How to force package users to use struct composite literals with field names?

Package developers can put a non-exported zero-size field in a struct type definition, so that compilers will forbid package users using composite literals with some field items but without field names to create values of the struct type. An example: // foo.go package foo type Config struct { _ [0]int Name string Size int } // main.go package main import "foo" func main() { //_ = foo.Config{[0]int{}, "bar", 123} // error _ = foo.Config{Name: "bar", Size: 123} // compile ok } An example: Please try not to place the zero-size non-exported field as the last field in the struct, for doing so might enlarge the size of the struct type.

How to make a struct type incomparable?

package main type T struct { dummy [0]func() AnotherField int } var x map[T]int // compile error: invalid map key type T func main() { var a, b T _ = a == b // compile error: invalid operation: } Sometimes, we want to avoid a custom struct type being used a map key types, then we can put a field of a non-exported zero-size incomparable type in a struct type to make the struct type incomparable. For example:

Don't use value assignments with expressions interacting with each other.

Currently (Go 1.15), there are some evaluation orders in a multi-value assignment are unspecified when the expressions involved in the multi-value assignment interact with each other. So try to split a multi-value assignment into multiple single value assignments if there are, or you can't make sure whether or not there are, dependencies between the involved expressions. In fact, in some bad-written single-value assignments, there are also expression evaluation order ambiguities. For example, the following program might print [7 0 9] , [0 8 9] , or [7 8 9] , depending on compiler implementations. package main import "fmt" var a = &[]int{1, 2, 3} var i int func f() int { i = 1 a = &[]int{7, 8, 9} return 0 } func main() { // The evaluation order of "a", "i" // and "f()" is unspecified. (*a)[i] = f() fmt.Println(*a) } In fact, in some bad-written single-value assignments, there are also expression evaluation order ambiguities. For example, the following program might print, or, depending on compiler implementations. In other words, a function call in a value assignment may the evaluation results of the non-function-call expressions in the same assignment. Please read evaluation orders in Go for details.

How to simulate for i in 0..N in some other languages?

package main import "fmt" func main() { const N = 5 for i := range [N]struct{}{} { fmt.Println(i) } for i := range [N][0]int{} { fmt.Println(i) } for i := range (*[N]int)(nil) { fmt.Println(i) } } We can range over an array with zero-size element or a nil array pointer to simulate such a loop. For example:

We should reset the pointers in the element slots which are freed up in all kinds of slice manipulations to avoid memory leaking if we can't make sure if the freed-up element slots will be reused later.

Please read how to delete slice elements and kind-of memory leaking caused by not resetting pointers in dead slice elements for details.

Values of some types in standard packages are not expected to be copied.

Values of the bytes.Buffer type, strings.Builder type and the types in the sync standard package are not recommended to be copied. (They really should not be copied, though it is no problems to copy them under some specified circumstances.) The implementation of strings.Builder will detect invalid strings.Builder value copies. Once such a copy is found, panic will occur. For example: package main import "strings" func main() { var b strings.Builder b.WriteString("hello ") var b2 = b b2.WriteString("world!") // panic here } The implementation ofwill detect invalidvalue copies. Once such a copy is found, panic will occur. For example: Copying values of the types in the sync standard package will be warned by the go vet command provided in Go Toolchain. // demo.go package demo import "sync" func f(m sync.Mutex) { // warning m.Lock() defer m.Unlock() // do something ... } $ go vet demo.go ./demo.go:5: f passes lock by value: sync.Mutex Copying values of the types in thestandard package will be warned by thecommand provided in Go Toolchain. Copying bytes.Buffer values will never be detected at run time nor by the go vet command. Just be careful not to do this.

We can use the memclr optimization to reset some contiguous elements in an array or slice.

Please read the memclr optimization for details.

How to check if a value has a method without importing the reflect package?

M(int) string .) package main import "fmt" type A int type B int func (b B) M(x int) string { return fmt.Sprint(b, ": ", x) } func check(v interface{}) bool { _, has := v.(interface{M(int) string}) return has } func main() { var a A = 123 var b B = 789 fmt.Println(check(a)) // false fmt.Println(check(b)) // true } Use the way in the following example. (Assume the prototype of the method needed to be checked is.)

How to efficiently and perfectly clone a slice?

We should use the three-index subslice form at some scenarios.

func NewX(...Option) *X function, and the implementation of this function will merge the input options with some internal default options, then the following implementation is not recommended. func NewX(opts ...Option) *X { options := append(opts, defaultOpts...) // Use the merged options to build and return a X. // ... } Assume a package provides afunction, and the implementation of this function will merge the input options with some internal default options, then the following implementation is not recommended. The reason why the above implementation is not recommended is the append call may modify the underlying Option sequence of the argument opts . For most scenarios, it is not a problem. But for some special scenarios, it may cause some unexpected results. To avoid modifying the underlying Option sequence of the input argument, we should use the following way instead. func NewX(opts ...Option) *X { opts = append(opts[:len(opts):len(opts)], defaultOpts...) // Use the merged options to build and return a X. // ... } To avoid modifying the underlyingsequence of the input argument, we should use the following way instead. On the other hand, for the callers of the NewX function, it is not a good idea to think and rely on the NewX function will not modify the underlying elements of the passed slice arguments, so it is best to pass these arguments with the three-index subslice form. Another scenario at which we should use three-index subslice form is mentioned in this wiki article. One drawback of three-index subslice forms is they are some verbose. In fact, I ever made a proposal to make it less verbose, but it was declined.

Use anonymous functions to make some deferred function calls be executed earlier.

Please read this article for details.

Make sure and show a custom defined type implements a specified interface type.

We can assign a value of the custom defined type to a variable of type of the specified interface type to make sure the custom type implements the specified interface type, and more importantly, to show the custom type is intended to implement which interface types. Sometimes, writing docs in runnable code is much better than in comments. package myreader import "io" type MyReader uint16 func NewMyReader() *MyReader { var mr MyReader return &mr } func (mr *MyReader) Read(data []byte) (int, error) { switch len(data) { default: *mr = MyReader(data[0]) << 8 | MyReader(data[1]) return 2, nil case 2: *mr = MyReader(data[0]) << 8 | MyReader(data[1]) case 1: *mr = MyReader(data[0]) case 0: } return len(data), io.EOF } // Any of the following three lines ensures // type *MyReader implements io.Reader. var _ io.Reader = NewMyReader() var _ io.Reader = (*MyReader)(nil) func _() {_ = io.Reader(nil).(*MyReader)}

Some compile-time assertion tricks.

Besides the above one, there are more compile-time assertion tricks. Several ways to guarantee a constant N is not smaller than another constant M at compile time: // Any of the following lines can guarantee N >= M func _(x []int) {_ = x[N-M]} func _(){_ = []int{N-M: 0}} func _([N-M]int){} var _ [N-M]int const _ uint = N-M type _ [N-M]int // If M and N are guaranteed to be positive integers. var _ uint = N/M - 1 One more way which is stolen from var _ = map[bool]struct{}{false: struct{}{}, N>=M: struct{}{}} The above way looks some verbose but it is more general. It can be used to assert any conditions. It can be less verbose but needs a little more (negligible) memory: var _ = map[bool]int{false: 0, N>=M: 1} Several ways to guarantee a constantis not smaller than another constantat compile time:One more way which is stolen from @lukechampine . It makes use of the rule that duplicate constant keys can't appear in the same composite literal The above way looks some verbose but it is more general. It can be used to assert any conditions. It can be less verbose but needs a little more (negligible) memory: Similarly, ways to assert two integer constants are equal to each other: var _ [N-M]int; var _ [M-N]int type _ [N-M]int; type _ [M-N]int const _, _ uint = N-M, M-N func _([N-M]int, [M-N]int) {} var _ = map[bool]int{false: 0, M==N: 1} var _ = [1]int{M-N: 0} // the only valid index is 0 var _ = [1]int{}[M-N] // the only valid index is 0 var _ [N-M]int = [M-N]int{} Similarly, ways to assert two integer constants are equal to each other: The last line is also inspired by one of Luke Champine's tweets. Ways of how to assert a constant string is not blank: type _ [len(aStringConstant)-1]int var _ = map[bool]int{false: 0, aStringConstant != "": 1} var _ = aStringConstant[:1] var _ = aStringConstant[0] const _ = 1/len(aStringConstant) Ways of how to assert a constant string is not blank: The last line is stolen from Jan Mercl's clever idea. Sometimes, to avoid package-level variables consuming too much memory, we can put assertion code in a function declared with the blank identifier. For example, func _() { var _ = map[bool]int{false: 0, N>=M: 1} var _ [N-M]int } Sometimes, to avoid package-level variables consuming too much memory, we can put assertion code in a function declared with the blank identifier. For example,

How to declare maximum int and uint constants?

const MaxUint = ^uint(0) const MaxInt = int(^uint(0) >> 1)

How to detect native word size at compile time?

const Is64bitArch = ^uint(0) >> 63 == 1 const Is32bitArch = ^uint(0) >> 63 == 0 const WordBits = 32 << (^uint(0) >> 63) // 64 or 32 This tip is Go unrelated.

How to guarantee that the 64-bit value operated by a 64-bit atomic function call is always 64-bit aligned on 32-bit architectures?

Avoid boxing large-size values into interface values.

When a non-interface value is assigned to an interface value, a copy of the non-interface value will be boxed into the interface value. The copy cost depends on the size of the non-interface value. The larger the size, the higher the copy cost. So please try to avoid boxing large-size values into interface values. In the following example, the costs of the latter two print calls are much lower than the former two. package main import "fmt" func main() { var a [1000]int // This cost of the two lines is high. fmt.Println(a) // a is copied fmt.Printf("Type of a: %T

", a) // a is copied // The cost of the two lines is low. fmt.Printf("%v

", a[:]) fmt.Println("Type of a:", fmt.Sprintf("%T", &a)[1:]) } In the following example, the costs of the latter two print calls are much lower than the former two. About value sizes of different types, please read value copy costs in Go.

Optimize Go code by making use of BCE (bounds check elimination).