A const declaration gives names to constants, that is, values that are fixed at compile time. The value of a constant must be a number, string, or boolean.
A constant declaration may specify a type as well as a value, but in the absence of an explicit type, the type is inferred from the expression on the right-hand side.
When a sequence of constants is declared as a group, the right-hand side expression may be omitted for all but the first of the group, implying that the previous expression and its type should be used again. For example:
a = 1
c = 2
fmt.Println(a, b, c, d) // “1 1 2 2”
A const declaration may use the constant generator iota, which is used to create a sequence of related values without spelling out each one explicitly. In a const declaration, the value of iota begins at zero and increments by one for each item in the sequence. For example:
type Weekday int
Sunday Weekday = iota
) This declares Sunday to be 0, Monday to be 1, and so on.
Constants in Go are a bit unusual. Although a constant can have any of the basic data types like int or float64, including named basic types like time.Duration, many constants are not committed to a particular type. The compiler represents these uncommitted constants with much greater numeric precision than values of basic types, and arithmetic on them is more precise than machine arithmetic; you may assume at least 256 bits of precision. There are six flavors of these uncommitted constants, called untyped boolean, untyped integer, untyped rune, untyped floating-point, untyped complex, and untyped string.
Only constants can be untyped. When an untyped constant is assigned to a variable, as in the first statement below, or appears on the right-hand side of a variable declaration with an explicit type, as in the other three statements, the constant is implicitly converted to the type of that variable if possible.
var f float64 = 3 + 0i // untyped complex -> float64
f = 2 // untyped integer -> float64
f = 1e123 // untyped floating-point -> float64
f = ‘a’ // untyped rune -> float64
The statements above are thus equivalent to these:
var f float64 = float64(3 + 0i)
f = float64(2)
f = float64(1e123)
f = float64(‘a’)
Whether implicit or explicit, converting a constant from one type to another requires that the target type can represent the original value. Rounding is allowed for real and complex floating-point numbers:
deadbeef = 0xdeadbeef // untyped int with value 3735928559
a = uint32(deadbeef) // uint32 with value 3735928559
b = float32(deadbeef) // float32 with value 3735928576 (rounded up)
c = float64(deadbeef) // float64 with value 3735928559 (exact)
d = int32(deadbeef) // compile error: constant overflows int32
e = float64(1e309) // compile error: constant overflows float64
f = uint(-1) // compile error: constant underflows uint )
In a variable declaration without an explicit type (including short variable declarations), the flavor of the untyped constant implicitly determines the default type of the variable, as in these examples:
i := 0 // untyped integer; implicit int(0)
r := ‘\000’ // untyped rune; implicit rune(‘\000’)
f := 0.0 // untyped floating-point; implicit float64(0.0)
c := 0i // untyped complex; implicit complex128(0i)
Note the asymmetry: untyped integers are converted to int, whose size is not guaranteed, but untyped floating-point and complex numbers are converted to the explicitly sized types float64 and complex128. The language has no unsized float and complex types analogous to unsized int, because it is very difficult to write correct numerical algorithms without knowing the size of one’s floating-point data types.
To give the variable a different type, we must explicitly convert the untyped constant to the desired type or state the desired type in the variable declaration, as in these examples:
var i = int8(0)
var i int8 = 0
These defaults are particularly important when converting an untyped constant to an interface value since they determine its dynamic type.
fmt.Printf(“%T\n”, 0) // “int”
fmt.Printf(“%T\n”, 0.0) // “float64”
fmt.Printf(“%T\n”, 0i) // “complex128”
fmt.Printf(“%T\n”, ‘\000’) // “int32” (rune)