How to use const in go language

In the Go language, the const keyword can be used to define constants, which are used to store data that will not change. The syntax for defining a single constant is "const constant name = constant value", and the syntax for defining multiple constants in batches is "const (Constant name 1 = constant value 1 Constant name 2 = constant value 2...)". Constants are created at compile time, even if defined inside a function, and can only be of type boolean, number (integer, floating point, and complex number) and string.

The operating environment of this tutorial: Windows 7 system, GO version 1.18, Dell G3 computer.

The const keyword is used to define constants in the Go language.

Constants are used to store data that will not change. Constants are created at compile time, even if they are defined inside a function, and can only be Boolean, numeric (integer, floating) dot type and plural type) and string type. Due to compile-time restrictions, the expression defining a constant must be a constant expression that can be evaluated by the compiler.

The definition format of constants is similar to the declaration syntax of variables: const name [type] = value, for example:

const pi = 3.14159 // 相当于 math.Pi 的近似值

In Go language, you can omit the type specifier [type], because The compiler can infer the type of a variable based on its value.

• Explicit type definition: const b string = "abc"

• Implicit type definition: const b = "abc"

The value of a constant must be able to be determined at compile time, and the calculation process can be involved in its assignment expression, but all uses The calculated value must be available at compile time.

• Correct approach:

const c1 = 2/3

• Wrong approach:

const c2 = getNumber() // 引发构建错误: getNumber() 用做值

Same as variable declaration, multiple constants can be declared in batches:

const (
    e  = 2.7182818
    pi = 3.1415926
)

All constant operations can be completed at compile time, which not only reduces runtime work, but also facilitates the compilation and optimization of other codes. When the operands are constants, some runtime errors can also be discovered at compile time, such as integer division by zero, string index out of bounds, any operation that results in an invalid floating point number, etc.

The results of all arithmetic operations, logical operations and comparison operations between constants are also constants. Type conversion operations on constants or the following function calls return constant results: len, cap, real, imag, complex and unsafe .Sizeof.

Because their values ​​are determined at compile time, constants can be part of the type, for example, used to specify the length of an array type:

const IPv4Len = 4
// parseIPv4 解析一个 IPv4 地址 (d.d.d.d).
func parseIPv4(s string) IP {
    var p [IPv4Len]byte
    // ...
}

The declaration of a constant can also contain A type and a value, but if no type is specified explicitly, the type is inferred from the expression on the right. In the following code, time.Duration is a named type, the underlying type is int64, and time.Minute is a constant of the corresponding type. The two constants declared below are of type time.Duration, and the type information can be printed through the %T parameter:

const noDelay time.Duration = 0
const timeout = 5 * time.Minute
fmt.Printf("%T %[1]v\n", noDelay)     // "time.Duration 0"
fmt.Printf("%T %[1]v\n", timeout)     // "time.Duration 5m0s"
fmt.Printf("%T %[1]v\n", time.Minute) // "time.Duration 1m0s"

If they are constants declared in batches, except for the first one, the initialization expressions on the right side of the other constants are It can be omitted. If the initialization expression is omitted, it means using the initialization expression of the previous constant, and the corresponding constant type is also the same. For example:

const (
    a = 1
    b
    c = 2
    d
)
fmt.Println(a, b, c, d) // "1 1 2 2"

[Related recommendations: Go video tutorial, Programming teaching]

The above is the detailed content of How to use const in go language. For more information, please follow other related articles on the PHP Chinese website!