Talking About Types, Variables, Pointers, and More

In my endeavor to get more of the specifics of Go figured out, I’ve been diving a bit more in depth to each of the specific features, capabilities, and characteristics of the language. I’ll be doing the same for Cassandra over the coming months and producing a blog entry series about it. This new series however is about Go, and this first in the series is about types, variables, pointers, and naming of these things. Enjoy.

Reserved Words, Keywords & Related Systemic Parts of Go

First off let’s talk about the reserved words in Go. The list includes 25 keywords that can’t be used as variables or names of functions or anything like that. This list includes:

if	import	interface	map
select	return	range	package
struct	switch	type	var
for	func	go	goto
default	defer	else	fallthrough
case	const	chan	continue
break

Beyond these 25 keywords there are also some predeclared names that can be but usually shouldn’t be used for naming. These include:

Constants

true

false

iota

nil

Types

int	int8	int16	int32
int64	uint	uint8	uint16
uint32	uint64	uintptr	float32
float64	complex128	complex64	bool
byte	rune	string	error

Functions

make	len	cap	new
append	copy	close	delete
complex	real	imag	panic
recover

When declaring an entity within a function, it’s scope is limited to that function. When declaring it outside of the function it is available to the contents of the package it is declared inside of. The case of the first letter also has the effect of allowing access within or across package boundaries. If the name begins with an upper case character it is exported for other packages. Package names themselves are always lower case however.

Camel Casing

It’s also important to know about the emphasis and standard within Go to use camel casing. Except it appears that for acronyms and such like; HTML, XML, JSON, or such things, it is routine to use names like HTMLthingy, XMLjunkHere, or rockStarJSON. Go is somewhat particular about these types of things, so it is extremely important to use the Go format standards. To help with this, there’s also the gofmt function, but it’s always good to just use the proper formatting during writing of the code anyway.

The Four Declarations

The four declarations are kind of like the four horsement, except not really. Forget that analogy as it was already going nowhere. The four declarations include:

var

const

type

func

Here are some code samples of various declarations. First use of the func. In any new Go program there’s always the main() function. Declared like this.

[sourcecode language=”javascript”]
func main() {
// Stuff goes here.
}
[/sourcecode]

Another example of a function declaration with parameters passed and expected results to return. The signature works like func followed by the name of the function, then in parenthesis parameter name followed by its type, for as may parameters as is needed. Then after the parenthesis the type of the expected result of the function. Of course, if no result is expected to be returned, then there would be no tpe specified here. Just as if no parameters, the parenthesis would still be used but no parameters specified.

[sourcecode language=”javascript”]
func doSomeComputation(someValue float64, anotherFloaty float64) float64 {
return someValue * anotherFloat
}
[/sourcecode]

Using the var for declaration of variables looks like this.

[sourcecode language=”javascript”]
var f = 212.0
var result = 2 + 2
var celsius = (f – 32) * 5 / 9
[/sourcecode]

Variable types can be defined in this way. Using var keyword, followed by the variable name, then followed by its type, and then if desired assigned via = and then the specific value or function with a result to assign to the variable.

var name type = expression

Without an assignment, the variable is simply created with the intent of assigning a value later. Such as these variable declarations.

[sourcecode language=”javascript”]
var f float32
var result float64
var celsius float64
var someVariable int
var junk string
[/sourcecode]

A shorter version of the creation and assignment of variables looks like this. It however can only be used within a function.

name := expression

Actual use of this shorter version would look like this.

[sourcecode language=”javascript”]
f := 212.0
randomStuff := rand.Float64()
wordContent := “This is some character string nonsense.”
[/sourcecode]

One can also do some other tricks with this shorter method of assignment. Let’s say some variables are needed that will have integers assigned to them. You could set up the variables with values like this. I’ve noticed some get a bit confused about this and tuple assignments, this is not the same thing. This merely gives you three variables: k, l, and m and assigns the values 0, 1, and 2 to the respective variable.

[sourcecode language=”javascript”]
k, l, m := 0, 1, 2
[/sourcecode]

It’s also important to note, a short declaration doesn’t always declare all of the variables on the left-hand side. Some, if already declared in the same lexical block, it then acts as an assignment to the variables instead.

Declaring a constant value with const would look like this.

[sourcecode language=”javascript”]
const fahrenheitBoiling = 212.0
[/sourcecode]

Pointers

A variable holds a particular value, where as a pointer holds the address in memory of a particular value. Sometimes, it gets a bit difficult to mentally keep track of pointers, so let’s look at a few examples. First I’ll start with a plain old variable.

[sourcecode language=”javascript”]
someVariable := “The original value of this variable.”
[/sourcecode]

Then I’ll add another variable, which will point at the address and respective value of someVariable.

[sourcecode language=”javascript”]
someVariable := “The original value of this variable.”
anotherVariable := &someVariable
fmt.Println(*anotherVariable)
[/sourcecode]

In this situation what does the fmt.Println(*anotherVariable) function print out? The result would be “The original value of this variable.” because anotherVariable is a variable that points to the value of someVariable. Which means anotherVariable doesn’t take the resources that someVariable does, but merely points to it.

The reasoning here is, if you have variable for example that stores a very large value in memory. One can merely point to that data with a pointer and the large value doesn’t have to change it’s memory space. Imagine if that variable were a gigabyte of data and it had to be moved every time something needed referenced or calculated with it, with a pointer we don’t have to move it. It can be used in various ways without it getting shifted around.

Now if I take the pointer value anotherVariable and assign it a value, like this.

[sourcecode language=”javascript”]
*anotherVariable = “42”
[/sourcecode]

Then what happens? The pointer now is re-allocated and assigned the new value of “42”. It no longer references the address space of the pointer and instead now has the value 42 assigned to the variable.

The Fancy new Function

There’s also another function, a built-in one, that creates a variable. Using the expression new(type) will create an unnamed variable of type, initializes it, and returns the address of the value of type. In pointer speak, it’d be *type. Here are some examples.

[sourcecode language=”javascript”]
pinto := new(int)
fmt.Println(*pinto)
[/sourcecode]

This code will create a variable of type int, then initialize the value to zero since it is an int. Let’s say we want to actually give it a value after it’s created though. We could assign pinto like this.

[sourcecode language=”javascript”]
*pinto = 42
fmt.Println(*p)
[/sourcecode]

This will then print out the value 42, of type integer. Note, this new is just sugar, a syntactical convenience feature really. They can be useful however, and there are a few caveats that need to be noted. For example, check out this gotcha.

If I declare two new values using new.

[sourcecode language=”javascript”]
firstThing := new(int)
secondThing := new(int)
fmt.Println(firstThing == secondThing)
[/sourcecode]

In this case, the firstThing and secondThing don’t actually match, because the comparison is on the address space, and they’re different address spaces!

Alright, that’s it for now, just a few important nuances around how variables, pointers, and Go features work.