go/doc/go_for_cpp_programmers.html

<!-- Go For C++ Programmers -->

<p>
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.

<p>
For a more general introduction to Go, see the
<a href="go_tutorial.html">Go tutorial</a>.

<p>
For a detailed description of the Go language, see the
<a href="go_spec.html">Go spec</a>.

<p>
There is more <a href="./">documentation about go</a>.

<h2 id="Conceptual_Differences">Conceptual Differences</h2>

<ul>
<li>Go does not have classes with constructors or destructors.
    Instead of class methods, a class inheritance hierarchy,
    and virtual functions, Go provides <em>interfaces</em>, which are
    <a href="#Interfaces">discussed in more detail below</a>.
    Interfaces are also used where C++ uses templates.

<li>Go uses garbage collection. It is not necessary (or currently possible)
    to explicitly release memory. The garbage collection is (intended to be)
    incremental and highly efficient on modern processors.

<li>Go supports pointers, but does not support pointer arithmetic. You cannot
    use a pointer variable to walk through the bytes of a string.

<li>Arrays in Go are first class values. When an array is used as a
    function parameter, the function receives a copy of the array,
    not a pointer to it. However, in practice functions often use
    slices for parameters, rather than arrays. This is discussed further
    below.

<li>Strings are provided by the language. They may not change once they
    have been created.

<li>Hash tables are provided by the language. They are called maps.

<li>Processes, and communication channels between them, are provided by
    the language. This is discussed further below.

<li>Certain types (maps, channels, and slices, all described further below)
    are passed by reference, not by value. That is, passing a map to a
    function does not copy the map, and if the function changes the map
    the change will be seen by the caller.

<li>Go does not use header files. Instead, each source file is part of a
    defined <em>package</em>. When a package defines an object
    (type, constant, variable, function) with a name which starts with an
    uppercase letter, than object is visible to any other file which
    imports that package.

<li>Go does not support implicit type conversion. Operations that mix
    different types require casts (called conversions in Go).

<li>Go does not support function overloading and does not support user
    defined operators.

<li>Go does not support <code>const</code> or <code>volatile</code> qualifiers.

<li>Go uses <code>nil</code> for invalid pointers, where C++ uses
    <code>NULL</code> or simply <code>0</code>.
</ul>

<h2 id="Syntax">Syntax</h2>

<p>
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.

<pre>
<b>Go                           C++</b>
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&lt;string, int&gt;* v7;
var v8 func(a int) int;   // int (*v8)(int a);
</pre>

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

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

<pre>
var (i int; m float)
</pre>

<p>
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:

<pre>
func f (i, j, k int);
</pre>

<p>
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.

<pre>
var v = *p;
</pre>

<p>
See also the <a href="#Constants">discussion of constants, below</a>.
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.

<p>
Within a function, a simple declaration syntax is available with
<code>:=</code> .

<pre>
v1 := v2;
</pre>

<p>
This is equivalent to

<pre>
var v1 = v2;
</pre>

<p>
Go permits multiple assignments which are done in parallel.

<pre>
i, j = j, i;   // Swap i and j.
</pre>

<p>
Functions may have multiple return values, indicating by a list in
parentheses.

<pre>
func f() (i int, j int);
v1, v2 = f();
</pre>

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

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

<p>
When using a pointer, you use <code>.</code> instead of <code>-&gt;</code>.
Thus syntactically
speaking there is no difference between a structure and a pointer to a
structure.

<pre>
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)
</pre>

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

<pre>
if a &lt; b { f() }    // Valid
if (a &lt; b) { f() }  // Valid
if (a &lt; b) f();     // INVALID
</pre>

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

<p>
Go permits <code>break</code> and <code>continue</code> to specify a label.
The label must
refer to a <code>for</code>, <code>switch</code>, or <code>select</code>
statement.

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

<pre>
switch i { case 0: case 1: f() } // f is not called when i == 0!
</pre>

<p>
But a <code>case</code> can have multiple values.

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

<p>
The values in a <code>case</code> 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 <code>switch</code>
value is omitted it defaults to <code>true</code>.

<pre>
switch { case i &lt; 0: f1() case i == 0: f2() case i &gt; 0: f3() }
</pre>

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

<h2 id="Constants">Constants </h2>

<p>
In Go constants may be <i>untyped</i>. This applies even to constants
named with a <code>const</code> declaration if no
type is given in the declaration and the initializer expression uses only
untyped constants.
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.

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

<p>
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.

<pre>
const huge = 1 &lt;&lt; 100; f(huge &gt;&gt; 98)
</pre>

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

<pre>
const ( red = iota; blue; green ) // red == 0, blue == 1, green == 2
</pre>

<h2 id="Slices">Slices</h2>

<p>
A slice is a pointer to an array, a length, and a capacity.  Slices support
the <code>[]</code> operator to access elements.  The builtin
<code>len</code> function returns the
length of the slice.  The builtin <code>cap</code> function returns the
capacity.

<p>
Given an array, or another slice, a new slice is created via
<code>a[I:J]</code>.  This
creates a new slice which refers to <code>a</code>, starts at
index <code>I</code>, and ends at index
<code>J - 1</code>.  It has length <code>J - I</code>.
If <code>a</code> is itself a slice, the new slice refers to the same array
to which <code>a</code>
refers.  That is, changes made using the new slice may be seen using
<code>a</code>.  The
capacity of the new slice is simply the capacity of <code>a</code> minus
<code>I</code>.  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 <code>var s []int; var a[10] int</code>,
<code>s = &amp;a</code> is more or
less the same as <code>s = a[0:len(a)]</code>.

<p>
What this means is that Go uses slices for some cases where C++ uses pointers.
If you create a value of type <code>[100]byte</code> (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 <code>[]byte</code>, 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
<code>len</code>.

<p>
The slice syntax may also be used with a string.  It returns a new string,
whose value is a substring of the original string.

<h2 id="Making_values">Making values</h2>

<p>
Go has a builtin function <code>new</code> which takes a type and
allocates space
on the heap. The allocated space will be zero-initialized for the type.
For example, <code>new(int)</code> returns a new object of type
<code>*int</code>,
allocated on the heap and initialized with the value <code>0</code>.
Unlike C++, <code>new</code> is a function, not an operator;
<code>new int</code> is a syntax error.

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

<p>
The <code>make</code> 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,
<code>make([]int, 10, 20)</code>.  This is identical to
<code>new([20]int)[0:10]</code>.  Since
Go uses garbage collection, the newly allocated array will be discarded
sometime after there are no references to the returned slice.

<h2 id="Interfaces">Interfaces</h2>

<p>
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.

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

<pre>
type my_type struct { i int }
func (p *my_type) get() int { return p.i }
</pre>

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

<p>
Given this interface:

<pre>
type my_interface interface {
  get() int;
  set(i int);
}
</pre>

<p>
we can make <code>my_type</code> satisfy the interface by additionally writing

<pre>
func (p *my_type) set(i int) { p.i = i }
</pre>

<p>
Now any function which takes <code>my_interface</code> as a parameter
will accept a
variable of type <code>*my_type</code>.

<pre>
func get_and_set(x my_interface);
func f1() {
  var p my_type;
  get_and_set(&amp;p);
}
</pre>

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

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

<pre>
type my_child_type struct { my_type; j int }
func (p *my_child_type) get() int { p.j++; return (&amp;p.my_type).get() }
</pre>

<p>
This effectively implements <code>my_child_type</code> as a child of
<code>my_type</code>.

<pre>
func f2() {
   var p my_child_type;
   get_and_set(&amp;p)
}
</pre>

<p>
The <code>set</code> method is effectively inherited from
<code>my_child_type</code>, because
methods associated with the anonymous type are promoted to become methods
of the enclosing type.  In this case, because <code>my_child_type</code> has an
anonymous field of type <code>my_type</code>, the methods of
<code>my_type</code> also become methods of <code>my_child_type</code>.
In this example, the <code>get</code> method was
overridden, and the <code>set</code> method was inherited.

<p>
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.

<p>
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++ <code>dynamic_cast</code>.  Unlike
<code>dynamic_cast</code>, there does
not need to be any declared relationship between the two interfaces.

<pre>
type my_compare_interface interface {
  print();
}
func f3(x my_interface) {
  x.(my_compare_interface).print()
}
</pre>

<p>
The conversion to <code>my_compare_interface</code> is entirely dynamic.
It will
work as long as the underlying type of x (the "dynamic type") defines
a <code>print</code> method.

<p>
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.

<pre>
type Any interface { }
</pre>

<p>
Containers may be written in terms of <code>Any</code>, 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.

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

<h2 id="Processes">Processes</h2>

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

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

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

<p>
Processes are (intended to be) cheap.

<p>
Function literals can be useful with the <code>go</code> statement.

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

<h2 id="Channels">Channels</h2>

<p>
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 <code>&lt;-</code> as a binary
operator.  To
receive a value on a channel, use <code>&lt;-</code> as a unary operator.
When calling
functions, channels are passed by reference.

<p>
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.

<pre>
type cmd struct { get bool; val int }
func manager(ch chan cmd) {
  var val int = 0;
  for {
    c := &lt;- ch
    if c.get { c.val = val; ch &lt;- c }
    else { val = c.val }
  }
}
</pre>

<p>
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.

<pre>
type cmd2 struct { get bool; val int; ch &lt;- chan int; }
func manager2(ch chan cmd2) {
  var val int = 0;
  for {
    c := &lt;- ch
    if c.get { c.ch &lt;- val }
    else { val = c.val }
  }
}
</pre>

<p>
To use manager2, given a channel to it:

<pre>
func f4(ch &lt;- chan cmd2) int {
  my_ch := make(chan int);
  c := cmd2 { true, 0, my_ch };  // Composite literal syntax.
  ch &lt;- c;
  return &lt;- my_ch;
}
</pre>