Go is a new language. Although it borrows ideas from existing languages, it has unusual properties that make effective Go programs different in character from programs written in its relatives. A straightforward translation of a C++ or Java program into Go is unlikely to produce a satisfactory result—Java programs are written in Java, not Go. On the other hand, thinking about the problem from a Go perspective could produce a successful but quite different program. In other words, to write Go well, it's important to understand its properties and idioms. It's also important to know the established conventions for programming in Go, such as naming, formatting, program construction, and so on, so that programs you write will be easy for other Go programmers to understand.
This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first.
The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. Moreover, many of the packages contain working, self-contained executable examples you can run directly from the golang.org web site, such as this one (if necessary, click on the word "Example" to open it up). If you have a question about how to approach a problem or how something might be implemented, the documentation, code and examples in the library can provide answers, ideas and background.
Formatting issues are the most contentious but the least consequential. People can adapt to different formatting styles but it's better if they don't have to, and less time is devoted to the topic if everyone adheres to the same style. The problem is how to approach this Utopia without a long prescriptive style guide.
With Go we take an unusual
approach and let the machine
take care of most formatting issues.
The gofmt program
(also available as go fmt, which
operates at the package level rather than source file level)
reads a Go program
and emits the source in a standard style of indentation
and vertical alignment, retaining and if necessary
reformatting comments.
If you want to know how to handle some new layout
situation, run gofmt; if the answer doesn't
seem right, rearrange your program (or file a bug about gofmt),
don't work around it.
As an example, there's no need to spend time lining up
the comments on the fields of a structure.
Gofmt will do that for you. Given the
declaration
type T struct {
name string // name of the object
value int // its value
}
gofmt will line up the columns:
type T struct {
name string // name of the object
value int // its value
}
All Go code in the standard packages has been formatted with gofmt.
Some formatting details remain. Very briefly:
gofmt emits them by default.
Use spaces only if you must.
if,
for, switch) do not have parentheses in
their syntax.
Also, the operator precedence hierarchy is shorter and clearer, so
x<<8 + y<<16means what the spacing implies, unlike in the other languages.
Go provides C-style /* */ block comments
and C++-style // line comments.
Line comments are the norm;
block comments appear mostly as package comments, but
are useful within an expression or to disable large swaths of code.
The program—and web server—godoc processes
Go source files to extract documentation about the contents of the
package.
Comments that appear before top-level declarations, with no intervening newlines,
are extracted along with the declaration to serve as explanatory text for the item.
The nature and style of these comments determines the
quality of the documentation godoc produces.
Every package should have a package comment, a block
comment preceding the package clause.
For multi-file packages, the package comment only needs to be
present in one file, and any one will do.
The package comment should introduce the package and
provide information relevant to the package as a whole.
It will appear first on the godoc page and
should set up the detailed documentation that follows.
/*
Package regexp implements a simple library for regular expressions.
The syntax of the regular expressions accepted is:
regexp:
concatenation { '|' concatenation }
concatenation:
{ closure }
closure:
term [ '*' | '+' | '?' ]
term:
'^'
'$'
'.'
character
'[' [ '^' ] character-ranges ']'
'(' regexp ')'
*/
package regexp
If the package is simple, the package comment can be brief.
// Package path implements utility routines for // manipulating slash-separated filename paths.
Comments do not need extra formatting such as banners of stars.
The generated output may not even be presented in a fixed-width font, so don't depend
on spacing for alignment—godoc, like gofmt,
takes care of that.
The comments are uninterpreted plain text, so HTML and other
annotations such as _this_ will reproduce verbatim and should
not be used.
One adjustment godoc does do is to display indented
text in a fixed-width font, suitable for program snippets.
The package comment for the
fmt package uses this to good effect.
Depending on the context, godoc might not even
reformat comments, so make sure they look good straight up:
use correct spelling, punctuation, and sentence structure,
fold long lines, and so on.
Inside a package, any comment immediately preceding a top-level declaration serves as a doc comment for that declaration. Every exported (capitalized) name in a program should have a doc comment.
Doc comments work best as complete sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared.
// Compile parses a regular expression and returns, if successful, a Regexp
// object that can be used to match against text.
func Compile(str string) (regexp *Regexp, err error) {
If the name always begins the comment, the output of godoc
can usefully be run through grep.
Imagine you couldn't remember the name "Compile" but were looking for
the parsing function for regular expressions, so you ran
the command,
$ godoc regexp | grep parse
If all the doc comments in the package began, "This function...", grep
wouldn't help you remember the name. But because the package starts each
doc comment with the name, you'd see something like this,
which recalls the word you're looking for.
$ godoc regexp | grep parse
Compile parses a regular expression and returns, if successful, a Regexp
parsed. It simplifies safe initialization of global variables holding
cannot be parsed. It simplifies safe initialization of global variables
$
Go's declaration syntax allows grouping of declarations. A single doc comment can introduce a group of related constants or variables. Since the whole declaration is presented, such a comment can often be perfunctory.
// Error codes returned by failures to parse an expression.
var (
ErrInternal = errors.New("regexp: internal error")
ErrUnmatchedLpar = errors.New("regexp: unmatched '('")
ErrUnmatchedRpar = errors.New("regexp: unmatched ')'")
...
)
Grouping can also indicate relationships between items, such as the fact that a set of variables is protected by a mutex.
var (
countLock sync.Mutex
inputCount uint32
outputCount uint32
errorCount uint32
)
Names are as important in Go as in any other language. They even have semantic effect: the visibility of a name outside a package is determined by whether its first character is upper case. It's therefore worth spending a little time talking about naming conventions in Go programs.
When a package is imported, the package name becomes an accessor for the contents. After
import "bytes"
the importing package can talk about bytes.Buffer. It's
helpful if everyone using the package can use the same name to refer to
its contents, which implies that the package name should be good:
short, concise, evocative. By convention, packages are given
lower case, single-word names; there should be no need for underscores
or mixedCaps.
Err on the side of brevity, since everyone using your
package will be typing that name.
And don't worry about collisions a priori.
The package name is only the default name for imports; it need not be unique
across all source code, and in the rare case of a collision the
importing package can choose a different name to use locally.
In any case, confusion is rare because the file name in the import
determines just which package is being used.
Another convention is that the package name is the base name of
its source directory;
the package in src/pkg/encoding/base64
is imported as "encoding/base64" but has name base64,
not encoding_base64 and not encodingBase64.
The importer of a package will use the name to refer to its contents,
so exported names in the package can use that fact
to avoid stutter.
(Don't use the import . notation, which can simplify
tests that must run outside the package they are testing, but should otherwise be avoided.)
For instance, the buffered reader type in the bufio package is called Reader,
not BufReader, because users see it as bufio.Reader,
which is a clear, concise name.
Moreover,
because imported entities are always addressed with their package name, bufio.Reader
does not conflict with io.Reader.
Similarly, the function to make new instances of ring.Ring—which
is the definition of a constructor in Go—would
normally be called NewRing, but since
Ring is the only type exported by the package, and since the
package is called ring, it's called just New,
which clients of the package see as ring.New.
Use the package structure to help you choose good names.
Another short example is once.Do;
once.Do(setup) reads well and would not be improved by
writing once.DoOrWaitUntilDone(setup).
Long names don't automatically make things more readable.
A helpful doc comment can often be more valuable than an extra long name.
Go doesn't provide automatic support for getters and setters.
There's nothing wrong with providing getters and setters yourself,
and it's often appropriate to do so, but it's neither idiomatic nor necessary
to put Get into the getter's name. If you have a field called
owner (lower case, unexported), the getter method should be
called Owner (upper case, exported), not GetOwner.
The use of upper-case names for export provides the hook to discriminate
the field from the method.
A setter function, if needed, will likely be called SetOwner.
Both names read well in practice:
owner := obj.Owner()
if owner != user {
obj.SetOwner(user)
}
By convention, one-method interfaces are named by
the method name plus an -er suffix or similar modification
to construct an agent noun: Reader,
Writer, Formatter,
CloseNotifier etc.
There are a number of such names and it's productive to honor them and the function
names they capture.
Read, Write, Close, Flush,
String and so on have
canonical signatures and meanings. To avoid confusion,
don't give your method one of those names unless it
has the same signature and meaning.
Conversely, if your type implements a method with the
same meaning as a method on a well-known type,
give it the same name and signature;
call your string-converter method String not ToString.
Finally, the convention in Go is to use MixedCaps
or mixedCaps rather than underscores to write
multiword names.
Like C, Go's formal grammar uses semicolons to terminate statements, but unlike in C, those semicolons do not appear in the source. Instead the lexer uses a simple rule to insert semicolons automatically as it scans, so the input text is mostly free of them.
The rule is this. If the last token before a newline is an identifier
(which includes words like int and float64),
a basic literal such as a number or string constant, or one of the
tokens
break continue fallthrough return ++ -- ) }
the lexer always inserts a semicolon after the token. This could be summarized as, “if the newline comes after a token that could end a statement, insert a semicolon”.
A semicolon can also be omitted immediately before a closing brace, so a statement such as
go func() { for { dst <- <-src } }()
needs no semicolons.
Idiomatic Go programs have semicolons only in places such as
for loop clauses, to separate the initializer, condition, and
continuation elements. They are also necessary to separate multiple
statements on a line, should you write code that way.
One consequence of the semicolon insertion rules
is that you cannot put the opening brace of a
control structure (if, for, switch,
or select) on the next line. If you do, a semicolon
will be inserted before the brace, which could cause unwanted
effects. Write them like this
if i < f() {
g()
}
not like this
if i < f() // wrong!
{ // wrong!
g()
}
The control structures of Go are related to those of C but differ
in important ways.
There is no do or while loop, only a
slightly generalized
for;
switch is more flexible;
if and switch accept an optional
initialization statement like that of for;
break and continue statements
take an optional label to identify what to break or continue;
and there are new control structures including a type switch and a
multiway communications multiplexer, select.
The syntax is also slightly different:
there are no parentheses
and the bodies must always be brace-delimited.
In Go a simple if looks like this:
if x > 0 {
return y
}
Mandatory braces encourage writing simple if statements
on multiple lines. It's good style to do so anyway,
especially when the body contains a control statement such as a
return or break.
Since if and switch accept an initialization
statement, it's common to see one used to set up a local variable.
if err := file.Chmod(0664); err != nil {
log.Print(err)
return err
}
In the Go libraries, you'll find that
when an if statement doesn't flow into the next statement—that is,
the body ends in break, continue,
goto, or return—the unnecessary
else is omitted.
f, err := os.Open(name)
if err != nil {
return err
}
codeUsing(f)
This is an example of a common situation where code must guard against a
sequence of error conditions. The code reads well if the
successful flow of control runs down the page, eliminating error cases
as they arise. Since error cases tend to end in return
statements, the resulting code needs no else statements.
f, err := os.Open(name)
if err != nil {
return err
}
d, err := f.Stat()
if err != nil {
f.Close()
return err
}
codeUsing(f, d)
An aside: The last example in the previous section demonstrates a detail of how the
:= short declaration form works.
The declaration that calls os.Open reads,
f, err := os.Open(name)
This statement declares two variables, f and err.
A few lines later, the call to f.Stat reads,
d, err := f.Stat()
which looks as if it declares d and err.
Notice, though, that err appears in both statements.
This duplication is legal: err is declared by the first statement,
but only re-assigned in the second.
This means that the call to f.Stat uses the existing
err variable declared above, and just gives it a new value.
In a := declaration a variable v may appear even
if it has already been declared, provided:
v
(if v is already declared in an outer scope, the declaration will create a new variable §),v, and
This unusual property is pure pragmatism,
making it easy to use a single err value, for example,
in a long if-else chain.
You'll see it used often.
§ It's worth noting here that in Go the scope of function parameters and return values is the same as the function body, even though they appear lexically outside the braces that enclose the body.
The Go for loop is similar to—but not the same as—C's.
It unifies for
and while and there is no do-while.
There are three forms, only one of which has semicolons.
// Like a C for
for init; condition; post { }
// Like a C while
for condition { }
// Like a C for(;;)
for { }
Short declarations make it easy to declare the index variable right in the loop.
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
If you're looping over an array, slice, string, or map,
or reading from a channel, a range clause can
manage the loop.
for key, value := range oldMap {
newMap[key] = value
}
If you only need the first item in the range (the key or index), drop the second:
for key := range m {
if key.expired() {
delete(m, key)
}
}
If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:
sum := 0
for _, value := range array {
sum += value
}
The blank identifier has many uses, as described in a later section.
For strings, the range does more work for you, breaking out individual
Unicode code points by parsing the UTF-8.
Erroneous encodings consume one byte and produce the
replacement rune U+FFFD.
(The name (with associated builtin type) rune is Go terminology for a
single Unicode code point.
See the language specification
for details.)
The loop
for pos, char := range "日本\x80語" { // \x80 is an illegal UTF-8 encoding
fmt.Printf("character %#U starts at byte position %d\n", char, pos)
}
prints
character U+65E5 '日' starts at byte position 0 character U+672C '本' starts at byte position 3 character U+FFFD '�' starts at byte position 6 character U+8A9E '語' starts at byte position 7
Finally, Go has no comma operator and ++ and --
are statements not expressions.
Thus if you want to run multiple variables in a for
you should use parallel assignment (although that precludes ++ and --).
// Reverse a
for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
a[i], a[j] = a[j], a[i]
}
Go's switch is more general than C's.
The expressions need not be constants or even integers,
the cases are evaluated top to bottom until a match is found,
and if the switch has no expression it switches on
true.
It's therefore possible—and idiomatic—to write an
if-else-if-else
chain as a switch.
func unhex(c byte) byte {
switch {
case '0' <= c && c <= '9':
return c - '0'
case 'a' <= c && c <= 'f':
return c - 'a' + 10
case 'A' <= c && c <= 'F':
return c - 'A' + 10
}
return 0
}
There is no automatic fall through, but cases can be presented in comma-separated lists.
func shouldEscape(c byte) bool {
switch c {
case ' ', '?', '&', '=', '#', '+', '%':
return true
}
return false
}
Although they are not nearly as common in Go as some other C-like
languages, break statements can be used to terminate
a switch early.
Sometimes, though, it's necessary to break out of a surrounding loop,
not the switch, and in Go that can be accomplished by putting a label
on the loop and "breaking" to that label.
This example shows both uses.
Loop:
for n := 0; n < len(src); n += size {
switch {
case src[n] < sizeOne:
if validateOnly {
break
}
size = 1
update(src[n])
case src[n] < sizeTwo:
if n+1 >= len(src) {
err = errShortInput
break Loop
}
if validateOnly {
break
}
size = 2
update(src[n] + src[n+1]<<shift)
}
}
Of course, the continue statement also accepts an optional label
but it applies only to loops.
To close this section, here's a comparison routine for byte slices that uses two
switch statements:
// Compare returns an integer comparing the two byte slices,
// lexicographically.
// The result will be 0 if a == b, -1 if a < b, and +1 if a > b
func Compare(a, b []byte) int {
for i := 0; i < len(a) && i < len(b); i++ {
switch {
case a[i] > b[i]:
return 1
case a[i] < b[i]:
return -1
}
}
switch {
case len(a) > len(b):
return 1
case len(a) < len(b):
return -1
}
return 0
}
A switch can also be used to discover the dynamic type of an interface
variable. Such a type switch uses the syntax of a type
assertion with the keyword type inside the parentheses.
If the switch declares a variable in the expression, the variable will
have the corresponding type in each clause.
It's also idiomatic to reuse the name in such cases, in effect declaring
a new variable with the same name but a different type in each case.
var t interface{}
t = functionOfSomeType()
switch t := t.(type) {
default:
fmt.Printf("unexpected type %T", t) // %T prints whatever type t has
case bool:
fmt.Printf("boolean %t\n", t) // t has type bool
case int:
fmt.Printf("integer %d\n", t) // t has type int
case *bool:
fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool
case *int:
fmt.Printf("pointer to integer %d\n", *t) // t has type *int
}
One of Go's unusual features is that functions and methods
can return multiple values. This form can be used to
improve on a couple of clumsy idioms in C programs: in-band
error returns such as -1 for EOF
and modifying an argument passed by address.
In C, a write error is signaled by a negative count with the
error code secreted away in a volatile location.
In Go, Write
can return a count and an error: “Yes, you wrote some
bytes but not all of them because you filled the device”.
The signature of the Write method on files from
package os is:
func (file *File) Write(b []byte) (n int, err error)
and as the documentation says, it returns the number of bytes
written and a non-nil error when n
!= len(b).
This is a common style; see the section on error handling for more examples.
A similar approach obviates the need to pass a pointer to a return value to simulate a reference parameter. Here's a simple-minded function to grab a number from a position in a byte slice, returning the number and the next position.
func nextInt(b []byte, i int) (int, int) {
for ; i < len(b) && !isDigit(b[i]); i++ {
}
x := 0
for ; i < len(b) && isDigit(b[i]); i++ {
x = x*10 + int(b[i]) - '0'
}
return x, i
}
You could use it to scan the numbers in an input slice b like this:
for i := 0; i < len(b); {
x, i = nextInt(b, i)
fmt.Println(x)
}
The return or result "parameters" of a Go function can be given names and
used as regular variables, just like the incoming parameters.
When named, they are initialized to the zero values for their types when
the function begins; if the function executes a return statement
with no arguments, the current values of the result parameters are
used as the returned values.
The names are not mandatory but they can make code shorter and clearer:
they're documentation.
If we name the results of nextInt it becomes
obvious which returned int
is which.
func nextInt(b []byte, pos int) (value, nextPos int) {
Because named results are initialized and tied to an unadorned return, they can simplify
as well as clarify. Here's a version
of io.ReadFull that uses them well:
func ReadFull(r Reader, buf []byte) (n int, err error) {
for len(buf) > 0 && err == nil {
var nr int
nr, err = r.Read(buf)
n += nr
buf = buf[nr:]
}
return
}
Go's defer statement schedules a function call (the
deferred function) to be run immediately before the function
executing the defer returns. It's an unusual but
effective way to deal with situations such as resources that must be
released regardless of which path a function takes to return. The
canonical examples are unlocking a mutex or closing a file.
// Contents returns the file's contents as a string.
func Contents(filename string) (string, error) {
f, err := os.Open(filename)
if err != nil {
return "", err
}
defer f.Close() // f.Close will run when we're finished.
var result []byte
buf := make([]byte, 100)
for {
n, err := f.Read(buf[0:])
result = append(result, buf[0:n]...) // append is discussed later.
if err != nil {
if err == io.EOF {
break
}
return "", err // f will be closed if we return here.
}
}
return string(result), nil // f will be closed if we return here.
}
Deferring a call to a function such as Close has two advantages. First, it
guarantees that you will never forget to close the file, a mistake
that's easy to make if you later edit the function to add a new return
path. Second, it means that the close sits near the open,
which is much clearer than placing it at the end of the function.
The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes. Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions. Here's a silly example.
for i := 0; i < 5; i++ {
defer fmt.Printf("%d ", i)
}
Deferred functions are executed in LIFO order, so this code will cause
4 3 2 1 0 to be printed when the function returns. A
more plausible example is a simple way to trace function execution
through the program. We could write a couple of simple tracing
routines like this:
func trace(s string) { fmt.Println("entering:", s) }
func untrace(s string) { fmt.Println("leaving:", s) }
// Use them like this:
func a() {
trace("a")
defer untrace("a")
// do something....
}
We can do better by exploiting the fact that arguments to deferred
functions are evaluated when the defer executes. The
tracing routine can set up the argument to the untracing routine.
This example:
func trace(s string) string {
fmt.Println("entering:", s)
return s
}
func un(s string) {
fmt.Println("leaving:", s)
}
func a() {
defer un(trace("a"))
fmt.Println("in a")
}
func b() {
defer un(trace("b"))
fmt.Println("in b")
a()
}
func main() {
b()
}
prints
entering: b in b entering: a in a leaving: a leaving: b
For programmers accustomed to block-level resource management from
other languages, defer may seem peculiar, but its most
interesting and powerful applications come precisely from the fact
that it's not block-based but function-based. In the section on
panic and recover we'll see another
example of its possibilities.
new
Go has two allocation primitives, the built-in functions
new and make.
They do different things and apply to different types, which can be confusing,
but the rules are simple.
Let's talk about new first.
It's a built-in function that allocates memory, but unlike its namesakes
in some other languages it does not initialize the memory,
it only zeros it.
That is,
new(T) allocates zeroed storage for a new item of type
T and returns its address, a value of type *T.
In Go terminology, it returns a pointer to a newly allocated zero value of type
T.
Since the memory returned by new is zeroed, it's helpful to arrange
when designing your data structures that the
zero value of each type can be used without further initialization. This means a user of
the data structure can create one with new and get right to
work.
For example, the documentation for bytes.Buffer states that
"the zero value for Buffer is an empty buffer ready to use."
Similarly, sync.Mutex does not
have an explicit constructor or Init method.
Instead, the zero value for a sync.Mutex
is defined to be an unlocked mutex.
The zero-value-is-useful property works transitively. Consider this type declaration.
type SyncedBuffer struct {
lock sync.Mutex
buffer bytes.Buffer
}
Values of type SyncedBuffer are also ready to use immediately upon allocation
or just declaration. In the next snippet, both p and v will work
correctly without further arrangement.
p := new(SyncedBuffer) // type *SyncedBuffer var v SyncedBuffer // type SyncedBuffer
Sometimes the zero value isn't good enough and an initializing
constructor is necessary, as in this example derived from
package os.
func NewFile(fd int, name string) *File {
if fd < 0 {
return nil
}
f := new(File)
f.fd = fd
f.name = name
f.dirinfo = nil
f.nepipe = 0
return f
}
There's a lot of boiler plate in there. We can simplify it using a composite literal, which is an expression that creates a new instance each time it is evaluated.
func NewFile(fd int, name string) *File {
if fd < 0 {
return nil
}
f := File{fd, name, nil, 0}
return &f
}
Note that, unlike in C, it's perfectly OK to return the address of a local variable; the storage associated with the variable survives after the function returns. In fact, taking the address of a composite literal allocates a fresh instance each time it is evaluated, so we can combine these last two lines.
return &File{fd, name, nil, 0}
The fields of a composite literal are laid out in order and must all be present.
However, by labeling the elements explicitly as field:value
pairs, the initializers can appear in any
order, with the missing ones left as their respective zero values. Thus we could say
return &File{fd: fd, name: name}
As a limiting case, if a composite literal contains no fields at all, it creates
a zero value for the type. The expressions new(File) and &File{} are equivalent.
Composite literals can also be created for arrays, slices, and maps,
with the field labels being indices or map keys as appropriate.
In these examples, the initializations work regardless of the values of Enone,
Eio, and Einval, as long as they are distinct.
a := [...]string {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
s := []string {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
make
Back to allocation.
The built-in function make(T, args) serves
a purpose different from new(T).
It creates slices, maps, and channels only, and it returns an initialized
(not zeroed)
value of type T (not *T).
The reason for the distinction
is that these three types represent, under the covers, references to data structures that
must be initialized before use.
A slice, for example, is a three-item descriptor
containing a pointer to the data (inside an array), the length, and the
capacity, and until those items are initialized, the slice is nil.
For slices, maps, and channels,
make initializes the internal data structure and prepares
the value for use.
For instance,
make([]int, 10, 100)
allocates an array of 100 ints and then creates a slice
structure with length 10 and a capacity of 100 pointing at the first
10 elements of the array.
(When making a slice, the capacity can be omitted; see the section on slices
for more information.)
In contrast, new([]int) returns a pointer to a newly allocated, zeroed slice
structure, that is, a pointer to a nil slice value.
These examples illustrate the difference between new and
make.
var p *[]int = new([]int) // allocates slice structure; *p == nil; rarely useful var v []int = make([]int, 100) // the slice v now refers to a new array of 100 ints // Unnecessarily complex: var p *[]int = new([]int) *p = make([]int, 100, 100) // Idiomatic: v := make([]int, 100)
Remember that make applies only to maps, slices and channels
and does not return a pointer.
To obtain an explicit pointer allocate with new or take the address
of a variable explicitly.
Arrays are useful when planning the detailed layout of memory and sometimes can help avoid allocation, but primarily they are a building block for slices, the subject of the next section. To lay the foundation for that topic, here are a few words about arrays.
There are major differences between the ways arrays work in Go and C. In Go,
[10]int
and [20]int are distinct.
The value property can be useful but also expensive; if you want C-like behavior and efficiency, you can pass a pointer to the array.
func Sum(a *[3]float64) (sum float64) {
for _, v := range *a {
sum += v
}
return
}
array := [...]float64{7.0, 8.5, 9.1}
x := Sum(&array) // Note the explicit address-of operator
But even this style isn't idiomatic Go. Use slices instead.
Slices wrap arrays to give a more general, powerful, and convenient interface to sequences of data. Except for items with explicit dimension such as transformation matrices, most array programming in Go is done with slices rather than simple arrays.
Slices hold references to an underlying array, and if you assign one
slice to another, both refer to the same array.
If a function takes a slice argument, changes it makes to
the elements of the slice will be visible to the caller, analogous to
passing a pointer to the underlying array. A Read
function can therefore accept a slice argument rather than a pointer
and a count; the length within the slice sets an upper
limit of how much data to read. Here is the signature of the
Read method of the File type in package
os:
func (file *File) Read(buf []byte) (n int, err error)
The method returns the number of bytes read and an error value, if
any. To read into the first 32 bytes of a larger buffer
b, slice (here used as a verb) the buffer.
n, err := f.Read(buf[0:32])
Such slicing is common and efficient. In fact, leaving efficiency aside for the moment, the following snippet would also read the first 32 bytes of the buffer.
var n int
var err error
for i := 0; i < 32; i++ {
nbytes, e := f.Read(buf[i:i+1]) // Read one byte.
if nbytes == 0 || e != nil {
err = e
break
}
n += nbytes
}
The length of a slice may be changed as long as it still fits within
the limits of the underlying array; just assign it to a slice of
itself. The capacity of a slice, accessible by the built-in
function cap, reports the maximum length the slice may
assume. Here is a function to append data to a slice. If the data
exceeds the capacity, the slice is reallocated. The
resulting slice is returned. The function uses the fact that
len and cap are legal when applied to the
nil slice, and return 0.
func Append(slice, data[]byte) []byte {
l := len(slice)
if l + len(data) > cap(slice) { // reallocate
// Allocate double what's needed, for future growth.
newSlice := make([]byte, (l+len(data))*2)
// The copy function is predeclared and works for any slice type.
copy(newSlice, slice)
slice = newSlice
}
slice = slice[0:l+len(data)]
for i, c := range data {
slice[l+i] = c
}
return slice
}
We must return the slice afterwards because, although Append
can modify the elements of slice, the slice itself (the run-time data
structure holding the pointer, length, and capacity) is passed by value.
The idea of appending to a slice is so useful it's captured by the
append built-in function. To understand that function's
design, though, we need a little more information, so we'll return
to it later.
Go's arrays and slices are one-dimensional. To create the equivalent of a 2D array or slice, it is necessary to define an array-of-arrays or slice-of-slices, like this:
type Transform [3][3]float64 // A 3x3 array, really an array of arrays. type LinesOfText [][]byte // A slice of byte slices.
Because slices are variable-length, it is possible to have each inner
slice be a different length.
That can be a common situation, as in our LinesOfText
example: each line has an independent length.
text := LinesOfText{
[]byte("Now is the time"),
[]byte("for all good gophers"),
[]byte("to bring some fun to the party."),
}
Sometimes it's necessary to allocate a 2D slice, a situation that can arise when processing scan lines of pixels, for instance. There are two ways to achieve this. One is to allocate each slice independently; the other is to allocate a single array and point the individual slices into it. Which to use depends on your application. If the slices might grow or shrink, they should be allocated independently to avoid overwriting the next line; if not, it can be more efficient to construct the object with a single allocation. For reference, here are sketches of the two methods. First, a line a time:
// Allocate the top-level slice.
picture := make([][]uint8, YSize) // One row per unit of y.
// Loop over the rows, allocating the slice for each row.
for i := range picture {
picture[i] = make([]uint8, XSize)
}
And now as one allocation, sliced into lines:
// Allocate the top-level slice, the same as before.
picture := make([][]uint8, YSize) // One row per unit of y.
// Allocate one large slice to hold all the pixels.
pixels := make([]uint8, XSize*YSize) // Has type []uint8 even though picture is [][]uint8.
// Loop over the rows, slicing each row from the front of the remaining pixels slice.
for i := range picture {
picture[i], pixels = pixels[:XSize], pixels[XSize:]
}
Maps are a convenient and powerful built-in data structure that associate values of one type (the key) with values of another type (the element or value) The key can be of any type for which the equality operator is defined, such as integers, floating point and complex numbers, strings, pointers, interfaces (as long as the dynamic type supports equality), structs and arrays. Slices cannot be used as map keys, because equality is not defined on them. Like slices, maps hold references to an underlying data structure. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller.
Maps can be constructed using the usual composite literal syntax with colon-separated key-value pairs, so it's easy to build them during initialization.
var timeZone = map[string]int{
"UTC": 0*60*60,
"EST": -5*60*60,
"CST": -6*60*60,
"MST": -7*60*60,
"PST": -8*60*60,
}
Assigning and fetching map values looks syntactically just like doing the same for arrays and slices except that the index doesn't need to be an integer.
offset := timeZone["EST"]
An attempt to fetch a map value with a key that
is not present in the map will return the zero value for the type
of the entries
in the map. For instance, if the map contains integers, looking
up a non-existent key will return 0.
A set can be implemented as a map with value type bool.
Set the map entry to true to put the value in the set, and then
test it by simple indexing.
attended := map[string]bool{
"Ann": true,
"Joe": true,
...
}
if attended[person] { // will be false if person is not in the map
fmt.Println(person, "was at the meeting")
}
Sometimes you need to distinguish a missing entry from
a zero value. Is there an entry for "UTC"
or is that the empty string because it's not in the map at all?
You can discriminate with a form of multiple assignment.
var seconds int var ok bool seconds, ok = timeZone[tz]
For obvious reasons this is called the “comma ok” idiom.
In this example, if tz is present, seconds
will be set appropriately and ok will be true; if not,
seconds will be set to zero and ok will
be false.
Here's a function that puts it together with a nice error report:
func offset(tz string) int {
if seconds, ok := timeZone[tz]; ok {
return seconds
}
log.Println("unknown time zone:", tz)
return 0
}
To test for presence in the map without worrying about the actual value,
you can use the blank identifier (_)
in place of the usual variable for the value.
_, present := timeZone[tz]
To delete a map entry, use the delete
built-in function, whose arguments are the map and the key to be deleted.
It's safe to do this even if the key is already absent
from the map.
delete(timeZone, "PDT") // Now on Standard Time
Formatted printing in Go uses a style similar to C's printf
family but is richer and more general. The functions live in the fmt
package and have capitalized names: fmt.Printf, fmt.Fprintf,
fmt.Sprintf and so on. The string functions (Sprintf etc.)
return a string rather than filling in a provided buffer.
You don't need to provide a format string. For each of Printf,
Fprintf and Sprintf there is another pair
of functions, for instance Print and Println.
These functions do not take a format string but instead generate a default
format for each argument. The Println versions also insert a blank
between arguments and append a newline to the output while
the Print versions add blanks only if the operand on neither side is a string.
In this example each line produces the same output.
fmt.Printf("Hello %d\n", 23)
fmt.Fprint(os.Stdout, "Hello ", 23, "\n")
fmt.Println("Hello", 23)
fmt.Println(fmt.Sprint("Hello ", 23))
The formatted print functions fmt.Fprint
and friends take as a first argument any object
that implements the io.Writer interface; the variables os.Stdout
and os.Stderr are familiar instances.
Here things start to diverge from C. First, the numeric formats such as %d
do not take flags for signedness or size; instead, the printing routines use the
type of the argument to decide these properties.
var x uint64 = 1<<64 - 1
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x))
prints
18446744073709551615 ffffffffffffffff; -1 -1
If you just want the default conversion, such as decimal for integers, you can use
the catchall format %v (for “value”); the result is exactly
what Print and Println would produce.
Moreover, that format can print any value, even arrays, slices, structs, and
maps. Here is a print statement for the time zone map defined in the previous section.
fmt.Printf("%v\n", timeZone) // or just fmt.Println(timeZone)
which gives output
map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]
For maps the keys may be output in any order, of course.
When printing a struct, the modified format %+v annotates the
fields of the structure with their names, and for any value the alternate
format %#v prints the value in full Go syntax.
type T struct {
a int
b float64
c string
}
t := &T{ 7, -2.35, "abc\tdef" }
fmt.Printf("%v\n", t)
fmt.Printf("%+v\n", t)
fmt.Printf("%#v\n", t)
fmt.Printf("%#v\n", timeZone)
prints
&{7 -2.35 abc def}
&{a:7 b:-2.35 c:abc def}
&main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200}
(Note the ampersands.)
That quoted string format is also available through %q when
applied to a value of type string or []byte.
The alternate format %#q will use backquotes instead if possible.
(The %q format also applies to integers and runes, producing a
single-quoted rune constant.)
Also, %x works on strings, byte arrays and byte slices as well as
on integers, generating a long hexadecimal string, and with
a space in the format (% x) it puts spaces between the bytes.
Another handy format is %T, which prints the type of a value.
fmt.Printf("%T\n", timeZone)
prints
map[string] int
If you want to control the default format for a custom type, all that's required is to define
a method with the signature String() string on the type.
For our simple type T, that might look like this.
func (t *T) String() string {
return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c)
}
fmt.Printf("%v\n", t)
to print in the format
7/-2.35/"abc\tdef"
(If you need to print values of type T as well as pointers to T,
the receiver for String must be of value type; this example used a pointer because
that's more efficient and idiomatic for struct types.
See the section below on pointers vs. value receivers for more information.)
Our String method is able to call Sprintf because the
print routines are fully reentrant and can be wrapped this way.
There is one important detail to understand about this approach,
however: don't construct a String method by calling
Sprintf in a way that will recur into your String
method indefinitely. This can happen if the Sprintf
call attempts to print the receiver directly as a string, which in
turn will invoke the method again. It's a common and easy mistake
to make, as this example shows.
type MyString string
func (m MyString) String() string {
return fmt.Sprintf("MyString=%s", m) // Error: will recur forever.
}
It's also easy to fix: convert the argument to the basic string type, which does not have the method.
type MyString string
func (m MyString) String() string {
return fmt.Sprintf("MyString=%s", string(m)) // OK: note conversion.
}
In the initialization section we'll see another technique that avoids this recursion.
Another printing technique is to pass a print routine's arguments directly to another such routine.
The signature of Printf uses the type ...interface{}
for its final argument to specify that an arbitrary number of parameters (of arbitrary type)
can appear after the format.
func Printf(format string, v ...interface{}) (n int, err error) {
Within the function Printf, v acts like a variable of type
[]interface{} but if it is passed to another variadic function, it acts like
a regular list of arguments.
Here is the implementation of the
function log.Println we used above. It passes its arguments directly to
fmt.Sprintln for the actual formatting.
// Println prints to the standard logger in the manner of fmt.Println.
func Println(v ...interface{}) {
std.Output(2, fmt.Sprintln(v...)) // Output takes parameters (int, string)
}
We write ... after v in the nested call to Sprintln to tell the
compiler to treat v as a list of arguments; otherwise it would just pass
v as a single slice argument.
There's even more to printing than we've covered here. See the godoc documentation
for package fmt for the details.
By the way, a ... parameter can be of a specific type, for instance ...int
for a min function that chooses the least of a list of integers:
func Min(a ...int) int {
min := int(^uint(0) >> 1) // largest int
for _, i := range a {
if i < min {
min = i
}
}
return min
}
Now we have the missing piece we needed to explain the design of
the append built-in function. The signature of append
is different from our custom Append function above.
Schematically, it's like this:
func append(slice []T, elements ...T) []T
where T is a placeholder for any given type. You can't
actually write a function in Go where the type T
is determined by the caller.
That's why append is built in: it needs support from the
compiler.
What append does is append the elements to the end of
the slice and return the result. The result needs to be returned
because, as with our hand-written Append, the underlying
array may change. This simple example
x := []int{1,2,3}
x = append(x, 4, 5, 6)
fmt.Println(x)
prints [1 2 3 4 5 6]. So append works a
little like Printf, collecting an arbitrary number of
arguments.
But what if we wanted to do what our Append does and
append a slice to a slice? Easy: use ... at the call
site, just as we did in the call to Output above. This
snippet produces identical output to the one above.
x := []int{1,2,3}
y := []int{4,5,6}
x = append(x, y...)
fmt.Println(x)
Without that ..., it wouldn't compile because the types
would be wrong; y is not of type int.
Although it doesn't look superficially very different from initialization in C or C++, initialization in Go is more powerful. Complex structures can be built during initialization and the ordering issues among initialized objects, even among different packages, are handled correctly.
Constants in Go are just that—constant.
They are created at compile time, even when defined as
locals in functions,
and can only be numbers, characters (runes), strings or booleans.
Because of the compile-time restriction, the expressions
that define them must be constant expressions,
evaluatable by the compiler. For instance,
1<<3 is a constant expression, while
math.Sin(math.Pi/4) is not because
the function call to math.Sin needs
to happen at run time.
In Go, enumerated constants are created using the iota
enumerator. Since iota can be part of an expression and
expressions can be implicitly repeated, it is easy to build intricate
sets of values.
type ByteSize float64
const (
_ = iota // ignore first value by assigning to blank identifier
KB ByteSize = 1 << (10 * iota)
MB
GB
TB
PB
EB
ZB
YB
)
The ability to attach a method such as String to any
user-defined type makes it possible for arbitrary values to format themselves
automatically for printing.
Although you'll see it most often applied to structs, this technique is also useful for
scalar types such as floating-point types like ByteSize.
func (b ByteSize) String() string {
switch {
case b >= YB:
return fmt.Sprintf("%.2fYB", b/YB)
case b >= ZB:
return fmt.Sprintf("%.2fZB", b/ZB)
case b >= EB:
return fmt.Sprintf("%.2fEB", b/EB)
case b >= PB:
return fmt.Sprintf("%.2fPB", b/PB)
case b >= TB:
return fmt.Sprintf("%.2fTB", b/TB)
case b >= GB:
return fmt.Sprintf("%.2fGB", b/GB)
case b >= MB:
return fmt.Sprintf("%.2fMB", b/MB)
case b >= KB:
return fmt.Sprintf("%.2fKB", b/KB)
}
return fmt.Sprintf("%.2fB", b)
}
The expression YB prints as 1.00YB,
while ByteSize(1e13) prints as 9.09TB.
The use here of Sprintf
to implement ByteSize's String method is safe
(avoids recurring indefinitely) not because of a conversion but
because it calls Sprintf with %f,
which is not a string format: Sprintf will only call
the String method when it wants a string, and %f
wants a floating-point value.
Variables can be initialized just like constants but the initializer can be a general expression computed at run time.
var (
home = os.Getenv("HOME")
user = os.Getenv("USER")
gopath = os.Getenv("GOPATH")
)
Finally, each source file can define its own niladic init function to
set up whatever state is required. (Actually each file can have multiple
init functions.)
And finally means finally: init is called after all the
variable declarations in the package have evaluated their initializers,
and those are evaluated only after all the imported packages have been
initialized.
Besides initializations that cannot be expressed as declarations,
a common use of init functions is to verify or repair
correctness of the program state before real execution begins.
func init() {
if user == "" {
log.Fatal("$USER not set")
}
if home == "" {
home = "/home/" + user
}
if gopath == "" {
gopath = home + "/go"
}
// gopath may be overridden by --gopath flag on command line.
flag.StringVar(&gopath, "gopath", gopath, "override default GOPATH")
}
As we saw with ByteSize,
methods can be defined for any named type (except a pointer or an interface);
the receiver does not have to be a struct.
In the discussion of slices above, we wrote an Append
function. We can define it as a method on slices instead. To do
this, we first declare a named type to which we can bind the method, and
then make the receiver for the method a value of that type.
type ByteSlice []byte
func (slice ByteSlice) Append(data []byte) []byte {
// Body exactly the same as above
}
This still requires the method to return the updated slice. We can
eliminate that clumsiness by redefining the method to take a
pointer to a ByteSlice as its receiver, so the
method can overwrite the caller's slice.
func (p *ByteSlice) Append(data []byte) {
slice := *p
// Body as above, without the return.
*p = slice
}
In fact, we can do even better. If we modify our function so it looks
like a standard Write method, like this,
func (p *ByteSlice) Write(data []byte) (n int, err error) {
slice := *p
// Again as above.
*p = slice
return len(data), nil
}
then the type *ByteSlice satisfies the standard interface
io.Writer, which is handy. For instance, we can
print into one.
var b ByteSlice
fmt.Fprintf(&b, "This hour has %d days\n", 7)
We pass the address of a ByteSlice
because only *ByteSlice satisfies io.Writer.
The rule about pointers vs. values for receivers is that value methods
can be invoked on pointers and values, but pointer methods can only be
invoked on pointers. This is because pointer methods can modify the
receiver; invoking them on a copy of the value would cause those
modifications to be discarded.
By the way, the idea of using Write on a slice of bytes
is central to the implementation of bytes.Buffer.
Interfaces in Go provide a way to specify the behavior of an
object: if something can do this, then it can be used
here. We've seen a couple of simple examples already;
custom printers can be implemented by a String method
while Fprintf can generate output to anything
with a Write method.
Interfaces with only one or two methods are common in Go code, and are
usually given a name derived from the method, such as io.Writer
for something that implements Write.
A type can implement multiple interfaces.
For instance, a collection can be sorted
by the routines in package sort if it implements
<