Swift generic operator -


i wrote class one: 

struct size {     var width:double=0     var height:double=0     init(width:double=0,height:double=0)     {         self.width=width         self.height=height     }     [...] }

now want ability divide size number, , use generics function each type convertible double. example int, cgfloat, float

but when insert function:

func /<t>(lhs:size,rhs:t)->size {     return size(width:lhs.width/double(rhs),height:lhs.height/double(rhs)) }

i error 

error: cannot invoke 'init' argument list of type '(width: double, height: double)'         return size(width:double(lhs.width/rhs),height:double(lhs.height/rhs))                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

that weird because list passed of exact types defined in class (double)..

if rewrite in way:

func /<t>(lhs:size,rhs:t)->size {     let drhs=double(rhs)     return size(width:lhs.width/drhs,height:lhs.height/drhs) }

then error:

error: cannot invoke 'init' argument of type 't'     let drhs=double(rhs)              ^~~~~~~~~~~

which weirder because swift library file has got lot of initializers double, int, or float types:

extension double {     init(_ v: uint8)     init(_ v: int8)     init(_ v: uint16)     init(_ v: int16)     init(_ v: uint32)     init(_ v: int32)     init(_ v: uint64)     init(_ v: int64)     init(_ v: uint)     init(_ v: int) }  extension double {     init(_ v: float)     init(_ v: float80) }

what wrong in code?

in swift generics require type safety done constraining through protocols. better in c++ safety cannot assumed generics. usually, can use protocols standard library implement these generics unfortunately in case need define special protocol , extend types want able use. i've done int , float can same thing types want use. 

protocol doubleconvertible {     func todouble() -> double } extension int : doubleconvertible {     func todouble() -> double     {         return double(self)     } } extension float: doubleconvertible {     func todouble() -> double     {         return double(self)     } } func /<t : doubleconvertible>(lhs: size, rhs: t) -> size {     return size(width: lhs.width / rhs.todouble(), height: lhs.height / rhs.todouble()) }

the reason have manually because none of standard library protocols (that know of) define requirement such numbers can converted double.


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