What is the difference between the const and immutable type qualifiers in D?
Something that is const cannot be mutated via that reference but could be mutated by a mutable reference to the same data. Something that is immutable can't be mutated by any reference to that data. So, if you have
const C c = foo();
then you know that you cannot mutate the object referred to by c through c, but other references to the object referred to by c may exist in your code, and if they're mutable, they could mutate it and therefore change what c sees. But if you have
immutable C c = foo();
then you know that it's not possible for the object referred to by c to change. Once an immutable object has been constructed, it's illegal for it to be mutated, and unless you subvert the type system via casting, it's not even possible to have a mutable reference to an immutable object. And since immutable objects can be put into read-only memory if the compiler chooses to, you could actually get segfaults and the like if you ever tried to cast away immutable and mutate the object. The same goes for const, since a const reference could actually refer to an immutable object. Casting away either const or immutable and then mutating the then mutable object is undefined behavior and should basically never be done.
And since an immutable object can never be mutated even by another reference, reading an immutable object from multiple threads is completely thread-safe. So, immutable objects are implicitly shared across threads, whereas everything else which isn't explicitly marked with shared is considered thread-local. immutable also provides better optimization opportunities to the compiler than const does, because it's guaranteed to never change, whereas a const object can change through another reference to the same data.
For value types, there isn't really much difference between const and immutable (since you can't have mutable references to non-mutable value types), but for reference types, there is a significant difference.
When you declare something as const, you promise that you won't modify it. When something is declared as immutable, you get promised that it won't get modified somewhere else(and ofcourse, you can't modify it either)
They are different in that immutable data, could actually placed in read-only sections of memory, and hence, any attempts to modify the data it will fail.
Something declared const (and not immutable) on the other hand exists in the r/w section and the value can still be changed via a different non-const reference to it.
So, "const-ness" can be bypassed in such a case, while immutability cannot.
(Reference)
A variable declared of type const can accept a mutable value or immutable value. This definition is relevant for referenced types like arrays and objects or pointers. It would typically be used for function arguments. So in D const is a kind of wildcard attribute for mutable and immutable values.
It doesn't have much sense for values that are copied with an assignment like a char, int or float.
The concept of const and immutable is very different from the one found in C and C++. I was very confused by this.
Related
I'm learning Swift and I cannot find a way to achieve a C++ equivalent of a reference to const.
I'd like my object to be immutable instead of a immutable reference achieved by let.
Is there a way to have that in Swift?
As others have mentioned in the comments, structs are the way to go with Swift if you want to ensure immutability. Regardless their properties are let or var, as long as the struct is passed by value (i.e. not inout), you are guaranteed that you won't be able to change the struct instance that was passed to your function. Even if you mutate it within your function, you'll only mutate a copy.
Now, back to C++, things can go horribly wrong even with const references, since you can easily bypass them. Take for example this code:
struct Immutable {
public:
int someIntValue = 9;
};
void doSomething(const Immutable &immutable) {
//immutable->someIntValue = 10; // won't compile
Immutable *mutableInstance = (Immutable *)&immutable;
mutableInstance->someIntValue = 10;
}
int main() {
Immutable immutable;
printf("%d\n", immutable.someIntValue); // 9
doSomething(immutable);
printf("%d\n", immutable.someIntValue); // 10... hmm.. did my const reference just got mutated?
}
Clearly, the commented out line won't compile, as you'll be attempting to change a const reference, however as soon as you cast to a normal pointer you'll be able to alter the instance as you like.
Now, constructs like the above one are not possible in Swift, due to value vs. reference semantics. Struct's passed by reference are in no way alterable from the caller, unless the caller allows it (inout).
Conclusion: Swift has better mechanisms when it comes to readonly references, and these come in the form of value types. As long as you use value types you're guaranteed to have the control that you want. When you start circulating reference types... well... it's not much differences of what you have in C++.
In Swift, when you pass a value type, say an Array to a function. A copy of the array is made for the function to use.
However the documentation at https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ClassesAndStructures.html#//apple_ref/doc/uid/TP40014097-CH13-XID_134 also says:
The description above refers to the “copying” of strings, arrays, and
dictionaries. The behavior you see in your code will always be as if a
copy took place. However, Swift only performs an actual copy behind
the scenes when it is absolutely necessary to do so. Swift manages all
value copying to ensure optimal performance, and you should not avoid
assignment to try to preempt this optimization.
So does it mean that the copying actually only takes placed when the passed value type is modified?
Is there a way to demonstrate that this is actually the underlying behavior?
Why this is important? If I create a large immutable array and want to pass it in from function to function, I certainly do not want to keep making copies of it. Should I just use NSArrray in this case or would the Swift Array work fine as long as I do not try to manipulate the passed in Array?
Now as long as I do not explicitly make the variables in the function editable by using var or inout, then the function can not modify the array anyway. So does it still make a copy? Granted that another thread can modify the original array elsewhere (only if it is mutable), making a copy at the moment the function is called necessary (but only if the array passed in is mutable). So if the original array is immutable and the function is not using var or inout, there is no point in Swift creating a copy. Right? So what does Apple mean by the phrase above?
TL;DR:
So does it mean that the copying actually only takes placed when the passed value type is modified?
Yes!
Is there a way to demonstrate that this is actually the underlying behavior?
See the first example in the section on the copy-on-write optimization.
Should I just use NSArrray in this case or would the Swift Array work fine
as long as I do not try to manipulate the passed in Array?
If you pass your array as inout, then you'll have a pass-by-reference semantics,
hence obviously avoiding unnecessary copies.
If you pass your array as a normal parameter,
then the copy-on-write optimization will kick in and you shouldn't notice any performance drop
while still benefiting from more type safety that what you'd get with a NSArray.
Now as long as I do not explicitly make the variables in the function editable
by using var or inout, then the function can not modify the array anyway.
So does it still make a copy?
You will get a "copy", in the abstract sense.
In reality, the underlying storage will be shared, thanks to the copy-on-write mechanism,
hence avoiding unnecessary copies.
If the original array is immutable and the function is not using var or inout,
there is no point in Swift creating a copy. Right?
Exactly, hence the copy-on-write mechanism.
So what does Apple mean by the phrase above?
Essentially, Apple means that you shouldn't worry about the "cost" of copying value types,
as Swift optimizes it for you behind the scene.
Instead, you should just think about the semantics of value types,
which is that get a copy as soon as you assign or use them as parameters.
What's actually generated by Swift's compiler is the Swift's compiler business.
Value types semantics
Swift does indeed treat arrays as value types (as opposed to reference types),
along with structures, enumerations and most other built-in types
(i.e. those that are part of the standard library and not Foundation).
At the memory level, these types are actually immutable plain old data objects (POD),
which enables interesting optimizations.
Indeed, they are typically allocated on the stack rather than the heap [1],
(https://en.wikipedia.org/wiki/Stack-based_memory_allocation).
This allows the CPU to very efficiently manage them,
and to automatically deallocate their memory as soon as the function exits [2],
without the need for any garbage collection strategy.
Values are copied whenever assigned or passed as a function.
This semantics has various advantages,
such as avoiding the creation of unintended aliases,
but also as making it easier for the compiler to guarantee the lifetime of values
stored in a another object or captured by a closure.
We can think about how hard it can be to manage good old C pointers to understand why.
One may think it's an ill-conceived strategy,
as it involves copying every single time a variable is assigned or a function is called.
But as counterintuitive it may be,
copying small types is usually quite cheap if not cheaper than passing a reference.
After all, a pointer is usually the same size as an integer...
Concerns are however legitimate for large collections (i.e. arrays, sets and dictionaries),
and very large structures to a lesser extent [3].
But the compiler has has a trick to handle these, namely copy-on-write (see later).
What about mutating
Structures can define mutating methods,
which are allowed to mutate the fields of the structure.
This doesn't contradict the fact that value types are nothing more than immutable PODs,
as in fact calling a mutating method is merely a huge syntactic sugar
for reassigning a variable to a brand new value that's identical to the previous ones,
except for the fields that were mutated.
The following example illustrates this semantical equivalence:
struct S {
var foo: Int
var bar: Int
mutating func modify() {
foo = bar
}
}
var s1 = S(foo: 0, bar: 10)
s1.modify()
// The two lines above do the same as the two lines below:
var s2 = S(foo: 0, bar: 10)
s2 = S(foo: s2.bar, bar: s2.bar)
Reference types semantics
Unlike value types, reference types are essentially pointers to the heap at the memory level.
Their semantics is closer to what we would get in reference-based languages,
such as Java, Python or Javascript.
This means they do not get copied when assigned or passed to a function, their address is.
Because the CPU is no longer able to manage the memory of these objects automatically,
Swift uses a reference counter to handle garbage collection behind the scenes
(https://en.wikipedia.org/wiki/Reference_counting).
Such semantics has the obvious advantage to avoid copies,
as everything is assigned or passed by reference.
The drawback is the danger of unintended aliases,
as in almost any other reference-based language.
What about inout
An inout parameter is nothing more than a read-write pointer to the expected type.
In the case of value types, it means the function won't get a copy of the value,
but a pointer to such values,
so mutations inside the function will affect the value parameter (hence the inout keyword).
In other terms, this gives value types parameters a reference semantics in the context of the function:
func f(x: inout [Int]) {
x.append(12)
}
var a = [0]
f(x: &a)
// Prints '[0, 12]'
print(a)
In the case of reference types, it will make the reference itself mutable,
pretty much as if the passed argument was a the address of the address of the object:
func f(x: inout NSArray) {
x = [12]
}
var a: NSArray = [0]
f(x: &a)
// Prints '(12)'
print(a)
Copy-on-write
Copy-on-write (https://en.wikipedia.org/wiki/Copy-on-write) is an optimization technique that
can avoid unnecessary copies of mutable variables,
which is implemented on all Swift's built-in collections (i.e. array, sets and dictionaries).
When you assign an array (or pass it to a function),
Swift doesn't make a copy of the said array and actually uses a reference instead.
The copy will take place as soon as the your second array is mutated.
This behavior can be demonstrated with the following snippet (Swift 4.1):
let array1 = [1, 2, 3]
var array2 = array1
// Will print the same address twice.
array1.withUnsafeBytes { print($0.baseAddress!) }
array2.withUnsafeBytes { print($0.baseAddress!) }
array2[0] = 1
// Will print a different address.
array2.withUnsafeBytes { print($0.baseAddress!) }
Indeed, array2 doesn't get a copy of array1 immediately,
as shown by the fact it points to the same address.
Instead, the copy is triggered by the mutation of array2.
This optimization also happens deeper in the structure,
meaning that if for instance your collection is made of other collections,
the latter will also benefit from the copy-on-write mechanism,
as demonstrated by the following snippet (Swift 4.1):
var array1 = [[1, 2], [3, 4]]
var array2 = array1
// Will print the same address twice.
array1[1].withUnsafeBytes { print($0.baseAddress!) }
array2[1].withUnsafeBytes { print($0.baseAddress!) }
array2[0] = []
// Will print the same address as before.
array2[1].withUnsafeBytes { print($0.baseAddress!) }
Replicating copy-on-write
It is in fact rather easy to implement the copy-on-write mechanism in Swift,
as some of the its reference counter API is exposed to the user.
The trick consists of wrapping a reference (e.g. a class instance) within a structure,
and to check whether that reference is uniquely referenced before mutating it.
When that's the case, the wrapped value can be safely mutated,
otherwise it should be copied:
final class Wrapped<T> {
init(value: T) { self.value = value }
var value: T
}
struct CopyOnWrite<T> {
init(value: T) { self.wrapped = Wrapped(value: value) }
var wrapped: Wrapped<T>
var value: T {
get { return wrapped.value }
set {
if isKnownUniquelyReferenced(&wrapped) {
wrapped.value = newValue
} else {
wrapped = Wrapped(value: newValue)
}
}
}
}
var a = CopyOnWrite(value: SomeLargeObject())
// This line doesn't copy anything.
var b = a
However, there is an import caveat here!
Reading the documentation for isKnownUniquelyReferenced we get this warning:
If the instance passed as object is being accessed by multiple threads simultaneously,
this function may still return true.
Therefore, you must only call this function from mutating methods
with appropriate thread synchronization.
This means the implementation presented above isn't thread safe,
as we may encounter situations where it'd wrongly assumes the wrapped object can be safely mutated,
while in fact such mutation would break invariant in another thread.
Yet this doesn't mean Swift's copy-on-write is inherently flawed in multithreaded programs.
The key is to understand what "accessed by multiple threads simultaneously" really means.
In our example, this would happen if the same instance of CopyOnWrite was shared across multiple threads,
for instance as part of a shared global variable.
The wrapped object would then have a thread safe copy-on-write semantics,
but the instance holding it would be subject to data race.
The reason is that Swift must establish unique ownership
to properly evaluate isKnownUniquelyReferenced [4],
which it can't do if the owner of the instance is itself shared across multiple threads.
Value types and multithreading
It is Swift's intention to alleviate the burden of the programmer
when dealing with multithreaded environments, as stated on Apple's blog
(https://developer.apple.com/swift/blog/?id=10):
One of the primary reasons to choose value types over reference types
is the ability to more easily reason about your code.
If you always get a unique, copied instance,
you can trust that no other part of your app is changing the data under the covers.
This is especially helpful in multi-threaded environments
where a different thread could alter your data out from under you.
This can create nasty bugs that are extremely hard to debug.
Ultimately, the copy-on-write mechanism is a resource management optimization that,
like any other optimization technique,
one shouldn't think about when writing code [5].
Instead, one should think in more abstract terms
and consider values to be effectively copied when assigned or passed as arguments.
[1]
This holds only for values used as local variables.
Values used as fields of a reference type (e.g. a class) are also stored in the heap.
[2]
One could get confirmation of that by checking the LLVM byte code that's produced
when dealing with value types rather than reference types,
but the Swift compiler being very eager to perform constant propagation,
building a minimal example is a bit tricky.
[3]
Swift doesn't allow structures to reference themselves,
as the compiler would be unable to compute the size of such type statically.
Therefore, it is not very realistic to think of a structure that is so large
that copying it would become a legitimate concern.
[4]
This is, by the way, the reason why isKnownUniquelyReferenced accepts an inout parameter,
as it's currently Swift's way to establish ownership.
[5]
Although passing copies of value-type instances should be safe,
there's a open issue that suggests some problems with the current implementation
(https://bugs.swift.org/browse/SR-6543).
I don't know if that's the same for every value type in Swift, but for Arrays I'm pretty sure it's a copy-on-write, so it doesn't copy it unless you modify it, and as you said if you pass it around as a constant you don't run that risk anyway.
p.s. In Swift 1.2 there are new APIs you can use to implement copy-on-write on your own value-types too
In Swift, when you pass a value type, say an Array to a function. A copy of the array is made for the function to use.
However the documentation at https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ClassesAndStructures.html#//apple_ref/doc/uid/TP40014097-CH13-XID_134 also says:
The description above refers to the “copying” of strings, arrays, and
dictionaries. The behavior you see in your code will always be as if a
copy took place. However, Swift only performs an actual copy behind
the scenes when it is absolutely necessary to do so. Swift manages all
value copying to ensure optimal performance, and you should not avoid
assignment to try to preempt this optimization.
So does it mean that the copying actually only takes placed when the passed value type is modified?
Is there a way to demonstrate that this is actually the underlying behavior?
Why this is important? If I create a large immutable array and want to pass it in from function to function, I certainly do not want to keep making copies of it. Should I just use NSArrray in this case or would the Swift Array work fine as long as I do not try to manipulate the passed in Array?
Now as long as I do not explicitly make the variables in the function editable by using var or inout, then the function can not modify the array anyway. So does it still make a copy? Granted that another thread can modify the original array elsewhere (only if it is mutable), making a copy at the moment the function is called necessary (but only if the array passed in is mutable). So if the original array is immutable and the function is not using var or inout, there is no point in Swift creating a copy. Right? So what does Apple mean by the phrase above?
TL;DR:
So does it mean that the copying actually only takes placed when the passed value type is modified?
Yes!
Is there a way to demonstrate that this is actually the underlying behavior?
See the first example in the section on the copy-on-write optimization.
Should I just use NSArrray in this case or would the Swift Array work fine
as long as I do not try to manipulate the passed in Array?
If you pass your array as inout, then you'll have a pass-by-reference semantics,
hence obviously avoiding unnecessary copies.
If you pass your array as a normal parameter,
then the copy-on-write optimization will kick in and you shouldn't notice any performance drop
while still benefiting from more type safety that what you'd get with a NSArray.
Now as long as I do not explicitly make the variables in the function editable
by using var or inout, then the function can not modify the array anyway.
So does it still make a copy?
You will get a "copy", in the abstract sense.
In reality, the underlying storage will be shared, thanks to the copy-on-write mechanism,
hence avoiding unnecessary copies.
If the original array is immutable and the function is not using var or inout,
there is no point in Swift creating a copy. Right?
Exactly, hence the copy-on-write mechanism.
So what does Apple mean by the phrase above?
Essentially, Apple means that you shouldn't worry about the "cost" of copying value types,
as Swift optimizes it for you behind the scene.
Instead, you should just think about the semantics of value types,
which is that get a copy as soon as you assign or use them as parameters.
What's actually generated by Swift's compiler is the Swift's compiler business.
Value types semantics
Swift does indeed treat arrays as value types (as opposed to reference types),
along with structures, enumerations and most other built-in types
(i.e. those that are part of the standard library and not Foundation).
At the memory level, these types are actually immutable plain old data objects (POD),
which enables interesting optimizations.
Indeed, they are typically allocated on the stack rather than the heap [1],
(https://en.wikipedia.org/wiki/Stack-based_memory_allocation).
This allows the CPU to very efficiently manage them,
and to automatically deallocate their memory as soon as the function exits [2],
without the need for any garbage collection strategy.
Values are copied whenever assigned or passed as a function.
This semantics has various advantages,
such as avoiding the creation of unintended aliases,
but also as making it easier for the compiler to guarantee the lifetime of values
stored in a another object or captured by a closure.
We can think about how hard it can be to manage good old C pointers to understand why.
One may think it's an ill-conceived strategy,
as it involves copying every single time a variable is assigned or a function is called.
But as counterintuitive it may be,
copying small types is usually quite cheap if not cheaper than passing a reference.
After all, a pointer is usually the same size as an integer...
Concerns are however legitimate for large collections (i.e. arrays, sets and dictionaries),
and very large structures to a lesser extent [3].
But the compiler has has a trick to handle these, namely copy-on-write (see later).
What about mutating
Structures can define mutating methods,
which are allowed to mutate the fields of the structure.
This doesn't contradict the fact that value types are nothing more than immutable PODs,
as in fact calling a mutating method is merely a huge syntactic sugar
for reassigning a variable to a brand new value that's identical to the previous ones,
except for the fields that were mutated.
The following example illustrates this semantical equivalence:
struct S {
var foo: Int
var bar: Int
mutating func modify() {
foo = bar
}
}
var s1 = S(foo: 0, bar: 10)
s1.modify()
// The two lines above do the same as the two lines below:
var s2 = S(foo: 0, bar: 10)
s2 = S(foo: s2.bar, bar: s2.bar)
Reference types semantics
Unlike value types, reference types are essentially pointers to the heap at the memory level.
Their semantics is closer to what we would get in reference-based languages,
such as Java, Python or Javascript.
This means they do not get copied when assigned or passed to a function, their address is.
Because the CPU is no longer able to manage the memory of these objects automatically,
Swift uses a reference counter to handle garbage collection behind the scenes
(https://en.wikipedia.org/wiki/Reference_counting).
Such semantics has the obvious advantage to avoid copies,
as everything is assigned or passed by reference.
The drawback is the danger of unintended aliases,
as in almost any other reference-based language.
What about inout
An inout parameter is nothing more than a read-write pointer to the expected type.
In the case of value types, it means the function won't get a copy of the value,
but a pointer to such values,
so mutations inside the function will affect the value parameter (hence the inout keyword).
In other terms, this gives value types parameters a reference semantics in the context of the function:
func f(x: inout [Int]) {
x.append(12)
}
var a = [0]
f(x: &a)
// Prints '[0, 12]'
print(a)
In the case of reference types, it will make the reference itself mutable,
pretty much as if the passed argument was a the address of the address of the object:
func f(x: inout NSArray) {
x = [12]
}
var a: NSArray = [0]
f(x: &a)
// Prints '(12)'
print(a)
Copy-on-write
Copy-on-write (https://en.wikipedia.org/wiki/Copy-on-write) is an optimization technique that
can avoid unnecessary copies of mutable variables,
which is implemented on all Swift's built-in collections (i.e. array, sets and dictionaries).
When you assign an array (or pass it to a function),
Swift doesn't make a copy of the said array and actually uses a reference instead.
The copy will take place as soon as the your second array is mutated.
This behavior can be demonstrated with the following snippet (Swift 4.1):
let array1 = [1, 2, 3]
var array2 = array1
// Will print the same address twice.
array1.withUnsafeBytes { print($0.baseAddress!) }
array2.withUnsafeBytes { print($0.baseAddress!) }
array2[0] = 1
// Will print a different address.
array2.withUnsafeBytes { print($0.baseAddress!) }
Indeed, array2 doesn't get a copy of array1 immediately,
as shown by the fact it points to the same address.
Instead, the copy is triggered by the mutation of array2.
This optimization also happens deeper in the structure,
meaning that if for instance your collection is made of other collections,
the latter will also benefit from the copy-on-write mechanism,
as demonstrated by the following snippet (Swift 4.1):
var array1 = [[1, 2], [3, 4]]
var array2 = array1
// Will print the same address twice.
array1[1].withUnsafeBytes { print($0.baseAddress!) }
array2[1].withUnsafeBytes { print($0.baseAddress!) }
array2[0] = []
// Will print the same address as before.
array2[1].withUnsafeBytes { print($0.baseAddress!) }
Replicating copy-on-write
It is in fact rather easy to implement the copy-on-write mechanism in Swift,
as some of the its reference counter API is exposed to the user.
The trick consists of wrapping a reference (e.g. a class instance) within a structure,
and to check whether that reference is uniquely referenced before mutating it.
When that's the case, the wrapped value can be safely mutated,
otherwise it should be copied:
final class Wrapped<T> {
init(value: T) { self.value = value }
var value: T
}
struct CopyOnWrite<T> {
init(value: T) { self.wrapped = Wrapped(value: value) }
var wrapped: Wrapped<T>
var value: T {
get { return wrapped.value }
set {
if isKnownUniquelyReferenced(&wrapped) {
wrapped.value = newValue
} else {
wrapped = Wrapped(value: newValue)
}
}
}
}
var a = CopyOnWrite(value: SomeLargeObject())
// This line doesn't copy anything.
var b = a
However, there is an import caveat here!
Reading the documentation for isKnownUniquelyReferenced we get this warning:
If the instance passed as object is being accessed by multiple threads simultaneously,
this function may still return true.
Therefore, you must only call this function from mutating methods
with appropriate thread synchronization.
This means the implementation presented above isn't thread safe,
as we may encounter situations where it'd wrongly assumes the wrapped object can be safely mutated,
while in fact such mutation would break invariant in another thread.
Yet this doesn't mean Swift's copy-on-write is inherently flawed in multithreaded programs.
The key is to understand what "accessed by multiple threads simultaneously" really means.
In our example, this would happen if the same instance of CopyOnWrite was shared across multiple threads,
for instance as part of a shared global variable.
The wrapped object would then have a thread safe copy-on-write semantics,
but the instance holding it would be subject to data race.
The reason is that Swift must establish unique ownership
to properly evaluate isKnownUniquelyReferenced [4],
which it can't do if the owner of the instance is itself shared across multiple threads.
Value types and multithreading
It is Swift's intention to alleviate the burden of the programmer
when dealing with multithreaded environments, as stated on Apple's blog
(https://developer.apple.com/swift/blog/?id=10):
One of the primary reasons to choose value types over reference types
is the ability to more easily reason about your code.
If you always get a unique, copied instance,
you can trust that no other part of your app is changing the data under the covers.
This is especially helpful in multi-threaded environments
where a different thread could alter your data out from under you.
This can create nasty bugs that are extremely hard to debug.
Ultimately, the copy-on-write mechanism is a resource management optimization that,
like any other optimization technique,
one shouldn't think about when writing code [5].
Instead, one should think in more abstract terms
and consider values to be effectively copied when assigned or passed as arguments.
[1]
This holds only for values used as local variables.
Values used as fields of a reference type (e.g. a class) are also stored in the heap.
[2]
One could get confirmation of that by checking the LLVM byte code that's produced
when dealing with value types rather than reference types,
but the Swift compiler being very eager to perform constant propagation,
building a minimal example is a bit tricky.
[3]
Swift doesn't allow structures to reference themselves,
as the compiler would be unable to compute the size of such type statically.
Therefore, it is not very realistic to think of a structure that is so large
that copying it would become a legitimate concern.
[4]
This is, by the way, the reason why isKnownUniquelyReferenced accepts an inout parameter,
as it's currently Swift's way to establish ownership.
[5]
Although passing copies of value-type instances should be safe,
there's a open issue that suggests some problems with the current implementation
(https://bugs.swift.org/browse/SR-6543).
I don't know if that's the same for every value type in Swift, but for Arrays I'm pretty sure it's a copy-on-write, so it doesn't copy it unless you modify it, and as you said if you pass it around as a constant you don't run that risk anyway.
p.s. In Swift 1.2 there are new APIs you can use to implement copy-on-write on your own value-types too
I recently wanted to make use of std.container.Array and proceeded to create a class with a getter member function which returns a value from the Array class. I quickly realised that I was not able to const-qualify my getter, since opIndex is a mutable function.
I tried changing the source code to const-qualify Array.opIndex, and it built fine. However, some unit tests in std.algorithm did not pass, complaining that the return value of Array.opIndex is not an lvalue.
Here is the code for Array.opIndex:
ref T opIndex(size_t i)
{
version (assert) if (!_data.refCountedStore.isInitialized) throw new RangeError();
return _data._payload[i];
}
Is there something I'm missing here? Why is it not const-qualified?
There are a number of issues with making the containers const-correct, since const makes it so that they can't change anything in their internals, unlike in C++, where you could make some stuff mutable as long as you made sure that the functions were logically const. IIRC, there are operations that Array does which could theoretically be const but can't be due to how some of its internals work. And it wouldn't surprise me if because of that, the folks who have worked on it didn't make any of it const, even if some of it could be.
As for opIndex, I don't see anything obvious in that implementation which couldn't be const, and the fact that it compiled at all implies that it might work. However, if you do that, you need to overload it rather than simply make that particular overload const, or you won't be able to assign to it - which is presumably what std.algorithm was complaining about it. So, you'd need something like
ref T opIndex(size_t i) {...}
ref const(T) opIndex(size_t i) const {...}
so that it still works to assign to it - e.g. arr[5] = "foo"; - as long as the Array isn't const. However, since many of Array's operations can't be const due to how its implementation works, I don't know how useful it really is to make functions like opIndex const, because you'll be very limited in what you can do with a const Array!T even if every member function that can be const is const.
Is the answer to this :
Instantiation of the object uses 'val' instead of 'var'.
Each member variable of the object being created is also 'val' instead of 'var'. This is to prevent users updating an object value after its set.
An object is immutable if there is no way for the user of that object to mutate it. This means that it must have no public methods that reassign any of its member variables or mutate any objects referred to by those variables. If all the object's members are vals this ensures the former (i.e. they can't be reassigned), but not the latter (i.e. if the objects referred to by those variables are themselves mutable, they can still be mutated by calling mutating methods on them even if they're referred to only by vals).
Also note that even if the members are declared as vars, the object can still be immutable if none of the object's methods actually reassign the variables (or call mutating methods on them) - assuming of course, they're private.
So having only val members is neither necessary nor sufficient for an object being immutable. Whether the object is referred to by a val or a var (or both) makes no difference in that matter.
#sepp2k nicely and correctly explains the criteria for an object being technically immutable. One subtle point missing from his answer is that not all member variables correspond to externally visible state. A member may also be e.g. a cached internal value to store some local, hard to compute data which is not directly visible from outside (thus qualified as private[this] in Scala). An object can have such a var member e.g. to store a computed hash value. It can even be accessible via a public getter - as long as the behaviour of the accessor is purely functional, i.e. it always produces the same value for each invocation on the same object (except that it returns faster when reusing the internally cached value).
The Scala compiler is aware of this distinction so it can help one to implement an immutable class correctly, even when using mutable state internally. This is important when generic type variance comes into play. Namely, the compiler allows a generic type parameter to be covariant even if the class contains reassignable fields of this type - as long as these fields are private[this], ensuring that one cannot have a reference to a containing object that has a statically weaker type than the type the object was defined with (which would be a precondition for variance to cause type errors).
This is explained in more detail, with a code example, in section 19.7 of Programming in Scala.