Anonymous struct, difference between struct{}{} and {} - dictionary

I have string to struct map in golang defined in the following way:
var Foo = map[string]struct{}{
"foo": struct{}{},
}
Gogland by default marks this declaration as warning, saying "Redundant type declaration".
var Foo = map[string]struct{}{
"foo": {},
}
Above code solves the warning, but I couldn't find any information about the difference between struct{}{} and {} declaration. Is it kind of a "short notation"?
https://play.golang.org/p/0Akx98XtB4

This:
struct{}{}
is a composite literal which contains the type (struct{}) and the literal's value ({}).
This:
{}
is also a composite literal without the type, just the value.
Generally you have to specify / include the type in the composite literal to let the compiler know what kind of (what "type" of) composite literal you're creating, hence the syntax is:
CompositeLit = LiteralType LiteralValue .
But when you specify a map composite literal, the types of the key and value are known from the map type, and thus may be omitted if you intend to specify values of those types. This is mentioned in Spec: Composite literals:
Within a composite literal of array, slice, or map type T, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of T. Similarly, elements or keys that are addresses of composite literals may elide the &T when the element or key type is *T.
(Note: due to an oversight, this is only valid from Go 1.5.)

Related

Swiftui: how do you assign the value in a "String?" object to a "String" object?

Swiftui dictionaries have the feature that the value returned by using key access is always of type "optional". For example, a dictionary that has type String keys and type String values is tricky to access because each returned value is of type optional.
An obvious need is to assign x=myDictionary[key] where you are trying to get the String of the dictionary "value" into the String variable x.
Well this is tricky because the String value is always returned as an Optional String, usually identified as type String?.
So how is it possible to convert the String?-type value returned by the dictionary access into a plain String-type that can be assigned to a plain String-type variable?
I guess the problem is that there is no way to know for sure that there exists a dictionary value for the key. The key used to access the dictionary could be anything so somehow you have to deal with that.
As described in #jnpdx answer to this SO question (How do you assign a String?-type object to a String-type variable?), there are at least three ways to convert a String? to a String:
import SwiftUI
var x: Double? = 6.0
var a = 2.0
if x != nil {
a = x!
}
if let b = x {
a = x!
}
a = x ?? 0.0
Two key concepts:
Check the optional to see if it is nil
if the optional is not equal to nil, then go ahead
In the first method above, "if x != nil" explicitly checks to make sure x is not nil be fore the closure is executed.
In the second method above, "if let a = b" will execute the closure as long as b is not equal to nil.
In the third method above, the "nil-coalescing" operator ?? is employed. If x=nil, then the default value after ?? is assigned to a.
The above code will run in a playground.
Besides the three methods above, there is at least one other method using "guard let" but I am uncertain of the syntax.
I believe that the three above methods also apply to variables other than String? and String.

ASN.1: Can the SIZE constraint be used to constrain SEQUENCE (not SEQUENCE OF)

Is the following type definition valid ASN.1 syntax?
MyType ::= SEQUENCE SIZE(2) {
theID OBJECT IDENTIFIER,
someNumber INTEGER OPTIONAL,
someString PrintableString OPTIONAL
}
I want to formally constrain the sequence such that for each instantiation exactly one of both optional attributes must be present.
PS: As SEQUENCE and SEQUENCE OF are encoded exactly the same way, I have the slight hope that this syntax is valid.
This syntax is not valid. SEQUENCE is like a struct type with fields, where each field can be of different type. SEQUENCE OF is like an array, where all nested types are of same type. Thus, in SEQUENCE OF you can set constraint to array size. Same rules apply to SET and SET OF, just unordered.
I would solve your problem this way: create a CHOICE of all your optional fields (without OPTIONAL modifier) and add this CHOICE to your main type:
MyChoice ::= CHOICE {
someNumber INTEGER,
someString PrintableString
}
MyType ::= SEQUENCE {
theID OBJECT IDENTIFIER,
myChoice < MyChoice
}
In this case, theID field is mandatory and exactly one of MyChoice elements is required.

How does a pointer to a struct or array value in Go work?

Considering the following Go struct:
type Person struct {
Name string
Age int
Country string
}
I have encountered numerious times the following use:
p := &Person{"Adam", 33, "Argentina"}
Yet I can not see the point in pointing to a struct value, and I wonder, how does it differ from:
n := &999 // Error
My questions are:
How is it even possible to point to a value, even if it is a struct or array and not a primitive like a string or int? Strange enough, the following doesn't contribute to my understanding:
fmt.Println(p, &p) // outputs: &{Adam 33 Argentina} 0xc042084018
Why would a programmer want to declare a struct instance by a pointer? What could you achieve doing so?
&Person{} is a language "construct", it's part of the spec: it allocates a new variable of Person type, and provides you the address of that anonymous variable.
Spec: Composite literals:
Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.
Also: Spec: Variables:
Calling the built-in function new or taking the address of a composite literal allocates storage for a variable at run time.
&999 is not allowed by the language spec. The possible operands of the address operators are listed in the Spec: Address operators:
The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x may also be a (possibly parenthesized) composite literal.
p := Person{} creates a new variable p whose type will be Person. p := &Person{} creates a new variable whose type will be *Person.
See possible duplicate: How do I do a literal *int64 in Go?
When you print the values with the fmt package, it has certain rules how to print values of different types:
For compound objects, the elements are printed using these rules, recursively, laid out like this:
struct: {field0 field1 ...}
array, slice: [elem0 elem1 ...]
maps: map[key1:value1 key2:value2]
pointer to above: &{}, &[], &map[]
When you use fmt.Println(), the default formatting rules will be applied, which for a value of type *int is the %p verb, which will print the memory address in hexadecimal format, but for a pointer to struct it prints the struct value prepended with an & sign (&{}). You can read more about it in related question: Difference between golang pointers
If you want to print the pointed value, dereference the pointer and pass the pointed value, e.g.:
var p = new(int)
*p = 12
fmt.Println(*p) // Prints 12
As to why to create a pointer to a value (and not a value), see these related questions:
Pointers vs. values in parameters and return values
Why should constructor of Go return address?
Go, X does not implement Y (... method has a pointer receiver)

golang shortcut syntax for slice literal of pointers to structs

Given
type foo struct {
id int
}
the two look equivalent
var foos = []*foo{
{1},
{2},
{3}}
var foos = []*foo{
&foo{1},
&foo{2},
&foo{3}}
Why is that? Can't find mentioning of it in TGPL though.
https://play.golang.org/p/JXxZaybbWnV
You can skip the type in composite literals.
Within a composite literal of array, slice, or map type T, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of T. Similarly, elements or keys that are addresses of composite literals may elide the &T when the element or key type is *T.

How can I prevent a type being used as a map key?

I have a type that can be used as a map key, but I want to prevent this from occurring. I assumed that if the type contained a private member it wouldn't be possible from other packages, but this appears to work anyway. What's the best way to make the type unusable as a map key?
type MyType struct {
A *A
b b
preventUseAsKey ?
}
I don't see any benefit of disallowing a type being used as a key. It is just an option which may or may not be used, the type will not be any better or smaller or faster just because you forbid to use it as a map key.
But if you want to do it: Spec: Map types:
The comparison operators == and != must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice.
So if you can violate the terms of the comparison operators, you implicitly get what you want. You have a struct, terms for the struct types:
Struct values are comparable if all their fields are comparable. Two struct values are equal if their corresponding non-blank fields are equal.
So struct values are only comparable (and thus can only be used as keys in maps) if all their fields are comparable. Simply add a field whose type is not comparable.
Slice, map, and function values are not comparable.
So for example add a field whose type is a slice, and you're done:
type MyType struct {
S string
i int
notComparable []int
}
Attempting to use the above MyType as a key:
m := map[MyType]int{}
You get a compile-time error:
invalid map key type MyType
Note:
I wrote about not having any benefit of forbidding the type being a key. It's more than that: from now on you won't be able to use comparison operators on values of your type anymore (because of the extra, non-comparable field), so e.g. you lose the option to compare those values:
p1, p2 := MyType{}, MyType{}
fmt.Println(p1 == p2)
Compile-time error:
invalid operation: p1 == p2 (struct containing []int cannot be compared)
Note that with a little trick you could still preserve the comparable nature of your type, e.g. by not exporting your type but a wrapper type which embeds the original one; and add the extra, non-comparable type to the wrapper type, e.g.:
type myType struct {
S string
i int
}
type MyType struct {
myType
notComparable []int
}
func main() {
p1, p2 := MyType{}, MyType{}
fmt.Println(p1.myType == p2.myType)
}
This way your myType can remain comparable but still prevent the exported, wrapper MyType type to be used as key type.
Your type should not be comparable in order to be unfit as a map key.
Slice, map, and function values are not comparable
See Key Type:
Notably absent from the list are slices, maps, and functions; these types cannot be compared using ==, and may not be used as map keys.
So if your type is a slice, map or function, you should get what you need.
It could be an "alias" (defining a new named type):
type StringSliceWrap []string
type MyFunc func(i int)
That alias would not be used as a map key.
Update 2017: Brad Fitzpatrick give this tip (adding a slice in your struct) to make sure your type struct is not comparable: See play.golang.org:
package main
// disallowEqual is an uncomparable type.
// If you place it first in your struct, you prevent == from
// working on your struct without growing its size. (Don't put it
// at the end; that grows the size of the struct)
type disallowEqual [0]func()
type T struct {
_ disallowEqual
Foo string
Bar int
}
func main() {
var t1 T
var t2 T
println(t1 == t2)
}
T cannot be used as amp key now!

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