I need to use a third party C library whose source I cannot modify and which makes heavy use of implicit type casting and typedefs to set values for its structs. They are all ints underneath and this is the preferred way of interacting with this library. I've previously used it in Objective C code without issue so now I am mostly porting some of my old code, but it feels like I am constantly hitting a brick wall with Swift.
tl;dr: how can I assign a different typedef value to a C struct member in Swift while automatically handling the type conversions (all typedefs are ints underneath)?
For example, consider the following definitions:
struct library_struct {
int member;
};
typedef enum library_consts {
LIBRARY_DEFINED_VALUE = 0
} library_consts;
In C or Objective C it would be absolutely acceptable to perform the following:
library_struct a;
a.member = LIBRARY_DEFINED_VALUE
However, attempting to do the same thing in Swift
var a: library_struct = library_struct()
a.member = LIBRARY_DEFINED_VALUE
results in an error:
Cannot assign a value of type 'library_consts' to a value of type 'Int32'
I tried several approaches:
Cast using Int32(). This leads to a Cannot find an initializer for type 'Int32' that accepts and argument list of type (library_consts) error.
Use LIBRARY_DEFINED_VALUE.rawValue. This won't work, because rawValue will return an UInt32, so I'm going to get the following error: Cannot assign a value of type 'UInt32' to a value of type 'Int32'
The only alternative is to cast again the value returned by rawValue to an Int32 like this: Int32(LIBRARY_DEFINED_VALUE.rawValue)
This works, but it feels wrong to make a double cast and it doesn't solve more complicated situations such as assigning a value of a different type (but still an int underneath) to a struct member such as the following:
enum library_consts
{
LIB_FALSE=0,
LIB_TRUE=1
};
typedef int lib_bool_t;
typedef struct another_struct {
lib_bool_t aFlag;
}
var b: another_struct = another_struct()
a.aFlag = LIB_FALSE
This will error out with "Cannot assign a value of type 'library_consts' to a value of type 'lib_bool_t'"
I am afraid that there is no easier solution if you cannot change the
C interface. Using the "Generated Interface" view in Xcode 7 you can
see that
enum library_consts
{
LIB_FALSE=0,
LIB_TRUE=1
};
typedef int lib_bool_t;
are mapped to Swift as
struct library_consts : RawRepresentable {
init(_ rawValue: UInt32)
init(rawValue: UInt32)
var rawValue: UInt32
}
typealias lib_bool_t = Int32
(the C int type is Int32 in Swift).
Swift does no implicit type conversions, which means that you have
to convert the types explicitly. In the second case it would be
var b: another_struct = another_struct()
b.aFlag = lib_bool_t(LIB_FALSE.rawValue)
Related
How are they different? I get a bit confused because they seem to be similar concepts.
How does understanding them help with optimizing compilation time?
From Swift's own documentation:
Type Safety
Swift is a type-safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can’t pass it an Int by mistake.
var welcomeMessage: String
welcomeMessage = 22 // this would create an error because you
//already specified that it's going to be a String
Type Inference
If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.
var meaningOfLife = 42 // meaningOfLife is inferred to be of type Int
meaningOfLife = 55 // it Works, because 55 is an Int
Type Safety & Type Inference together
var meaningOfLife = 42 // 'Type inference' happened here, we didn't specify that this an Int, the compiler itself found out.
meaningOfLife = 55 // it Works, because 55 is an Int
meaningOfLife = "SomeString" // Because of 'Type Safety' ability you will get an
//error message: 'cannot assign value of type 'String' to type 'Int''
Tricky example for protocols with associated types:
Imagine the following protocol
protocol Identifiable {
associatedtype ID
var id: ID { get set }
}
You would adopt it like this:
struct Person: Identifiable {
typealias ID = String
var id: String
}
However you can also adopt it like this:
struct Website: Identifiable {
var id: URL
}
You can remove the typealias. The compiler will still infer the type.
For more see Generics - Associated Types
Thanks to Swift’s type inference, you don’t actually need to declare a
concrete Item of Int as part of the definition of IntStack. Because
IntStack conforms to all of the requirements of the Container
protocol, Swift can infer the appropriate Item to use, simply by
looking at the type of the append(_:) method’s item parameter and the
return type of the subscript. Indeed, if you delete the typealias Item
= Int line from the code above, everything still works, because it’s clear what type should be used for Item.
Type-safety and Generics
Suppose you have the following code:
struct Helper<T: Numeric> {
func adder(_ num1: T, _ num2: T) -> T {
return num1 + num2
}
var num: T
}
T can be anything that's numeric e.g. Int, Double, Int64, etc.
However as soon as you type let h = Helper(num: 10) the compiler will assume that T is an Int. It won't accept Double, Int64, for its adder func anymore. It will only accept Int.
This is again because of type-inference and type-safety.
type-inference: because it has to infer that that the generic is of type Int.
type-safety: because once the T is set to be of type Int, it will no longer accept Int64, Double...
As you can see in the screenshot the signature is now changed to only accept a parameter of type Int
Pro tip to optimize compiler performance:
The less type inference your code has to do the faster it compiles. Hence it's recommended to avoid collection literals. And the longer a collection gets, the slower its type inference becomes...
not bad
let names = ["John", "Ali", "Jane", " Taika"]
good
let names : [String] = ["John", "Ali", "Jane", " Taika"]
For more see this answer.
Also see Why is Swift compile time so slow?
The solution helped his compilation time go down from 10/15 seconds to a single second.
Playing with the sample code from Swift Language Guide: Extensions I've extedned struct Double like this
extension Double {
func someFunc() {
print("someFunc")
}
}
I was surprised that this statement
2.someFunc()
did not generate compile time error like: Value of type 'Int' has no member 'someFunc'. I've expected value of 2 to be implicitly casted to Int but Swift casted it to Double. Why is that ? How does Swift determine that value of 2 in this case is of type Double ?
Then I've tried calling someFunc() like that
let x = 2
x.someFunc()
Here I get the expected compile time error
Does'n this contradict statement in Swift Programming Language 3.0.1 : Language guide : The basics : Type Safety and Type Inference?
Type inference enables a compiler to deduce the type of a particular
expression automatically when it compiles your code, simply by
examining the values you provide.
EDIT
From the responses I've learned that it happens because Double conforms to ExpressibleByIntegerLiteral protocol. However Float struct also does conform to it and some other types as well. Below I have created my struct that also conforms to that protocol. In the end however Double is chosen at compile time. Why? How is the precedence of method from one extension determined over the method from other extension ?
struct someStruct: ExpressibleByIntegerLiteral{
var i:Int = 0
init(integerLiteral value: Int64){
i = Int(value)
}
}
extension someStruct {
func someFunc() {print("Somestruct someFunc") }
}
extension Double {
func someFunc() { print("Double someFunc") }
}
4.someFunc()
//prints: Double someFunc
Double is one of the types that conforms to the protocol ExpressibleByIntegerLiteral. Since 2 is an integer leteral, the compiler can check which of the types conforming to the protocol has a someFunc() and since only Double does, there is no abiguity in this context.
Double conforms to ExpressibleByIntegerLiteral. In your example, the compiler sees that of the all the types that conform to ExpressibleByIntegerLiteral, only Double contains someFunc(), so it knows to create a Double out of 2.
As you noticed in your second example, this behaviour doesn't define implicit casting between types. It only applies to literals.
How are they different? I get a bit confused because they seem to be similar concepts.
How does understanding them help with optimizing compilation time?
From Swift's own documentation:
Type Safety
Swift is a type-safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can’t pass it an Int by mistake.
var welcomeMessage: String
welcomeMessage = 22 // this would create an error because you
//already specified that it's going to be a String
Type Inference
If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.
var meaningOfLife = 42 // meaningOfLife is inferred to be of type Int
meaningOfLife = 55 // it Works, because 55 is an Int
Type Safety & Type Inference together
var meaningOfLife = 42 // 'Type inference' happened here, we didn't specify that this an Int, the compiler itself found out.
meaningOfLife = 55 // it Works, because 55 is an Int
meaningOfLife = "SomeString" // Because of 'Type Safety' ability you will get an
//error message: 'cannot assign value of type 'String' to type 'Int''
Tricky example for protocols with associated types:
Imagine the following protocol
protocol Identifiable {
associatedtype ID
var id: ID { get set }
}
You would adopt it like this:
struct Person: Identifiable {
typealias ID = String
var id: String
}
However you can also adopt it like this:
struct Website: Identifiable {
var id: URL
}
You can remove the typealias. The compiler will still infer the type.
For more see Generics - Associated Types
Thanks to Swift’s type inference, you don’t actually need to declare a
concrete Item of Int as part of the definition of IntStack. Because
IntStack conforms to all of the requirements of the Container
protocol, Swift can infer the appropriate Item to use, simply by
looking at the type of the append(_:) method’s item parameter and the
return type of the subscript. Indeed, if you delete the typealias Item
= Int line from the code above, everything still works, because it’s clear what type should be used for Item.
Type-safety and Generics
Suppose you have the following code:
struct Helper<T: Numeric> {
func adder(_ num1: T, _ num2: T) -> T {
return num1 + num2
}
var num: T
}
T can be anything that's numeric e.g. Int, Double, Int64, etc.
However as soon as you type let h = Helper(num: 10) the compiler will assume that T is an Int. It won't accept Double, Int64, for its adder func anymore. It will only accept Int.
This is again because of type-inference and type-safety.
type-inference: because it has to infer that that the generic is of type Int.
type-safety: because once the T is set to be of type Int, it will no longer accept Int64, Double...
As you can see in the screenshot the signature is now changed to only accept a parameter of type Int
Pro tip to optimize compiler performance:
The less type inference your code has to do the faster it compiles. Hence it's recommended to avoid collection literals. And the longer a collection gets, the slower its type inference becomes...
not bad
let names = ["John", "Ali", "Jane", " Taika"]
good
let names : [String] = ["John", "Ali", "Jane", " Taika"]
For more see this answer.
Also see Why is Swift compile time so slow?
The solution helped his compilation time go down from 10/15 seconds to a single second.
I'm new to Swift and I am confused about the following:
In the lines Int(something) and var x :Int = something, what is the difference between Int() and :Int?
In fact var x = Int(something) and var x : Int = something is exactly the same.
Unlike in Objective-C where int is a scalar type Int in Swift ist a struct and
structs must be initialized.
The former syntax is an explicit call of the initializer, the latter an implicit call by assigning the value
From a pure language perspective, the correct way to assign a value to an integer (or other numerical types) variable is:
let num = Int(16)
or any of its variants.
However Swift implements some syntactic sugar to make that less verbose - thanks to which you can rewrite the above statement as:
let num = 16
which is equivalent to:
let num: Int = 16
(thanks to type inference)
This is possible because the Int type implements the IntegerLiteralConvertible protocol, and by doing that the compiler is able to translate an integer literal into an initializer invocation.
There are several other protocols like that, for string, array, float, etc.
If you want to know more, I recommend reading Swift Literal Convertibles at NSHipster.
If you are wondering if you can do that on your own classes/structs, the answer is yes - you just have to implement the protocol corresponding to the literal type you want to use.
What is currently the best/preferred way to define explicit conversions in Swift? Of the top of my head I can think of two:
Creating custom initializers for the destination type via an extension, like this:
extension String {
init(_ myType: MyType) {
self = "Some Value"
}
}
This way, you could just use String(m) where m is of type MyType to convert m to a string.
Defining toType-Methods in the source type, like this:
class MyType {
func toString() -> String {
return "Some Value"
}
}
This is comparable to Swift's String.toInt(), which returns an Int?. But if this was the definitive way to go I would expect there to be protocols for the basic types for this, like an inversion of the already existing *LiteralConvertible protocols.
Sub-question: None of the two methods allow something like this to compile: let s: MyString = myTypeInstance (as String) (part in parentheses optional), but if I understand right, the as operator is only for downcasting within type hierarchies, is that correct?
The pattern used in swift is the initializer. So for instance when converting an Int to UInt, we have to write:
var i: Int = 10
var u: UInt = UInt(i)
I would stick with that pattern.
As for the subquestion, the documentation states that:
Type casting is a way to check the type of an instance, and/or to treat that instance as if it is a different superclass or subclass from somewhere else in its own class hierarchy.
and
You can also use type casting to check whether a type conforms to a protocol
so no, the as keyword can`t be used to transform a value of a certain type to another type.
That can be tested in a simple way:
var i: Int = 10
var u: UInt = i as UInt
That generates an error:
'Int' is not convertible to 'UInt'
More about Type Casting