1、不涉及到类型的转换。
取地址& ， 函数参数列表中加inout， 调用函数使用取地址&， 做实参传递。 函数里面，可以直接使用变量读取， 修改。

2、能够表现的类型T的转换， 用UnsafeMutablePointer, UnsafePointer

func t(block : ((UnsafeMutablePointer<Int>)->())?) {
    let p = UnsafeMutablePointer<Int>.allocate(capacity: 1)
    p.initialize(to: 50)
    print(p)
    print(p.pointee)
    block?(p)
    print(p)
    print(p.pointee)
}
t { (p) in
    p.pointee = 99
}

func t2(p : inout UnsafeMutablePointer<Int>) -> (){
    p.pointee = 333
    
    let p2 = UnsafeMutablePointer<Int>.allocate(capacity: 1)
    p2.initialize(to: 444)
    p = p2
}
var p = UnsafeMutablePointer<Int>.allocate(capacity: 1)
p.initialize(to: 111)
print(p.pointee)
t2(p: &p)
print(p)
print(p.pointee)

3、C类型的转换， 用Unmanaged
CFArray

4、函数类型的转换，函数类型转换为闭包

int fff(int (f)(int x,int y)) {
    return f(1,2)
}
let callback: @convention(c) (Int32, Int32) -> Int32 = {
    (x, y) -> Int32 in
    return x + y
}

5、不能够表现的类型的转换，用OpaquePointer