Go Home Page The Go Programming Language

Go For C++ Programmers

Go is a systems programming language intended as an alternative to 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 tutorial.

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

There is more documentation about go.

Conceptual Differences

Syntax

The declaration syntax is reversed compared to C++. You write the name followed by the type. Unlike 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;            // approximately const std::string v2;
var v3 [10]int;           // int v3[10];
var v4 []int;             // approximately int* v4;
var v5 struct { f int };  // struct { int f; } v5;
var v6 *int;              // int* v6;  // but no pointer arithmetic
var v7 map[string]int;       // approximately unordered_map<string, int>* v7;
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 begin declared; see the discussion of interfaces.

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

var (i int; m float)

When declaring a function, you must 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);

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, the type must be specified. In that case it will be implicitly initialized to 0 (or nil, or whatever). There are no uninitialized variables in Go.

Within a function, a simple 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, indicating by a list in parentheses. 

func f() (i int, j int);
v1, v2 = f();

Go treats semicolons as separators, not terminators. Moreover, a semicolon is not required after a curly brace ending a type declaration (e.g., var s struct {}) or a block. Semicolons are never required at the top level of a file (between global declarations). However, they are always permitted at the end of a statement, so you can continue using them as in C++.

Go treats semicolons as separators, not terminators. Moreover, a semicolon is not required after a curly brace ending a type declaration (e.g., var s struct {}) or a block. Semicolons are never required at the top level of a file (between global declarations). However, they are always permitted at the end of a statement, so you can continue using them as in C++.

When using a pointer, you use . instead of ->. Thus syntactically speaking there is no difference between a structure and a pointer to a structure. 

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

Go does not require parentheses around the condition of a 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
if (a < b) f();     // 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: 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)++.

Constants

In Go integer and floating-point constants have so-called ideal types. This applies even to constants named with a const declaration, if no type is given in the declaration. An ideal type becomes concrete when it is actually used. This permits constants to be used relatively freely without requiring general implicit type conversion. 

var a uint; f(a + 1)  // Ideal type of "1" becomes "uint".

The language does not impose any limits on the size of an abstract integer constant or constant expression. A limit is only applied when a constant expression 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; blue; green ) // red == 0, blue == 1, green == 2

Slices

A slice is a pointer to an array, a length, and a capacity. Slices support the [] operator to access elements. 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 at index J - 1. It has length J - I. If a is itself a slice, 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. You may also assign a pointer to an array to a variable of slice type; given var s []int; var a[10] int, s = &a is more or less the same as s = a[0:len(a)].

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 the address of the array. Unlike 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.

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) returns a new object of type *int, allocated on the heap and initialized with the value 0. Unlike C++, new is a function, not an operator; new int is a syntax error.

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 is the buffering capacity of the channel.

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 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 my_type struct { i int }
func (p *my_type) get() int { return p.i }

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

Given this interface: 

type my_interface interface {
  get() int;
  set(i int);
}

we can make my_type satisfy the interface by additionally writing 

func (p *my_type) set(i int) { p.i = i }

Now any function which takes my_interface as a parameter will accept a variable of type *my_type. 

func get_and_set(x my_interface);
func f1() {
  var p my_type;
  get_and_set(&p);
}

In other words, if we view my_interface as a C++ pure abstract base class, defining set and get for *my_type made *my_type automatically inherit from my_interface. A type may satisfy multiple interfaces.

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

type my_child_type struct { my_type; j int }
func (p *my_child_type) get() int { p.j++; return (&p.my_type).get() }

This effectively implements my_child_type as a child of my_type. 

func f2() {
   var p my_child_type;
   get_and_set(&p)
}

The set method is effectively inherited from my_child_type, because methods associated with the anonymous type are promoted to become methods of the enclosing type. In this case, because my_child_type has an anonymous field of type my_type, the methods of my_type also become methods of my_child_type. 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 parent method is called, it receives a pointer to the field in the child class. If the parent method calls some other method on its argument, it will call the method associated with the parent class, not the method associated with the child class. In other words, methods are not virtual functions. When you want the equivalent of a virtual function, use an interface.

A variable which has an interface type may be converted to have a different interface type. This conversion is implemented dynamically at runtime, like C++ dynamic_cast. Unlike dynamic_cast, there does not need to be any declared relationship between the two interfaces. 

type my_compare_interface interface {
  print();
}
func f3(x my_interface) {
  x.(my_compare_interface).print()
}

The conversion to my_compare_interface 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, e.g., manipulating values of the minimal interface. 

type Any interface { }

Containers may be written in terms of Any, and the caller may cast the values back to the desired 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 runtime, but all operations will involve a function call. 

type iterator interface {
  get() Any;
  set(v Any);
  increment();
  equal(arg *iterator) bool;
}

Processes

Go permits starting a new process (a "goroutine") using the go statement. The go statement runs a function in a different process. All processes in a single program share the same address space. 

func server(i int) { for { print(i); sys.sleep(10) } }
go server(1); go server(2);

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

Processes are (intended to be) cheap.

Function literals can be useful with the go statement. 

var g int // global variable
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 processes. 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 process 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) {
  var val int = 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 means that if two processes try to retrieve the value at the same time, the first process may read the response which was triggered by the second process's request. In simple cases that is fine. For more complex cases, pass in a channel. 

type cmd2 struct { get bool; val int; ch <- chan int; }
func manager2(ch chan cmd2) {
  var val int = 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 {
  my_ch := make(chan int);
  c := cmd2 { true, 0, my_ch };  // Composite literal syntax.
  ch <- c;
  return <- my_ch;
}