How to pass indefinite parameters in golang

Golang's variable parameters

The variable parameter is a placeholder. You can assign one or more parameters to this placeholder, so no matter The actual number of parameters can be handled by variable parameters. Let's take a look at the declaration of variable parameters:

func Printf(format string, a ...interface{}) (n int, err error) func Println(a ...interface{}) (n int, err error)

Variable parameters use name...Type The formal declaration is in the parameter list of the function, and it needs to be the last parameter in the parameter list. This is similar to other languages;

The variable parameters will be converted to the corresponding []Type type in the function, so we You can get the parameters passed to the function just like when using slice;

One thing worth noting is that golang's variable parameters do not need to force the occurrence of bound parameters.

For example, I want to implement a function sum that sums any integers in C language:

int sum(int num, ...) { // todo }

We can only use it if we specify at least one fixed formal parameter (num) first. ..Variable parameters do not need to be done in golang:

func sum(nums ...int) int { //todo }

This is also one of the embodiments of the concise syntax of golang.

Passing parameters to... variable parameters

There are two forms of passing parameters to functions with variable parameters. The first one is no different from the usual parameter passing. Take An example of sum in this section is:

sum(1, 2, 3) sum(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

It is the same as an ordinary function call except that the number of parameters changes dynamically.

The second form is to use the... operator to pass parameters in the form of a variable..., the variable here must be a slice of the same type as the variable parameter, and cannot be other types (yes ,

Arrays are not allowed either), look at an example:

numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10} sum(numbers...) // 和sum(1, 2, 3, 4, 5, 6, 7, 8, 9. 10)等价

The most commonly used place for this form is in the built-in function append:

result := []int{1, 3} data := []int{5, 7, 9} result = append(result, data...) // result == []int{1, 3, 5, 7, 9}

Is it related to python? The unpacking operation is very similar, yes, in most cases you can treat the... operator as golang's unpacking operation, but there are a few differences that you should pay attention to:

First, you can only use the... operator for the slice type:

arr := [...]int{1, 2, 3, 4, 5} sum(arr...) // 编译无法通过

You will see an error message like this:

cannot use arr (type [5]int) as type []int in argument to sum

This is because the variable parameter is actually a slice,... The operator is syntactic sugar. It copies the previous slice directly to the variable parameter, instead of unpacking it into independent n parameters and then passing them

. This is why I only say... The reason why the .operator looks like unpack.

The second thing to note is that you cannot mix independent parameter passing and... operators. Let’s look at another example:

slice := []int{2, 3, 4, 5} sum(1, slice...) // 无法通过编译

This time you will see a relatively long error message:

too many arguments in call to sum have (number, []int...) want (...int)

This is for the same reason as mentioned before. The... operator directly replaces the variable parameters with slice, which causes the previous independently given parameter to no longer be counted as variable parameters. Within the scope

, the parameter list of the function changes from (...int) to (int, ...int), which ultimately causes the function type mismatch and compilation failure.

The correct approach is also very simple. Do not mix and use... operators to pass parameters to variable parameters.

After reading this article and making some simple connections, I believe you will be able to master the use of golang variable parameters.

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