void Is one of the most common keywords in programming languages , Literally , It is “ Empty 、 An empty set 、 blank ” It means , A return value type most commonly used to represent functions .
There's a definition on Wikipedia ：
The void type, in several programming languages derived from C and Algol68, is the type for the result of a function that returns normally, but does not provide a result value to its caller.
stay C、Algol68 And their derived programming languages ,void A type is a type that a function returns normally , But it doesn't return a value to the caller .
Simply speaking ,void It's a type of （type）, But there is no specific value （value）.
What exactly does this mean ？
With Python For example, several common types of , We can see the law from the contrast ：int Is a type of integer , It has infinite possible integer values ;bool It's a boolean type , It has two possible values （True and False）;NoneType It's a sign of None The type of , It has only one value （None）.
as for void, It's a more abstract special type , But it doesn't contain any values .
After introducing the concept , We can get to the point . The question in the title can be further broken down into two ：
For the first question , We use C/C++ For example , Have a look first void Two usage scenarios of
When void When used in the parameter position of a function , It means that the function does not need to pass parameters .
first C Linguistic f() Indicates that the number of parameters is uncertain , In order to express “ No parameters required ” The semantics of the , So the introduction of f(void) As a limit . Later language （ Include Python） Basically not used in parameters void, It's direct use f() No need to pass the reference .C++ For compatibility C, That's why they support both grammars .
When void When used in front of a function , It means that the function has no return value .
stay C In language , If you do not declare the return type , be f() The function will return the integer value after compilation . To avoid chaos , When you don't need a return value , Just use void f() To limit .
meanwhile , More importantly , It also acts as a place holder , Indicates that the type of a function is known , This is helpful for code readability and compilation .
void As the null return value type of the function , This is used in C++/Java And it's been inherited from . in addition , stay Javascript There are also void The figure of , It's just that it becomes an operator , It plays a completely different role , Not here .
however ,Python From the beginning to the end void keyword .
Why is that? ？ Is it because of Python Isn't there a problem with other languages ？ Still say ,Python Has its own set of solutions ？
Take the two usages related to functions as examples .
When the representation function does not need to pass parameters ,f(void) This kind of writing is superfluous at all , therefore Python It uses the simplest and most straightforward nonparametric writing f().
As for the use of return value types , When we define a function , For example, the simplest def func():pass , To make its call result func() It's a legitimate object , Then it must have a valid type （type）.
This should be a common problem encountered by type based programming languages ,Python No exception .
This is the time , If the function itself is not explicitly return If you have an object , There are two possible solutions ：
Briefly ,Python The design idea is to directly reuse the existing NoneType type , And let the interpreter fill in the missing function types .
There are at least two benefits to this ： One is that there is no introduction of new void Types and keywords ; Second, the programmer does not need to declare the return type before the function , This is consistent with writing with explicit return values .
Just imagine , If Python If the function does not have a return value by default , It might be written as void def func():... Form like this , Then it becomes a special case of function definition . Compared with another special case function , Asynchronous functions asyc def func():... , It could cause chaos .
Overall speaking ,Python It seems that void Empty types are not so necessary to exist , Seems to be NoneType Type is enough , And when the return value is missing , It's extremely convenient to have the interpreter injected uniformly , And that's what we're seeing .
thus , The question of the title of the article is a satisfactory answer .
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