocks...
a := ""
b := "test"
var c
if a {
c = a
} else {
c = b
}
I think this is answer is going to be "no, this cannot be done", but I figured I would ask just in case I'm wrong. The example here is simplified. The variables a and b could have been defined long ago...
From golang FAQ, There is no ternary testing operation in Go. You may use the following to achieve the same result:
if expr {
n = trueVal
} else {
n = falseVal
}
In the code below I show union-like class S which contains two non-related structs B and C. I show how to instantiate the non-POD std::string and delete it again and then switch S to S::CC and set the num int.
#include <vector>
#include <string>
#include <iostream>
#include <memory>
struct B
{
B() {}
~B() {}
std::string str;
void Func1() {}
};
struct C
{
C() {}
~C() {}
int num;
void Func2() {}
};
struct S
{
S() { tag = CC; }
S( const S& s )
{
switch( s.tag )
{
case BB:
new ( &b.str ) std::string;
b.str = s.b.str;
break;
case CC:
c.num = s.c.num;
default:
break;
}
}
~S()
{
switch( tag )
{
case BB:
b.str.~basic_string< char >();
break;
case CC:
c.num = 0;
break;
default:
break;
}
}
enum { BB, CC } tag;
union
{
B b;
C c;
};
};
struct H
{
H( std::initializer_list< S > initializerList ) : initListVect( initializerList ) {}
std::vector< S > initListVect;
};
int main()
{
S s;
s.tag = S::BB;
new ( &s.b.str ) std::string; // docs say use new placement to create memory
s.b.str = "bbb";
s.b.str.~basic_string< char >(); // string usage in B ok
s.tag = S::CC;
s.c.num = 333; // int usage in C ok
H h { }; // what should the init list be if I wanted 3 list elements S::BB, S::CC, S::BB?
return 0;
}
My goal, however, is to use S in an std::initializer_list. I don’t know what the format should be for initializeing h. What should the arguments be if I wanted to initialize h with these S::BB, S::CC, S::BB?
My compiler is VS2015.
Edit:
This post’s history: my posting comes from a need for a definitive answer to the question of storing compile-time-deduceable heterogeneous objects in an std::initializer_list. This question has been asked many times before and there have been many attempts at answers (see Heterogeneous containers in C++). The most simplistic answer is to use polymorphism, but this ignores the power of being able to define a type at compile time (templates). Besides, heterogeneous, non-related objects grouped together polymorphically means a lot of derived data members are useless, which sows usage and maintenance confusion downstream. Other advice given was to use boost::any or boost::variant, but this has the same weakness as polymorphism and reduces message declaration clarity. Another attempt at container object heterogeneity was the use of std::tuple, but although an initializer_list can certainly contain tuples, this approach too ignores compile-time type resolution. I even found a paper written in 1999 called Heterogeneous, Nested STL Containers in C++ which uses template template arguments to solve the heterogeneity problem. After all this, I settled on class-like unions which led to my posting here. Class-like unions for non-related/heterogeneous container objects has perfect message declaration clarity, no object size ambiguity, and is compile time template-able, and it leads to excellent downstream maintenance scenarios.
Edit2: (5 weeks later) Here is what has happened. 1) I implemented a full class-like union solution given the advice in this posting. The result was tedious and unwieldy with ‘tag’ being used to identify which sub-method to call for each new functionality. Low grade regarding code maintenance. 2) c++17 has accepted std::variant. Since that is currently not yet implemented in VS2015 Update 2, I set about using boost::variant. See What is the right c++ variant syntax for calling a member function set to a particular variant? which uses the Visitor pattern to allow access to initialized variant members and member functions. This eliminates the ‘tag’ switches and variant ‘get’ calls. Bottom line: I dropped my class-like union and adopted variant for creating maintainable code that uses initializer_list to store variant member functionality all being initializable at compile time (read: highly maintainable).
Alright, I'm feeling generous and I've made custom unions myself so he're some stuff that'll get you set up. I've rewritten your S structure to be more compliant and usable. (I've made changes marked by comments)
struct S
{
S() : tag(CC) // initializer
{
new (&c) C; // make C object
}
S(int num) : tag(CC) // added integer constructor
{
new (&c) C;
c.num = num;
}
S(const std::string& str) : tag(BB) // added string constructor
{
new (&b) B;
b.str = str;
}
S( const S& s ) : tag(s.tag)
{
if (tag == CC)
{
new (&c) C; // construct c
c.num = s.c.num;
}
else if (tag == BB)
{
new (&b) B; // construct b, not b.str
b.str = s.b.str;
}
}
S& operator= (const S& s) // added assignment operator
{
if (tag == s.tag) // just copy b or c
{
if (tag == CC)
c = s.c;
else
b = s.b;
}
else // reconstruct b or c
{
if (tag == CC)
{
c.~C(); // destroy c
new (&b) B; // construct b
b.str = s.b.str;
}
else
{
b.~B(); // destroy b
new (&c) C; // construct c
c.num = s.c.num;
}
tag = s.tag;
}
return *this;
}
~S()
{
if (tag == CC)
{
c.~C(); // destroy c
}
else if (tag == BB)
{
b.~B(); // destroy b, not b.str
}
}
enum { BB, CC } tag;
union
{
B b;
C c;
};
};
One of the things that you were doing improperly was skipping the construction and destruction of B and C and going straight for the internal variables. You should always create and destroy types properly even when they may be trivial. While this may work out, not initializing these objects properly is only asking for trouble (It also makes it easier should you change B or C in the future).
To make using the class easier, I added in the proper constructors for std::string and int as well as an assignment operator. Because now that we can construct the objects how we want, your main() could look like this:
int main()
{
S s; // default S
s = std::string("bbb"); // set to string
s = 333; // set to number
// use initialization list
H h { std::string("bb"), 33, std::string("bb") };
return 0;
}
I encourage you to modify B and C to use constructors to build their internals rather than relying on S.
I've recently had a look at Haskell's Monad - State. I've been able to create functions that operate with this Monad, but I'm trying to encapsulate the behavior into a class, basically I'm trying to replicate in Haskell something like this:
class A {
public:
int x;
void setX(int newX) {
x = newX;
}
void getX() {
return x;
}
}
I would be very grateful if anyone can help with this. Thanks!
I would start off by noting that Haskell, to say the least, does not encourage traditional OO-style development; instead, it has features and characteristics that lend themselves well to the sort of 'pure functional' manipulation that you won't really find in many other languages; the short on this is that trying to 'bring over' concepts from other (traditional languages) can often be a very bad idea.
but I'm trying to encapsulate the behavior into a class
Hence, my first major question that comes to mind is why? Surely you must want to do something with this (traditional OO concept of a) class?
If an approximate answer to this question is: "I'd like to model some sort of data construct", then you'd be better off working with something like
data A = A { xval :: Int }
> let obj1 = A 5
> xval obj1
5
> let obj2 = obj1 { xval = 10 }
> xval obj2
10
Which demonstrates pure, immutable data structures, along with 'getter' functions and destructive updates (utilizing record syntax). This way, you'd do whatever work you need to do as some combination of functions mapping these 'data constructs' to new data constructs, as appropriate.
Now, if you absolutely needed some sort of model of State (and indeed, answering this question requires a bit of experience in knowing exactly what local versus global state is), only then would you delve into using the State Monad, with something like:
module StateGame where
import Control.Monad.State
-- Example use of State monad
-- Passes a string of dictionary {a,b,c}
-- Game is to produce a number from the string.
-- By default the game is off, a C toggles the
-- game on and off. A 'a' gives +1 and a b gives -1.
-- E.g
-- 'ab' = 0
-- 'ca' = 1
-- 'cabca' = 0
-- State = game is on or off & current score
-- = (Bool, Int)
type GameValue = Int
type GameState = (Bool, Int)
playGame :: String -> State GameState GameValue
playGame [] = do
(_, score) <- get
return score
playGame (x:xs) = do
(on, score) <- get
case x of
'a' | on -> put (on, score + 1)
'b' | on -> put (on, score - 1)
'c' -> put (not on, score)
_ -> put (on, score)
playGame xs
startState = (False, 0)
main = print $ evalState (playGame "abcaaacbbcabbab") startState
(shamelessly lifted from this tutorial). Note the use of the analogous 'pure immutable data structures' within the context of a state monad, in addition to 'put' and 'get' monadic functions, which facilitate access to the state contained within the State Monad.
Ultimately, I'd suggest you ask yourself: what is it that you really want to accomplish with this model of an (OO) class? Haskell is not your typical OO-language, and trying to map concepts over 1-to-1 will only frustrate you in the short (and possibly long) term. This should be a standard mantra, but I'd highly recommend learning from the book Real World Haskell, where the authors are able to delve into far more detailed 'motivation' for picking any one tool or abstraction over another. If you were adamant, you could model traditional OO constructs in Haskell, but I wouldn't suggest going about doing this - unless you have a really good reason for doing so.
It takes a bit of permuting to transform imperative code into a purely functional context.
A setter mutates an object. We're not allowed to do that directly in Haskell because of laziness and purity.
Perhaps, if we transcribe the State monad to another language it'll be more apparent. Your code is in C++, but because I at least want garbage collection I'll use java here.
Since Java never got around to defining anonymous functions, first, we'll define an interface for pure functions.
public interface Function<A,B> {
B apply(A a);
}
Then we can make up a pure immutable pair type.
public final class Pair<A,B> {
private final A a;
private final B b;
public Pair(A a, B b) {
this.a = a;
this.b = b;
}
public A getFst() { return a; }
public B getSnd() { return b; }
public static <A,B> Pair<A,B> make(A a, B b) {
return new Pair<A,B>(a, b);
}
public String toString() {
return "(" + a + ", " + b + ")";
}
}
With those in hand, we can actually define the State monad:
public abstract class State<S,A> implements Function<S, Pair<A, S> > {
// pure takes a value a and yields a state action, that takes a state s, leaves it untouched, and returns your a paired with it.
public static <S,A> State<S,A> pure(final A a) {
return new State<S,A>() {
public Pair<A,S> apply(S s) {
return new Pair<A,S>(a, s);
}
};
}
// we can also read the state as a state action.
public static <S> State<S,S> get() {
return new State<S,S>() {
public Pair<S,S> apply(S, s) {
return new Pair<S,S>(s, s);
}
}
}
// we can compose state actions
public <B> State<S,B> bind(final Function<A, State<S,B>> f) {
return new State<S,B>() {
public Pair<B,S> apply(S s0) {
Pair<A,S> p = State.this.apply(s0);
return f.apply(p.getFst()).apply(p.getSnd());
}
};
}
// we can also read the state as a state action.
public static <S> State<S,S> put(final S newS) {
return new State<S,S>() {
public Pair<S,S> apply(S, s) {
return new Pair<S,S>(s, newS);
}
}
}
}
Now, there does exist a notion of a getter and a setter inside of the state monad. These are called lenses. The basic presentation in Java would look like:
public abstract class Lens[A,B] {
public abstract B get(A a);
public abstract A set(B b, A a);
// .. followed by a whole bunch of concrete methods.
}
The idea is that a lens provides access to a getter that knows how to extract a B from an A, and a setter that knows how to take a B and some old A, and replace part of the A, yielding a new A. It can't mutate the old one, but it can construct a new one with one of the fields replaced.
I gave a talk on these at a recent Boston Area Scala Enthusiasts meeting. You can watch the presentation here.
To come back into Haskell, rather than talk about things in an imperative setting. We can import
import Data.Lens.Lazy
from comonad-transformers or one of the other lens libraries mentioned here. That link provides the laws that must be satisfied to be a valid lens.
And then what you are looking for is some data type like:
data A = A { x_ :: Int }
with a lens
x :: Lens A Int
x = lens x_ (\b a -> a { x_ = b })
Now you can write code that looks like
postIncrement :: State A Int
postIncrement = do
old_x <- access x
x ~= (old_x + 1)
return old_x
using the combinators from Data.Lens.Lazy.
The other lens libraries mentioned above provide similar combinators.
First of all, I agree with Raeez that this probably the wrong way to go, unless you really know why! If you want to increase some value by 42 (say), why not write a function that does that for you?
It's quite a change from the traditional OO mindset where you have boxes with values in them and you take them out, manipulate them and put them back in. I would say that until you start noticing the pattern "Hey! I always take some value as an argument, and at the end return it slightly modified, tupled with some other value and all the plumbing is getting messy!" you don't really need the State monad. Part of the fun of (learning) Haskell is finding new ways to get around the stateful OO thinking!
That said, if you absolutely want a box with an x of type Int in it, you could try making your own get and put variants, something like this:
import Control.Monad.State
data Point = Point { x :: Int, y :: Int } deriving Show
getX :: State Point Int
getX = get >>= return . x
putX :: Int -> State Point ()
putX newVal = do
pt <- get
put (pt { x = newVal })
increaseX :: State Point ()
increaseX = do
x <- getX
putX (x + 1)
test :: State Point Int
test = do
x1 <- getX
increaseX
x2 <- getX
putX 7
x3 <- getX
return $ x1 + x2 + x3
Now, if you evaluate runState test (Point 2 9) in ghci, you'll get back (12,Point {x = 7, y = 9}) (since 12 = 2 + (2+1) + 7 and the x in the state gets set to 7 at the end). If you don't care about the returned point, you can use evalState and you'll get just the 12.
There's also more advanced things to do here like abstracting out Point with a typeclass in case you have multiple datastructures which have something which behaves like x but that's better left for another question, in my opinion!