Compile-Time Assertions in Go (Golang)

The Go language compile-time assertion mechanism is used to enforce specific conditions or constraints at the compilation stage rather than at runtime. If the conditions are not met, the compilation process will fail and report an error, which helps to detect errors as early as possible and ensure that the program satisfies certain invariants or assumptions before execution.

Compile-time assertions are often used for:

• Make sure the data structure size is as expected.
• Verify that the value of a constant or expression is as expected.
• Enforce type constraints or other compile-time checks.

Compile-time assertions in Go language

The Go language itself does not directly support compile-time assertions like some other languages. However, we can achieve similar functionality with some clever techniques. Here are some common methods:

Assert that a constant boolean expression is true (or false) at compile time:

We can take advantage of the following features:

The Go language specification clearly stipulates that constant keys in map/slice/array composite literals cannot be repeated.

For example, the following code ensures that the constant Boolean expression aBoolConst is true. If it is false, the code will not compile.

<code class="language-go">const aBoolConst = true
var _ = map[bool]int{false: 0, aBoolConst: 1}</code>

Assert that the length of the constant string is 15:

<code class="language-go">const STR = "abcdefghij12345"
var _ = map[bool]int{false: 0, len(STR) == 15: 1}</code>

Assert that the constant integer is 15:

<code class="language-go">var _ = [1]int{len(STR) - 15: 0}
//或者
var _ = [1]int{}[len(STR) - 15]</code>

Assert that constant X is not less than constant Y:

<code class="language-go">const _ uint = X - Y
//或者
type _ [X - Y]int</code>

Assert that the constant string is not empty:

<code class="language-go">var _ = aStringConst[0]
//或者
const _ = 1 / len(aStringConst)</code>

Use array size check:

<code class="language-go">import "unsafe"

type MyStruct struct {
    A int64
    B int64
}

// 确保结构体大小为16字节
var _ = [1]int{int(unsafe.Sizeof(MyStruct{}) - 16): 0}</code>

Assert enumeration length:

<code class="language-go">type enumType int

const (
    EnumA enumType = iota
    EnumB
    EnumC
    end
)

var enumDescriptions = [...]string{
    EnumA: "first",
    EnumB: "second",
    EnumC: "third",
}

func (e enumType) String() string {
    if e == end {
        panic("invalid value")
    }
    return enumDescriptions[e]
}

var _ = [1]int{}[len(enumDescriptions) - int(end)]

func _() {
    var x [1]struct{}
    _ = x[EnumA - 0]
    _ = x[EnumB - 1]
    _ = x[EnumC - 2]
}</code>

Use the init function and panic:

While this is not strictly a compile-time assertion, you can use the init function to perform checks that will fail when the program starts (effectively acting as a runtime assertion during initialization):

<code class="language-go">const ExpectedSize = 8

var myInt int64

func init() {
    if unsafe.Sizeof(myInt) != ExpectedSize {
        panic("int size is not 8 bytes")
    }
}</code>

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