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Go is a systems programming language intended to be a general-purpose systems language, like C++. These are some notes on Go for experienced C++ programmers. This document discusses the differences between Go and C++, and says little to nothing about the similarities.

For a more general introduction to Go, see the Go Tour, How to Write Go Code and Effective Go.

For a detailed description of the Go language, see the Go spec.

Conceptual Differences

• Go does not have classes with constructors or destructors. Instead of class methods, a class inheritance hierarchy, and virtual functions, Go provides interfaces, which are discussed in more detail below. Interfaces are also used where C++ uses templates.

Syntax

The declaration syntax is reversed compared to C++. You write the name followed by the type. Unlike in C++, the syntax for a type does not match the way in which the variable is used. Type declarations may be read easily from left to right.

Go                           C++
var v1 int                // int v1;
var v2 string             // const std::string v2;  (approximately)
var v3 [10]int            // int v3[10];
var v4 []int              // int* v4;  (approximately)
var v5 struct { f int }   // struct { int f; } v5;
var v6 *int               // int* v6;  (but no pointer arithmetic)
var v7 map[string]int     // unordered_map<string, int>* v7;  (approximately)
var v8 func(a int) int    // int (*v8)(int a);

Declarations generally take the form of a keyword followed by the name of the object being declared. The keyword is one of var, func, const, or type. Method declarations are a minor exception in that the receiver appears before the name of the object being declared; see the discussion of interfaces.

You can also use a keyword followed by a series of declarations in parentheses.

var (
    i int
    m float64
)

When declaring a function, you must either provide a name for each parameter or not provide a name for any parameter; you can't omit some names and provide others. You may group several names with the same type:

func f(i, j, k int, s, t string)

A variable may be initialized when it is declared. When this is done, specifying the type is permitted but not required. When the type is not specified, the type of the variable is the type of the initialization expression.

var v = *p

See also the discussion of constants, below. If a variable is not initialized explicitly, the type must be specified. In that case it will be implicitly initialized to the type's zero value (0, nil, etc.). There are no uninitialized variables in Go.

Within a function, a short declaration syntax is available with := .

v1 := v2

This is equivalent to

var v1 = v2

Go permits multiple assignments, which are done in parallel.

i, j = j, i    // Swap i and j.

Functions may have multiple return values, indicated by a list in parentheses. The returned values can be stored by assignment to a list of variables.

func f() (i int, j int) { ... }
v1, v2 = f()

Go code uses very few semicolons in practice. Technically, all Go statements are terminated by a semicolon. However, Go treats the end of a non-blank line as a semicolon unless the line is clearly incomplete (the exact rules are in the language specification). A consequence of this is that in some cases Go does not permit you to use a line break. For example, you may not write

func g()
{                  // INVALID
}

A semicolon will be inserted after g(), causing it to be a function declaration rather than a function definition. Similarly, you may not write

if x {
}
else {             // INVALID
}

A semicolon will be inserted after the } preceding the else, causing a syntax error.

Since semicolons do end statements, you may continue using them as in C++. However, that is not the recommended style. Idiomatic Go code omits unnecessary semicolons, which in practice is all of them other than the initial for loop clause and cases where you want several short statements on a single line.

While we're on the topic, we recommend that rather than worry about semicolons and brace placement, you format your code with the gofmt program. That will produce a single standard Go style, and let you worry about your code rather than your formatting. While the style may initially seem odd, it is as good as any other style, and familiarity will lead to comfort.

When using a pointer to a struct, you use . instead of ->. Thus syntactically speaking a structure and a pointer to a structure are used in the same way.

type myStruct struct { i int }
var v9 myStruct              // v9 has structure type
var p9 *myStruct             // p9 is a pointer to a structure
f(v9.i, p9.i)

Go does not require parentheses around the condition of an if statement, or the expressions of a for statement, or the value of a switch statement. On the other hand, it does require curly braces around the body of an if or for statement.

if a < b { f() }             // Valid
if (a < b) { f() }           // Valid (condition is a parenthesized expression)
if (a < b) f()               // INVALID
for i = 0; i < 10; i++ {}    // Valid
for (i = 0; i < 10; i++) {}  // INVALID

Go does not have a while statement nor does it have a do/while statement. The for statement may be used with a single condition, which makes it equivalent to a while statement. Omitting the condition entirely is an endless loop.

Go permits break and continue to specify a label. The label must refer to a for, switch, or select statement.

In a switch statement, case labels do not fall through. You can make them fall through using the fallthrough keyword. This applies even to adjacent cases.

switch i {
case 0:  // empty case body
case 1:
    f()  // f is not called when i == 0!
}

But a case can have multiple values.

switch i {
case 0, 1:
    f()  // f is called if i == 0 || i == 1.
}

The values in a case need not be constants--or even integers; any type that supports the equality comparison operator, such as strings or pointers, can be used--and if the switch value is omitted it defaults to true.

switch {
case i < 0:
    f1()
case i == 0:
    f2()
case i > 0:
    f3()
}

The ++ and -- operators may only be used in statements, not in expressions. You cannot write c = *p++. *p++ is parsed as (*p)++.

The defer statement may be used to call a function after the function containing the defer statement returns.

fd := open("filename")
defer close(fd)         // fd will be closed when this function returns.

Constants

In Go constants may be untyped. This applies even to constants named with a const declaration, if no type is given in the declaration and the initializer expression uses only untyped constants. A value derived from an untyped constant becomes typed when it is used within a context that requires a typed value. This permits constants to be used relatively freely without requiring general implicit type conversion.

var a uint
f(a + 1)  // untyped numeric constant "1" becomes typed as uint

The language does not impose any limits on the size of an untyped numeric constant or constant expression. A limit is only applied when a constant is used where a type is required.

const huge = 1 << 100
f(huge >> 98)

Go does not support enums. Instead, you can use the special name iota in a single const declaration to get a series of increasing value. When an initialization expression is omitted for a const, it reuses the preceding expression.

const (
    red = iota   // red == 0
    blue         // blue == 1
    green        // green == 2
)

Slices

A slice is conceptually a struct with three fields: a pointer to an array, a length, and a capacity. Slices support the [] operator to access elements of the underlying array. The builtin len function returns the length of the slice. The builtin cap function returns the capacity.

Given an array, or another slice, a new slice is created via a[i:j]. This creates a new slice which refers to a, starts at index i, and ends before index j. It has length j-i. If i is omitted, the slice starts at 0. If j is omitted, the slice ends at len(a). The new slice refers to the same array to which a refers. That is, changes made using the new slice may be seen using a. The capacity of the new slice is simply the capacity of a minus i. The capacity of an array is the length of the array.

What this means is that Go uses slices for some cases where C++ uses pointers. If you create a value of type [100]byte (an array of 100 bytes, perhaps a buffer) and you want to pass it to a function without copying it, you should declare the function parameter to have type []byte, and pass a slice of the array (a[:] will pass the entire array). Unlike in C++, it is not necessary to pass the length of the buffer; it is efficiently accessible via len.

The slice syntax may also be used with a string. It returns a new string, whose value is a substring of the original string. Because strings are immutable, string slices can be implemented without allocating new storage for the slices's contents.

Making values

Go has a builtin function new which takes a type and allocates space on the heap. The allocated space will be zero-initialized for the type. For example, new(int) allocates a new int on the heap, initializes it with the value 0, and returns its address, which has type *int. Unlike in C++, new is a function, not an operator; new int is a syntax error.

Perhaps surprisingly, new is not commonly used in Go programs. In Go taking the address of a variable is always safe and never yields a dangling pointer. If the program takes the address of a variable, it will be allocated on the heap if necessary. So these functions are equivalent:

type S { I int }

func f1() *S {
	return new(S)
}

func f2() *S {
	var s S
	return &s
}

func f3() *S {
	// More idiomatic: use composite literal syntax.
	return &S{0}
}

Map and channel values must be allocated using the builtin function make. A variable declared with map or channel type without an initializer will be automatically initialized to nil. Calling make(map[int]int) returns a newly allocated value of type map[int]int. Note that make returns a value, not a pointer. This is consistent with the fact that map and channel values are passed by reference. Calling make with a map type takes an optional argument which is the expected capacity of the map. Calling make with a channel type takes an optional argument which sets the buffering capacity of the channel; the default is 0 (unbuffered).

The make function may also be used to allocate a slice. In this case it allocates memory for the underlying array and returns a slice referring to it. There is one required argument, which is the number of elements in the slice. A second, optional, argument is the capacity of the slice. For example, make([]int, 10, 20). This is identical to new([20]int)[0:10]. Since Go uses garbage collection, the newly allocated array will be discarded sometime after there are no references to the returned slice.

Interfaces

Where C++ provides classes, subclasses and templates, Go provides interfaces. A Go interface is similar to a C++ pure abstract class: a class with no data members, with methods which are all pure virtual. However, in Go, any type which provides the methods named in the interface may be treated as an implementation of the interface. No explicitly declared inheritance is required. The implementation of the interface is entirely separate from the interface itself.

A method looks like an ordinary function definition, except that it has a receiver. The receiver is similar to the this pointer in a C++ class method.

type myType struct { i int }
func (p *myType) Get() int { return p.i }

This declares a method Get associated with myType. The receiver is named p in the body of the function.

Methods are defined on named types. If you convert the value to a different type, the new value will have the methods of the new type, not the old type.

You may define methods on a builtin type by declaring a new named type derived from it. The new type is distinct from the builtin type.

type myInteger int
func (p myInteger) Get() int { return int(p) } // Conversion required.
func f(i int) { }
var v myInteger
// f(v) is invalid.
// f(int(v)) is valid; int(v) has no defined methods.

Given this interface:

type myInterface interface {
	Get() int
	Set(i int)
}

we can make myType satisfy the interface by adding

func (p *myType) Set(i int) { p.i = i }

Now any function which takes myInterface as a parameter will accept a variable of type *myType.

func GetAndSet(x myInterface) {}
func f1() {
	var p myType
	GetAndSet(&p)
}

In other words, if we view myInterface as a C++ pure abstract base class, defining Set and Get for *myType made *myType automatically inherit from myInterface. A type may satisfy multiple interfaces.

An anonymous field may be used to implement something much like a C++ child class.

type myChildType struct { myType; j int }
func (p *myChildType) Get() int { p.j++; return p.myType.Get() }

This effectively implements myChildType as a child of myType.

func f2() {
	var p myChildType
	GetAndSet(&p)
}

The set method is effectively inherited from myType, because methods associated with the anonymous field are promoted to become methods of the enclosing type. In this case, because myChildType has an anonymous field of type myType, the methods of myType also become methods of myChildType. In this example, the Get method was overridden, and the Set method was inherited.

This is not precisely the same as a child class in C++. When a method of an anonymous field is called, its receiver is the field, not the surrounding struct. In other words, methods on anonymous fields are not virtual functions. When you want the equivalent of a virtual function, use an interface.

A variable that has an interface type may be converted to have a different interface type using a special construct called a type assertion. This is implemented dynamically at run time, like C++ dynamic_cast. Unlike dynamic_cast, there does not need to be any declared relationship between the two interfaces.

type myPrintInterface interface {
	Print()
}
func f3(x myInterface) {
	x.(myPrintInterface).Print()  // type assertion to myPrintInterface
}

The conversion to myPrintInterface is entirely dynamic. It will work as long as the underlying type of x (the dynamic type) defines a print method.

Because the conversion is dynamic, it may be used to implement generic programming similar to templates in C++. This is done by manipulating values of the minimal interface.

type Any interface { }

Containers may be written in terms of Any, but the caller must unbox using a type assertion to recover values of the contained type. As the typing is dynamic rather than static, there is no equivalent of the way that a C++ template may inline the relevant operations. The operations are fully type-checked at run time, but all operations will involve a function call.

type Iterator interface {
	Get() Any
	Set(v Any)
	Increment()
	Equal(arg Iterator) bool
}

Note that Equal has an argument of type Iterator. This does not behave like a C++ template. See the FAQ.

Goroutines

Go permits starting a new thread of execution (a goroutine) using the go statement. The go statement runs a function in a different, newly created, goroutine. All goroutines in a single program share the same address space.

Internally, goroutines act like coroutines that are multiplexed among multiple operating system threads. You do not have to worry about these details.

func server(i int) {
	for {
		fmt.Print(i)
		time.Sleep(10 * time.Second)
	}
}
go server(1)
go server(2)

(Note that the for statement in the server function is equivalent to a C++ while (true) loop.)

Goroutines are (intended to be) cheap.

Function literals (which Go implements as closures) can be useful with the go statement.

var g int
go func(i int) {
	s := 0
	for j := 0; j < i; j++ { s += j }
	g = s
}(1000)  // Passes argument 1000 to the function literal.

Channels

Channels are used to communicate between goroutines. Any value may be sent over a channel. Channels are (intended to be) efficient and cheap. To send a value on a channel, use <- as a binary operator. To receive a value on a channel, use <- as a unary operator. When calling functions, channels are passed by reference.

The Go library provides mutexes, but you can also use a single goroutine with a shared channel. Here is an example of using a manager function to control access to a single value.

type Cmd struct { Get bool; Val int }
func Manager(ch chan Cmd) {
	val := 0
	for {
		c := <-ch
		if c.Get { c.Val = val; ch <- c }
		else { val = c.Val }
	}
}

In that example the same channel is used for input and output. This is incorrect if there are multiple goroutines communicating with the manager at once: a goroutine waiting for a response from the manager might receive a request from another goroutine instead. A solution is to pass in a channel.

type Cmd2 struct { Get bool; Val int; Ch <- chan int }
func Manager2(ch chan Cmd2) {
	val := 0
	for {
		c := <-ch
		if c.Get { c.ch <- val }
		else { val = c.Val }
	}
}

To use Manager2, given a channel to it:

func f4(ch <- chan Cmd2) int {
	myCh := make(chan int)
	c := Cmd2{ true, 0, myCh }   // Composite literal syntax.
	ch <- c
	return <-myCh
}