The Go language has built-in functions that you can use without importing. They can sometimes be used for different types of operations, such as len, cap, and append, or they must be used for system level operations, such as panic. Therefore, they need direct compiler support.
Download address of go1.13.7:
https://golang.org/dl/
There are 15 built-in functions defined in the source file builtin.go. Let's take a look at the builtin.go file
// Copyright 2011 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. /* Package builtin provides documentation for Go's predeclared identifiers. The items documented here are not actually in package builtin but their descriptions here allow godoc to present documentation for the language's special identifiers. */ package builtin // bool is the set of boolean values, true and false. type bool bool // true and false are the two untyped boolean values. const ( true = 0 == 0 // Untyped bool. false = 0 != 0 // Untyped bool. ) // uint8 is the set of all unsigned 8-bit integers. // Range: 0 through 255. type uint8 uint8 // uint16 is the set of all unsigned 16-bit integers. // Range: 0 through 65535. type uint16 uint16 // uint32 is the set of all unsigned 32-bit integers. // Range: 0 through 4294967295. type uint32 uint32 // uint64 is the set of all unsigned 64-bit integers. // Range: 0 through 18446744073709551615. type uint64 uint64 // int8 is the set of all signed 8-bit integers. // Range: -128 through 127. type int8 int8 // int16 is the set of all signed 16-bit integers. // Range: -32768 through 32767. type int16 int16 // int32 is the set of all signed 32-bit integers. // Range: -2147483648 through 2147483647. type int32 int32 // int64 is the set of all signed 64-bit integers. // Range: -9223372036854775808 through 9223372036854775807. type int64 int64 // float32 is the set of all IEEE-754 32-bit floating-point numbers. type float32 float32 // float64 is the set of all IEEE-754 64-bit floating-point numbers. type float64 float64 // complex64 is the set of all complex numbers with float32 real and // imaginary parts. type complex64 complex64 // complex128 is the set of all complex numbers with float64 real and // imaginary parts. type complex128 complex128 // string is the set of all strings of 8-bit bytes, conventionally but not // necessarily representing UTF-8-encoded text. A string may be empty, but // not nil. Values of string type are immutable. type string string // int is a signed integer type that is at least 32 bits in size. It is a // distinct type, however, and not an alias for, say, int32. type int int // uint is an unsigned integer type that is at least 32 bits in size. It is a // distinct type, however, and not an alias for, say, uint32. type uint uint // uintptr is an integer type that is large enough to hold the bit pattern of // any pointer. type uintptr uintptr // byte is an alias for uint8 and is equivalent to uint8 in all ways. It is // used, by convention, to distinguish byte values from 8-bit unsigned // integer values. type byte = uint8 // rune is an alias for int32 and is equivalent to int32 in all ways. It is // used, by convention, to distinguish character values from integer values. type rune = int32 // iota is a predeclared identifier representing the untyped integer ordinal // number of the current const specification in a (usually parenthesized) // const declaration. It is zero-indexed. const iota = 0 // Untyped int. // nil is a predeclared identifier representing the zero value for a // pointer, channel, func, interface, map, or slice type. var nil Type // Type must be a pointer, channel, func, interface, map, or slice type // Type is here for the purposes of documentation only. It is a stand-in // for any Go type, but represents the same type for any given function // invocation. type Type int // Type1 is here for the purposes of documentation only. It is a stand-in // for any Go type, but represents the same type for any given function // invocation. type Type1 int // IntegerType is here for the purposes of documentation only. It is a stand-in // for any integer type: int, uint, int8 etc. type IntegerType int // FloatType is here for the purposes of documentation only. It is a stand-in // for either float type: float32 or float64. type FloatType float32 // ComplexType is here for the purposes of documentation only. It is a // stand-in for either complex type: complex64 or complex128. type ComplexType complex64 // The append built-in function appends elements to the end of a slice. If // it has sufficient capacity, the destination is resliced to accommodate the // new elements. If it does not, a new underlying array will be allocated. // Append returns the updated slice. It is therefore necessary to store the // result of append, often in the variable holding the slice itself: // slice = append(slice, elem1, elem2) // slice = append(slice, anotherSlice...) // As a special case, it is legal to append a string to a byte slice, like this: // slice = append([]byte("hello "), "world"...) func append(slice []Type, elems ...Type) []Type // The copy built-in function copies elements from a source slice into a // destination slice. (As a special case, it also will copy bytes from a // string to a slice of bytes.) The source and destination may overlap. Copy // returns the number of elements copied, which will be the minimum of // len(src) and len(dst). func copy(dst, src []Type) int // The delete built-in function deletes the element with the specified key // (m[key]) from the map. If m is nil or there is no such element, delete // is a no-op. func delete(m map[Type]Type1, key Type) // The len built-in function returns the length of v, according to its type: // Array: the number of elements in v. // Pointer to array: the number of elements in *v (even if v is nil). // Slice, or map: the number of elements in v; if v is nil, len(v) is zero. // String: the number of bytes in v. // Channel: the number of elements queued (unread) in the channel buffer; // if v is nil, len(v) is zero. // For some arguments, such as a string literal or a simple array expression, the // result can be a constant. See the Go language specification's "Length and // capacity" section for details. func len(v Type) int // The cap built-in function returns the capacity of v, according to its type: // Array: the number of elements in v (same as len(v)). // Pointer to array: the number of elements in *v (same as len(v)). // Slice: the maximum length the slice can reach when resliced; // if v is nil, cap(v) is zero. // Channel: the channel buffer capacity, in units of elements; // if v is nil, cap(v) is zero. // For some arguments, such as a simple array expression, the result can be a // constant. See the Go language specification's "Length and capacity" section for // details. func cap(v Type) int // The make built-in function allocates and initializes an object of type // slice, map, or chan (only). Like new, the first argument is a type, not a // value. Unlike new, make's return type is the same as the type of its // argument, not a pointer to it. The specification of the result depends on // the type: // Slice: The size specifies the length. The capacity of the slice is // equal to its length. A second integer argument may be provided to // specify a different capacity; it must be no smaller than the // length. For example, make([]int, 0, 10) allocates an underlying array // of size 10 and returns a slice of length 0 and capacity 10 that is // backed by this underlying array. // Map: An empty map is allocated with enough space to hold the // specified number of elements. The size may be omitted, in which case // a small starting size is allocated. // Channel: The channel's buffer is initialized with the specified // buffer capacity. If zero, or the size is omitted, the channel is // unbuffered. func make(t Type, size ...IntegerType) Type // The new built-in function allocates memory. The first argument is a type, // not a value, and the value returned is a pointer to a newly // allocated zero value of that type. func new(Type) *Type // The complex built-in function constructs a complex value from two // floating-point values. The real and imaginary parts must be of the same // size, either float32 or float64 (or assignable to them), and the return // value will be the corresponding complex type (complex64 for float32, // complex128 for float64). func complex(r, i FloatType) ComplexType // The real built-in function returns the real part of the complex number c. // The return value will be floating point type corresponding to the type of c. func real(c ComplexType) FloatType // The imag built-in function returns the imaginary part of the complex // number c. The return value will be floating point type corresponding to // the type of c. func imag(c ComplexType) FloatType // The close built-in function closes a channel, which must be either // bidirectional or send-only. It should be executed only by the sender, // never the receiver, and has the effect of shutting down the channel after // the last sent value is received. After the last value has been received // from a closed channel c, any receive from c will succeed without // blocking, returning the zero value for the channel element. The form // x, ok := <-c // will also set ok to false for a closed channel. func close(c chan<- Type) // The panic built-in function stops normal execution of the current // goroutine. When a function F calls panic, normal execution of F stops // immediately. Any functions whose execution was deferred by F are run in // the usual way, and then F returns to its caller. To the caller G, the // invocation of F then behaves like a call to panic, terminating G's // execution and running any deferred functions. This continues until all // functions in the executing goroutine have stopped, in reverse order. At // that point, the program is terminated with a non-zero exit code. This // termination sequence is called panicking and can be controlled by the // built-in function recover. func panic(v interface{}) // The recover built-in function allows a program to manage behavior of a // panicking goroutine. Executing a call to recover inside a deferred // function (but not any function called by it) stops the panicking sequence // by restoring normal execution and retrieves the error value passed to the // call of panic. If recover is called outside the deferred function it will // not stop a panicking sequence. In this case, or when the goroutine is not // panicking, or if the argument supplied to panic was nil, recover returns // nil. Thus the return value from recover reports whether the goroutine is // panicking. func recover() interface{} // The print built-in function formats its arguments in an // implementation-specific way and writes the result to standard error. // Print is useful for bootstrapping and debugging; it is not guaranteed // to stay in the language. func print(args ...Type) // The println built-in function formats its arguments in an // implementation-specific way and writes the result to standard error. // Spaces are always added between arguments and a newline is appended. // Println is useful for bootstrapping and debugging; it is not guaranteed // to stay in the language. func println(args ...Type) // The error built-in interface type is the conventional interface for // representing an error condition, with the nil value representing no error. type error interface { Error() string }
append
The append built-in function appends the element to the end of the slice. If it has enough capacity, reallocate the destination to accommodate the new elements. If not, a new underlying array is assigned.
func append(slice []Type, elems ...Type) []Type
append usage:
1,slice = append(slice, elem1, elem2) 2,slice = append(slice, anotherSlice...)
Example:
package main import ( "fmt" ) func main() { var a []string b := append(a, "a") fmt.Println(b) c := append(b, "b", "c", "d", "e") fmt.Println(c) x := []int {1,2,3} y := []int {4,5,6} fmt.Println(append(x,4,5,6)) fmt.Println(append(x,y...)); }
Compilation result:
In the first usage, the first parameter is slice, and multiple parameters can be added later. The name of the second slice is followed by three points. In this case, append only supports two parameters and does not support any number of parameters.
copy
The copy built-in function copies elements from the source slice to the target slice.
func copy(dst, src []Type) int
Used to copy the data of the source slice (second parameter) to the target slice (first parameter).
Example:
package main import ( "fmt" ) func main() { var a = []int{0, 1, 2, 3, 4, 5, 6, 7} var s = make([]int, 6) //Source length is 8, target is 6, only the first 6 will be copied n1 := copy(s, a) fmt.Println("s - ", s) fmt.Println("n1 - ", n1) //Source length 7, target 6, replication index 1 to 6 n2 := copy(s, a[1:]) fmt.Println("s - ", s) fmt.Println("n2 - ", n2) }
Compilation result:
delete
The delete built-in function removes an element from the map with the specified key / / (mfkeyl). If m is nil, or there is no such element, delete it.
func delete(m map[Type]Type1, key Type)
Example:
package main import ( "fmt" ) func main() { map1 := make(map[string]int) map1["one"] = 100 map1["two"] = 200 map1["three"] = 300 map1["four"] = 400 fmt.Println(map1, len(map1)) delete(map1, "two") fmt.Println(map1, len(map1)) }
Compilation result:
len
func len(v Type) int
If v is an array: returns the number of elements of the array.
If V is a pointer to an array: returns the number of elements of * v.
If v is slice or map: returns the number of elements of v.
Example:
package main import "fmt" func main() { var arr1 [5]int for i:=0; i < len(arr1); i++ { arr1[i] = i * 2 } for i:=0; i < len(arr1); i++ { fmt.Printf("Array at index %d is %d\n", i, arr1[i]) } }
Compilation result:
cap
func cap(v Type) int
Array: returns the number of elements of the array, the same as len(v)
Pointer to array: the number of elements returned is * v, the same as len(v)
Slice: returns the maximum slice capacity, > = len (V)
package main import "fmt" func main() { var slice1 []int = make([]int, 10) for i := 0; i < len(slice1); i++ { slice1[i] = 5 * i } for i := 0; i < len(slice1); i++ { fmt.Printf("Slice at %d is %d\n", i, slice1[i]) } fmt.Printf("\nThe length of slice1 is %d\n", len(slice1)) fmt.Printf("The capacity of slice1 is %d\n", cap(slice1)) }
Compilation result:
make
func make(t Type, size ...IntegerType) Type
The make built-in function allocates and initializes objects of type slice, map, or Chan (only). Like new, the first parameter is a type, not a value. Unlike new, make has the same return type as its parameter, not a pointer to it.
Example:
package main import "fmt" func main() { slice1 := make([]int, 0, 10) for i := 0; i < cap(slice1); i++ { slice1 = slice1[0:i+1] slice1[i] = i fmt.Printf("The length of slice is %d\n", len(slice1)) } // Print slice: for i := 0; i < len(slice1); i++ { fmt.Printf("Slice at %d is %d\n", i, slice1[i]) } }
Compilation result:
new
func new(Type) *Type
The new built-in function allocates memory. The first parameter is a type, not a value, and the value returned is a pointer to the newly assigned zero value of that type.
Example:
package main import ( "fmt" ) func main() { i := new(int) s := new(string) arr := new([5]string) slice := new([]string) mp := new(map[string]string) ch := new(chan int) fmt.Printf("i type is %T\n", i) fmt.Printf("s type is %T,len is %v\n", s, len(*s)) fmt.Printf("arr type is %T,len is %v\n", arr, len(*arr)) fmt.Printf("slice type is %T,len is %v\n", slice, len(*slice)) fmt.Printf("mp type is %T,len is %v\n", mp, len(*mp)) fmt.Printf("ch type is %T,len is %v\n", ch, len(*ch)) }
Compilation result:
complex，real，imag
func complex(r, i FloatType) ComplexType
func real(c ComplexType) FloatType
func imag(c ComplexType) FloatType
Use the built-in complex function to build complex numbers, and use real and imag functions to return the real and virtual parts of complex numbers:
Example:
package main import ( "fmt" ) func main() { var x complex128 = complex(1, 2) // 1+2i var y complex128 = complex(3, 4) // 3+4i fmt.Println(x*y) // "(-5+10i)" fmt.Println(real(x*y)) // "-5" fmt.Println(imag(x*y)) // "10" }
Compilation result:
close
func close(c chan<- Type)
The close built-in function closes a channel that must be bidirectional or send only. It should only be performed by the sender, not the receiver, and the channel should be closed after receiving the last sent value.
panic,recover
func panic(v interface{})
If a panic statement is written in function F, the code to be executed after it will be terminated. If there is a list of defer functions to be executed in function F where panic is located, execute them in reverse order.
Returns the caller g of function F. in G, the code after calling function f statement will not execute. If there is a list of defer functions to execute in function g, execute them in reverse order of defer. Here, defer is similar to finally in try catch finally.
func recover() interface{}
Restoring built-in functionality allows programs to manage the behavior of panicky goroutine. Executing a function called to recover a delay (but not any function called by it) will stop the panic sequence y and resume normal execution, and retrieve the error value passed to all panic. If recover is called outside of the deferred function, it will not stop a panicky sequence.
Simply put: a panic exception can be thrown in go, and then it can be caught by recover in defer, and then handled normally.
Example:
package main import ( "fmt" ) func main() { fmt.Println("c") defer func() { // defer must be declared first, otherwise panic exception cannot be caught fmt.Println("d") if err := recover(); err != nil { fmt.Println(err) // err here is actually the incoming content of panic } fmt.Println("e") }() f() //Start calling f fmt.Println("f") //From here on, the following code will not be executed } func f() { fmt.Println("a") panic("Abnormal information") fmt.Println("b") //From here on, the following code will not be executed }
Compilation result:
Note: when using recover to process the panic instruction, defer must be declared before the panic, otherwise recover cannot capture the panic.
print,println
func print(args ...Type)
func println(args ...Type)
Reference resources:
Get started guide
go1.13.7 source code builtin.go
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