Conversions [complete list]

Basics

The expression T(x) converts the value x to the type T.

x := 5.1
n := int(x) // convert float to int

The conversion rules are extensive but predictable:

• all conversions between typed expressions must be explicitly stated,
• illegal conversions are caught by the compiler.

Conversions to and from numbers and strings may change the representation and have a run-time cost. All other conversions only change the type but not the representation of x.

Interfaces

To “convert” an interface to a string, struct or map you should use a type assertion or a type switch. A type assertion doesn’t really convert an interface to another data type, but it provides access to an interface’s concrete value, which is typically what you want. Type assertions and type switches

Integers

• When converting to a shorter integer type, the value is truncated to fit in the result type’s size.

• When converting to a longer integer type,

  • if the value is a signed integer, it is sign extended;
  • otherwise it is zero extended.

a := uint16(0x10fe) // 0001 0000 1111 1110
b := int8(a)        //           1111 1110 (truncated to -2)
c := uint16(b)      // 1111 1111 1111 1110 (sign extended to 0xfffe)

Floats

• When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero).
• When converting an integer or floating-point number to a floating-point type, the result value is rounded to the precision specified by the destination type.

var x float64 = 1.9
n := int64(x) // 1
n = int64(-x) // -1

n = 1234567890
y := float32(n) // 1.234568e+09

Warning: In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent. The Go Programming Language Specification: Conversions

Integer to string

• When converting an integer to a string, the value is interpreted as a Unicode code point, and the resulting string will contain the character represented by that code point, encoded in UTF-8.
• If the value does not represent a valid code point (for instance if it’s negative), the result will be "\ufffd", the Unicode replacement character �.

string(97) // "a"
string(-1) // "\ufffd" == "\xef\xbf\xbd"

Use strconv.Itoa to get the decimal string representation of an integer.

strconv.Itoa(97) // "97"

Strings and byte slices

• Converting a slice of bytes to a string type yields a string whose successive bytes are the elements of the slice.
• Converting a value of a string type to a slice of bytes type yields a slice whose successive elements are the bytes of the string.

string([]byte{97, 230, 151, 165}) // "a日"
[]byte("a日")                     // []byte{97, 230, 151, 165}

Strings and rune slices

• Converting a slice of runes to a string type yields a string that is the concatenation of the individual rune values converted to strings.
• Converting a value of a string type to a slice of runes type yields a slice containing the individual Unicode code points of the string.

string([]rune{97, 26085}) // "a日"
[]rune("a日")             // []rune{97, 26085}

Underlying type

A non-constant value can be converted to type T if it has the same underlying type as T.

In this example, the underlying type of int64, T1, and T2 is int64.

type (
	T1 int64
	T2 T1
)

It’s idiomatic in Go to convert the type of an expression to access a specific method.

var n int64 = 12345
fmt.Println(n)                // 12345
fmt.Println(time.Duration(n)) // 12.345µs

(The underlying type of time.Duration is int64, and the time.Duration type has a String method that returns the duration formatted as a time.)

Implicit conversions

The only implicit conversion in Go is when an untyped constant is used in a situation where a type is required.

In this example the untyped literals 1 and 2 are implicitly converted.

var x float64
x = 1 // Same as x = float64(1)

t := 2 * time.Second // Same as t := time.Duration(2) * time.Second

(The implicit conversions are necessary since there is no mixing of numeric types in Go. You can only multiply a time.Duration with another time.Duration.)

When the type can’t be inferred from the context, an untyped constant is converted to a bool, int, float64, complex128, string or rune depending on the syntactical format of the constant.

n := 1   // Same as n := int(1)
x := 1.0 // Same as x := float64(1.0)
s := "A" // Same as s := string("A")
c := 'A' // Same as c := rune('A')

Illegal implicit conversions are caught by the compiler.

var b byte = 256 // Same as var b byte = byte(256)

../main.go:2:6: constant 256 overflows byte

Pointers

The Go compiler does not allow conversions between pointers and integers. Package unsafe implements this functionality under restricted circumstances.

Warning: The built-in package unsafe, known to the compiler, provides facilities for low-level programming including operations that violate the type system. A package using unsafe must be vetted manually for type safety and may not be portable. The Go Programming Language Specification: Package unsafe

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