what is string is it a data type or class ?
var somename:String="sdsa"
somename.append("sd")
we can able to conclude that
somename is a variable but how does the append generated
it should been generated only when this happens
var somename =String()(we have instance so that we can access its properties)
but how is append came when var somename:String
what is the role of colon symbol
what is string is it a data type or class ?
A String is a struct. You can read about structs and how they differ from classes here: https://docs.swift.org/swift-book/LanguageGuide/ClassesAndStructures.html
it should been generated only when this happens
var somename =String()
A variable can be initialized with a literal. These are all identical:
var some1 = "Hello World"
var some2: String = "Hello World"
var some3 = String("Hello World")
what is the role of colon symbol
It's a type annotation, which you can read about here: https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html
how to create custom struct like String
and make that custom struct behave as like string Struct?
These questions are very general, and I can't be more specific in my answer than referring you to a few chapters in the Swift Book.
Structs are covered starting at the section I linked above and continue on with the following few chapters.
https://docs.swift.org/swift-book/LanguageGuide/ClassesAndStructures.html
I recommend reading over the entire Language Guide section in the Swift Book.