2 Program Structure

2.1 Names

1. 可见性

   1）声明**在函数内部**，是函数的本地值，相似private
   2）声明**在函数外部**，是对当前包可见(包内全部.go文件都可见)的全局值，相似protect
   3）声明**在函数外部且首字母大写**是全部包可见的全局值,相似public

2. 命名风格

   写短语要么全部大写。要么全部小写。比如htmlEscape，HTMLEscape，不能写成HtmlEscape。

2.2 Declarations

A declaration names a program entity and specifies some or all of its properties.
There are four major kinds of declarations: var,const,type and func.

Var/:=

Var

var name type = expression

1.If the expression is omitted,the initial value is the zero value for the type:

Type	Zero Value
numbers	0
boolean	0
strings	“”
interfaces	nil
reference types (slice,pointer,map,channel,function)	nil
aggregate types (array,struct)	the zero value of all of its elements or fields

2.function 内声明的var一定要用上，不然会报错；import的package一定要用上，不然会报错；const,type,func则不会发生上述问题。

3.

var i,j,k int // ok
var i,k = true, 2.3,"four" // ok & type(j)为float64
var i bool,j float64,k string = true,"four" // error

Short Variable Declarations (:=)

Within a function,an alternate form called a short variable declaration may be used to declare and initialize local variables.

name := expression

1.

i,j := 0, 1.0 // ok

2.声明语句中必须要有新变量声明

// 声明新变量 in,err
in,err := os.Open(infile)
// 声明新变量 out
out,err := os.Create(outfile)

// 而下面两个语句则不合法
f,err := os.Open(infile)

f,err := os.Create(outfile) // compile error: no new variables

// 应该改为
f,err = os.Create(outfile)

Pointers

A pointer value is the address of a variable.
Pointer就是address的Type。

x := 1
p := &x
fmt.Println(*p) // "1"
*p = 2
fmt.Println(x) // "2"

1.一个被引用的局部变量，即使在函数调用后仍然不会被回收：

var p = f()
fun f() *int{
    v := 1
    return &v
}

// Each call of f returns a distance value:
fmt.Println(f()==f()) // "false"

1. copy a reference types (pointer,slices,maps and channels) <=> create new aliases or ways to identify the same variable.

关于new

Another way to create a variable is to use the built-in function new. The expression new(T) creates an unnamed variable of type T,initializes it to the zero value of T,and returns its address,which is a value of type *T.

p := new(int)
fmt.Println(*p) // "0"
*p = 2
fmt.Println(*p) // "2"

1.new不需要delete
2.Each call to new return a distinct variable with a unique address. However,there is one exception to this rule: two variables whose type carries no information and is therefore of size zero,such as struct{} or [0]int,may,depending on the implementation,have the same address.

p := new(int)
q := new(int)
fmt.Println(p==q) // "false"

3.The new function is relatively rarely used because the most common unnamed variables are of struct types,for which the struct literal Syntax is more flexible.

Lifetime of Variables

a package-level variable: the entire execution of the program (即使是被import的package中的package-level variable);
a local variable have dynamic lifetime: a new instance is created each time the declaration statement is executed,and the variable lives on until it becomes unreachable,at which point its storage may be recycled.

1.Golang的垃圾回收机制

2.Golang中variables哪些放在栈上，哪些放在堆上，不取决于它的声明方式是var还是new。

const

type

func

用例

1. 读取/设置命令行的flags以及arguments。（P33）