This article will introduce constant and variable declarations in Go. The concept of untyped values and explicit conversions will also be introduced.
The literals introduced in the
last article are all called unnamed constants (or literal constants),
except false
and true
,
which are two predeclared (built-in) named constants.
Custom named constant declarations will be introduced below in this article.
In Go, some values are untyped. An untyped value means the type of the value has not been confirmed yet. On the contrary, the type of a typed value is determined.
For most untyped values, each of them has one default type.
The predeclared nil
is the only untyped value
which has no default type.
We will learn more about nil
in other Go 101 articles later.
All literal constants (unnamed constants) are untyped values. In fact, in Go,
most untyped values are literal constants and named constants (which will be introduced
below in the current article). The other untyped values include the just mentioned
nil
and some boolean results returned by some operations
which will be introduced in other articles later.
string
.
bool
.
int
.
rune
(a.k.a., int32
).
float64
.
complex128
.
Like many other languages, Go also supports value conversions.
We can use the form T(v)
to convert a value v
to the type denoted by T
(or simply speaking, type T
).
If the conversion T(v)
is legal,
Go compilers view T(v)
as a typed value of type T
.
Surely, for a certain type T
,
to make the conversion T(v)
legal,
the value v
can't be arbitrary.
The following mentioned rules apply for both the literal constants introduced in the last article and the untyped named constants which will be introduced soon.
v
,
there are two scenarios where T(v)
is legal.
v
(or the literal denoted by v
) is
representable
as a value of a basic type T
.
The result value is a typed constant of type T
.
v
is an integer type (int
or rune
) and T
is a string type.
The result of T(v)
is a string of type T
and contains the UTF-8 representation of the integer as a Unicode code point.
Integer values outside the range of valid Unicode code points
result strings represented by "\uFFFD"
(a.k.a., "\xef\xbf\xbd"
).
0xFFFD
is the code point for the Unicode replacement character.
The result string of a conversion from an integer always contains one and only one rune.
(Note, later Go version might only allow converting rune or byte integers to strings.
Since Go Toolchain 1.15, the go vet
command warns on conversions from non-rune and non-byte integers to strings.)
In fact, the second scenario doesn't require v
to be a constant.
If v
is a constant, then the result of the conversion is also a constant,
otherwise, the result is not a constant.
// Rounding happens in the following 3 lines.
complex128(1 + -1e-1000i) // 1.0+0.0i
float32(0.49999999) // 0.5
float32(17000000000000000)
// No rounding in the these lines.
float32(123)
uint(1.0)
int8(-123)
int16(6+0i)
complex128(789)
string(65) // "A"
string('A') // "A"
string('\u68ee') // "森"
string(-1) // "\uFFFD"
string(0xFFFD) // "\uFFFD"
string(0x2FFFFFFFF) // "\uFFFD"
And the following conversions are all illegal.
// 1.23 is not representable as a value of int.
int(1.23)
// -1 is not representable as a value of uint8.
uint8(-1)
// 1+2i is not representable as a value of float64.
float64(1+2i)
// Constant -1e+1000 overflows float64.
float64(-1e1000)
// Constant 0x10000000000000000 overflows int.
int(0x10000000000000000)
// The default type of 65.0 is float64,
// which is not an integer type.
string(65.0)
// The default type of 66+0i is complex128,
// which is not an integer type.
string(66+0i)
From the above examples, we know that an untyped constant,
(for example -1e1000
and 0x10000000000000000
),
may even not be able to represent as a value of its default type.
Please note,
sometimes, the form of explicit conversions must
be written as (T)(v)
to avoid ambiguities.
Such situations often happen in case of T
is not an identifier.
We will learn more explicit conversion rules later in other Go 101 articles.
Go supports type deduction. In other words, in many circumstances, programmers don't need to explicitly specify the types of some values in code. Go compilers will deduce the types for these values by context.
Type deduction is also often called type inference.
In Go code, if a place needs a value of a certain type and an untyped value (often a constant) is representable as a value of the certain type, then the untyped value can be used in the place. Go compilers will view the untyped value as a typed value of the certain type. Such places include an operand in an operator operation, an argument in a function call, a destination value or a source value in an assignment, etc.
Some circumstances have no requirements on the types of the used values. If an untyped value is used in such a circumstance, Go compilers will treat the untyped value as a typed value of its default type.
The two type deduction cases can be viewed as implicit conversions.
The below constant and variable declaration sections will show some type deduction cases. More type deduction rules and cases will be introduced in other articles.
const
is used to declare named constants.
The following program contains some constant declarations.
package main
// Declare two individual constants. Yes,
// non-ASCII letters can be used in identifiers.
const π = 3.1416
const Pi = π // <=> const Pi = 3.1416
// Declare multiple constants in a group.
const (
No = !Yes
Yes = true
MaxDegrees = 360
Unit = "radian"
)
func main() {
// Declare multiple constants in one line.
const TwoPi, HalfPi, Unit2 = π * 2, π * 0.5, "degree"
}
Go specification calls each of the lines containing a =
symbol in
the above constant declaration group as a constant specification.
In the above example,
the *
symbol is the multiplication operator
and the !
symbol is the boolean-not operator.
Operators will be introduced in the next article,
common operators.
The =
symbol means "bind" instead of "assign".
We should interpret each constant specification
as a declared identifier is bound to a corresponding basic value literal.
Please read the last section in the current article for more explanations.
In the above example, the name constants π
and Pi
are both bound to the literal 3.1416
.
The two named constants may be used at many places in code.
Without constant declarations, the literal 3.1416
would be
populated at those places. If we want to change the literal
to 3.14
later, many places need to be modified.
With the help of constant declarations, the literal 3.1416
will only appear in one constant declaration,
so only one place needs to be modified.
This is the main purpose of constant declarations.
Later, we use the terminology non-constant values to denote the values who are not constants. The to be introduced variables below, all belong to one kind of non-constant values.
Please note that, constants can be declared both at package level (out of any function body) and in function bodies. The constants declared in function bodies are called local constants. The variables declared out of any function body are called package-level constants. We also often call package-level constants as global constants.
The declaration orders of two package-level constants are not important.
In the above example, the declaration orders of
No
and Yes
can be exchanged.
All constants declared in the last example are untyped. The default type of a named untyped constant is the same as the literal bound to it.
X
and Y
are both float32
and the types of A
and B
are both int64
.
const X float32 = 3.14
const (
A, B int64 = -3, 5
Y float32 = 2.718
)
If multiple typed constants are declared in the same constant specification,
then their types must be the same, just as the constants
A
and B
in the above example.
X
, Y
, A
and B
are all typed constants.
const X = float32(3.14)
const (
A, B = int64(-3), int64(5)
Y = float32(2.718)
)
If a basic value literal is bound to a typed constant, the basic value
literal must be representable as a value of the type of the constant.
The following typed constant declarations are invalid.
// error: 256 overflows uint8
const a uint8 = 256
// error: 256 overflows uint8
const b = uint8(255) + uint8(1)
// error: 128 overflows int8
const c = int8(-128) / int8(-1)
// error: -1 overflows uint
const MaxUint_a = uint(^0)
// error: -1 overflows uint
const MaxUint_b uint = ^0
In the above and following examples ^
is bitwise-not operator.
uint
value has only 32 bits on 32-bit OSes.
(1 << 64) - 1
is not representable as 32-bit values.
(Here, <<
is bitwise-left-shift operator.)
const MaxUint uint = (1 << 64) - 1
Then how to declare a typed uint
constant and bind
the largest uint
value to it? Use the following way instead.
const MaxUint = ^uint(0)
Similarly, we can declare a typed int
constant
and bind the largest int
value to it.
(Here, >>
is bitwise-right-shift operator.)
const MaxInt = int(^uint(0) >> 1)
A similar method can be used to get the number of bits of a native word,
and check the current OS is 32-bit or 64-bit.
// NativeWordBits is 64 or 32.
const NativeWordBits = 32 << (^uint(0) >> 63)
const Is64bitOS = ^uint(0) >> 63 != 0
const Is32bitOS = ^uint(0) >> 32 == 0
Here, !=
and ==
are not-equal-to and equal-to operators.
const (
X float32 = 3.14
Y // here must be one identifier
Z // here must be one identifier
A, B = "Go", "language"
C, _
// In the above line, the blank identifier
// is required to be present.
)
as
const (
X float32 = 3.14
Y float32 = 3.14
Z float32 = 3.14
A, B = "Go", "language"
C, _ = "Go", "language"
)
iota
in constant declarationsiota
constant generator
feature brings much convenience to Go programming.
iota
is a predeclared constant
which can only be used in other constant declarations.
It is declared as
const iota = 0
But the value of an iota
in code may be not always 0
.
When the predeclared iota
constant is used in
a custom constant declaration, at compile time,
within the custom constant declaration,
its value will be reset to 0
at the first constant specification of each group of constants
and will increase 1
constant specification by constant specification.
In other words, in the nth constant specification
of a constant declaration, the value of iota
is n (starting from zero).
So iota
is only useful in group-style constant declarations.
iota
constant generator features.
Please read the comments to get what will happen at compile time.
The +
symbol in this example is the addition operator.
package main
func main() {
const (
k = 3 // now, iota == 0
m float32 = iota + .5 // m float32 = 1 + .5
n // n float32 = 2 + .5
p = 9 // now, iota == 3
q = iota * 2 // q = 4 * 2
_ // _ = 5 * 2
r // r = 6 * 2
s, t = iota, iota // s, t = 7, 7
u, v // u, v = 8, 8
_, w // _, w = 9, 9
)
const x = iota // x = 0
const (
y = iota // y = 0
z // z = 1
)
println(m) // +1.500000e+000
println(n) // +2.500000e+000
println(q, r) // 8 12
println(s, t, u, v, w) // 7 7 8 8 9
println(x, y, z) // 0 0 1
}
The above example is just to demo the rules of
the iota
constant generator feature.
Surely, in practice, we should use it in more meaningful ways.
For example,
const (
Failed = iota - 1 // == -1
Unknown // == 0
Succeeded // == 1
)
const (
Readable = 1 << iota // == 1
Writable // == 2
Executable // == 4
)
Here, the -
symbol is the subtraction operator,
and the <<
symbol is the left-shift operator.
Both of these operators will be introduced in the next article.
Variables are named values. Variables are stored in memory at run time. The value represented by a variable can be modified at run time.
All variables are typed values. When declaring a variable, there must be sufficient information provided for compilers to deduce the type of the variable.
The variables declared within function bodies are called local variables. The variables declared out of any function body are called package-level variables. We also often call package-level variables as global variables.
There are two basic variable declaration forms, the standard one and the short one. The short form can only be used to declare local variables.
Each standard declaration starts with the var
keyword,
which is followed by the declared variable name.
Variable names must be identifiers.
The following are some full standard declaration forms. In these declarations, the types and initial values of the declared variables are all specified.
var lang, website string = "Go", "https://golang.org"
var compiled, dynamic bool = true, false
var announceYear int = 2009
As we have found, multiple variables can be declared together in one variable declaration. Please note, there can be just one type specified in a variable declaration. So the types of the multiple variables declared in the same declaration line must be identical.
Full standard variable declaration forms are seldom used in practice, since they are verbose. In practice, the two standard variable declaration variant forms introduced below are used more often. In the two variants, either the types or the initial values of the declared variables are absent.
The following are some standard variable declarations without specifying variable types. Compilers will deduce the types of the declared variables as the types (or default types) of their respective initial values. The following declarations are equivalent to the above ones in fact. Please note, in the following declarations, the types of the multiple variables declared in the same declaration line can be different.// The types of the lang and dynamic variables
// will be deduced as built-in types "string"
// and "bool" by compilers, respectively.
var lang, dynamic = "Go", false
// The types of the compiled and announceYear
// variables will be deduced as built-in
// types "bool" and "int", respectively.
var compiled, announceYear = true, 2009
// The types of the website variable will be
// deduced as the built-in type "string".
var website = "https://golang.org"
The type deductions in the above example can be viewed as implicit conversions.
The following are some standard declarations without specifying variable initial values. In these declarations, all declared variables are initialized as the zero values of their respective types.// Both are initialized as blank strings.
var lang, website string
// Both are initialized as false.
var interpreted, dynamic bool
// n is initialized as 0.
var n int
Multiple variables can be grouped into one standard form declaration
by using ()
. For example:
var (
lang, bornYear, compiled = "Go", 2007, true
announceAt, releaseAt int = 2009, 2012
createdBy, website string
)
The above example is formatted by using the go fmt
command
provided in Go Toolchain.
In the above example, each of the three lines are enclosed in ()
this is known as variable specification.
Generally, declaring related variables together will make code more readable.
In the above variable declarations, the sign =
means assignment.
Once a variable is declared, we can modify its value by using pure value assignments.
Like variable declarations, multiple values can be assigned in a pure assignment.
The expression items at the left of =
symbol in a pure assignment
are called destination or target values.
They must be addressable values, map index expressions, or the blank identifier.
Value addresses and maps will be introduced in later articles.
Constants are immutable, so a constant can't show up at the left side of a pure assignment as a destination value, it can only appear at the right side as a source value. Variables can be used as both source values and destination values, so they can appear at both sides of pure value assignments.
Blank identifiers can also appear at the left side of pure value assignments as destination values, in which case, it means we ignore the destination values. Blank identifiers can't be used as source values in assignments.
Example:const N = 123
var x int
var y, z float32
N = 9 // error: constant N is not modifiable
y = N // ok: N is deduced as a float32 value
x = y // error: type mismatch
x = N // ok: N is deduced as an int value
y = x // error: type mismatch
z = y // ok
_ = y // ok
x, y = y, x // error: type mismatch
x, y = int(y), float32(x) // ok
z, y = y, z // ok
_, y = y, z // ok
z, _ = y, z // ok
_, _ = y, z // ok
x, y = 69, 1.23 // ok
The code at last line in the above example uses explicit conversions to make the corresponding destination and source values matched. The explicit conversion rules for non-constant numeric values are introduced below.
Go doesn't support assignment chain. For example, the following code is illegal.
var a, b int
a = b = 123 // syntax error
package main
func main() {
// Both lang and year are newly declared.
lang, year := "Go language", 2007
// Only createdBy is a new declared variable.
// The year variable has already been
// declared before, so here its value is just
// modified, or we can say it is redeclared.
year, createdBy := 2009, "Google Research"
// This is a pure assignment.
lang, year = "Go", 2012
print(lang, " is created by ", createdBy)
println(", and released at year", year)
}
Each short variable declaration must declare at least one new variable.
There are several differences between short and standard variable declarations.var
keyword and
variable types must be omitted.
:=
instead of =
.
:=
. But there must be
at least one new variable at the left.
Please note, comparing to pure assignments, there is a limit for short variable declarations.
In a short variable declaration, all items at the left of the
:=
sign must pure identifiers.
This means some other items which can be assigned to,
which will be introduced in other articles,
can't appear at the left of :=
.
These items include qualified identifiers, container elements,
pointer dereferences and struct field selectors.
Pure assignments have no such limit.
Later, when the word "assignment" is mentioned, it may mean a pure assignment, a short variable declaration, or a variable specification with initial values in a standard variable declaration. In fact, a more general definition also includes function argument passing introduced in a follow-up article.
We say x
is assignable to y
if y = x
is a legal statement (compiles okay).
Assume the type of y
is Ty
,
sometimes, for description convenience, we can also say
x
is assignable to type Ty
.
Generally, if x
is assignable to y
,
then y
should be mutable, and the types of x
and
y
are identical or x
can be implicitly converted to
the type of y
.
Surely, y
can also be the blank identifier _
.
Please note, the standard Go compiler and gccgo both don't allow local variables declared but not used. Package-level variables have no such limit.
If a local variable is only ever used as destination values, it will also be viewed as unused.
For example, in the following program, r
is only used as destination.
package main
// Some package-level variables.
var x, y, z = 123, true, "foo"
func main() {
var q, r = 789, false
r, s := true, "bar"
r = y // r is unused.
x = q // q is used.
}
Compiling the above program will result to the following compilation errors
(assume the source file is name example-unused.go
):
./example-unused.go:6:6: r declared and not used
./example-unused.go:7:16: s declared and not used
The fix is easy, we can assign r
and s
to blank identifiers to avoid compilation errors.
package main
var x, y, z = 123, true, "foo"
func main() {
var q, r = 789, false
r, s := true, "bar"
r = y
x = q
_, _ = r, s // make r and s used.
}
Generally, the above fix is not recommended to be used in production code. It should be used in development/debug phase only. It is not a good habit to leave unused local variables in code, for unused local variables have negative effects on both code readability and program execution performance.
For the following example,
var x, y = a+1, 5 // 8 5
var a, b, c = b+1, c+1, y // 7 6 5
the initialization order of the package-level variables are y = 5
,
c = y
, b = c+1
, a = b+1
,
and x = a+1
.
Here, the +
symbol is the addition operator,
which will be introduced in the next article.
Package-level variables can't be depended circularly in their declaration. The following code fails to compile.
var x, y = y, x
In Go, some values are addressable (there is an address to find them). All variables are addressable and all constants are unaddressable. We can learn more about addresses and pointers from the article pointers in Go and learn other addressable and unaddressable values from other articles later.
In Go, two typed values of two different basic types can't be assigned to each other. In other words, the types of the destination and source values in an assignment must be identical if the two values are both basic values. If the type of the source basic value is not same as the type of the destination basic value, then the source value must be explicitly converted to the type of the destination value.
As mentioned above, non-constant integer values can be converted to strings. Here we introduce two more legal non-constant numeric values related conversion cases.Unlike constant number conversions, overflows are allowed in non-constant number conversions. And when converting a non-constant floating-point value to an integer, rounding is also allowed. If a non-constant floating-point value doesn't overflow an integer type, the fraction part of the floating-point value will be discarded (towards zero) when it is converted to the integer type.
In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value, then the conversion succeeds but the result value is implementation-dependent.
In the following example, the intended implicit conversions at line 7 and line 18 both don't work. The explicit conversions at line 5 and line 16 are also disallowed.const a = -1.23
// The type of b is deduced as float64.
var b = a
// error: constant 1.23 truncated to integer.
var x = int32(a)
// error: cannot assign float64 to int32.
var y int32 = b
// okay: z == -1, and the type of z is int32.
// The fraction part of b is discarded.
var z = int32(b)
const k int16 = 255
// The type of n is deduced as int16.
var n = k
// error: constant 256 overflows uint8.
var f = uint8(k + 1)
// error: cannot assign int16 to uint8.
var g uint8 = n + 1
// okay: h == 0, and the type of h is uint8.
// n+1 overflows uint8 and is truncated.
var h = uint8(n + 1)
We can think that value a
at line 3 is implicitly
converted to its default type (float64
), so that the type
of b
is deducted as float64
.
More implicit conversion rules will be introduced in other articles later.
In Go, we can use a pair of {
and }
to form a code block.
A code block can nest other code blocks.
A variable or a named constant declared in an inner code block will shadow
the variables and constants declared with the same name in outer code blocks.
For examples, the following program declares three distinct variables,
all of them are called x
.
An inner x
shadows an outer one.
package main
const y = 789
var x int = 123
func main() {
// The x variable shadows the above declared
// package-level variable x.
var x = true
// A nested code block.
{
// Here, the left x and y are both
// new declared variable. The right
// ones are declared in outer blocks.
x, y := x, y
// In this code block, the just new
// declared x and y shadow the outer
// declared same-name identifiers.
x, z := !x, y/10 // only z is new declared
y /= 100
println(x, y, z) // false 7 78
}
println(x) // true
println(z) // error: z is undefined.
}
The scope (visibility range in code) of a package-level variable
(or a named constant) is the whole package of the variable
(or the named constant) is declared in.
The scope of a local variable (or a named constant) begins at the end of
its declaration and ends at the end of its innermost containing code block.
This is why the last line in the main
function
of the above example doesn't compile.
Code blocks and identifier scopes will be explained in detail in blocks and scopes later.
// 3 untyped named constants. Their bound
// values all overflow their respective
// default types. This is allowed.
const n = 1 << 64 // overflows int
const r = 'a' + 0x7FFFFFFF // overflows rune
const x = 2e+308 // overflows float64
func main() {
_ = n >> 2
_ = r - 0x7FFFFFFF
_ = x / 2
}
But the following code does't compile, for the constants are all typed.
// 3 typed named constants. Their bound
// values are not allowed to overflow their
// respective default types. The 3 lines
// all fail to compile.
const n int = 1 << 64 // overflows int
const r rune = 'a' + 0x7FFFFFFF // overflows rune
const x float64 = 2e+308 // overflows float64
Constant declarations can be viewed as enhanced #define
macros in C.
A constant declaration defines a named constant which represents a literal.
All the occurrences of a named constant will be replaced with the literal
it represents at compile time.
If the two operands of an operator operation are both constants, then the operation will be evaluated at compile time. Please read the next article common operators for details.
package main
const X = 3
const Y = X + X
var a = X
func main() {
b := Y
println(a, b, X, Y)
}
will be viewed as
package main
var a = 3
func main() {
b := 6
println(a, b, 3, 6)
}
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reflect
표준 패키지sync
표준 패키지sync/atomic
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