"variable or field declared void"

Can somebody explain the point of the compiler's message:"variable or field `a' declared void "?
I'm not alone. If you put this message without `a' into the google's browser, you will get 10 100 000 results.
I'm beyond the first steps in c++. At least, I thought so far. I must admit that I don't understand the datatype void at all.
Here's a simply, short code which doesn't work due to use of datatype void:

#include<iostream>
using namespace std;

int main(){

void a, b;

cout<<"a = ";cin>>a; cout<<"\n\tb = ";cin>>b;

cin.get();

}

To be exact, yet the compilation was failed.

If I'm not mistaken, you simply can't declare variables of "type" void. That is, void is not a type, it means lack of type (e.g where a function doesn't return anything).

If you've seen it in a function "argument" declaration:

Code:

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int foo(void);

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then again, it doesn't mean the function takes one argument of void type. It means the function doesn't take any arguments, and in C++ it has exactly the same meaning as

Code:

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int foo();

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Thanks. But I have already have some expierence in another a bit higher level language, namely Visual Basic 6.0 for application (Excel-but it doesn't matter). So it is sure that this confuses me. You know, in that language void is a datatype which cover somehow almost the same concept.
But there it works like a jolyjoker you can use this key-word as an implicit typecaster. When you edit your code and don't know what type of data you will put in the variable you use it simply.

So you say that this key-word has no such role in c-language that I detailed above. If I make out also this sense of your message, I don't make a mistake, do I?

Yes, void cannot be used in that way. However, when it comes to pointers, a pointer to void can be used in that way when dealing with pointers to objects.

Thanks a lot. It was a more concrete and I will visit the recommended links.

you can think of variables in C++ as balls on a pool table. solid-colored balls are pointers, and stripes are objects. every solid has a corresponding stripe of the same color, except the 8-ball, just like you can instantiate an object of any type you can create a pointer to, except a void pointer. there is no void object type.

That's a terrble analogy Elkvis.

You can't declare a void variable because it occupies no space. It would be like building a bridge out of a vacuum.

I would not suggest using void* either, however. If you need a function that takes a generic type, you should look into templates.

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Originally Posted by Elysia

I would not suggest using void* either, however. If you need a function that takes a generic type, you should look into templates.

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Unless size is a consideration or your platform doesn't support them...

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Originally Posted by jeffcobb

Unless size is a consideration or your platform doesn't support them...

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Or your building an API (no templates for APIs). Threading is another place where void*'s are pretty much unavoidable.

Void is a type. You can instantiate templates with it. You just can't have variables of (cv-qualified) void or void &.

Void pointer is special because any non-function pointer type is implicitly convertible to (and in C form) void * (as long cv correctness is preserved). This makes it a good generic pointer.

Quote:

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Originally Posted by Cogman

Or your building an API (no templates for APIs).

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That depends on what exactly you mean by an API: the standard generic algorithms can be said to form an API, and they certainly involve templates.

Quote:

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Originally Posted by Cogman

Threading is another place where void*'s are pretty much unavoidable.

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Take a look at Boost.Thread, which is currently in the process of being adapted to be part of the C++ standard library.

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Originally Posted by iMalc

That's a terrble analogy Elkvis.

You can't declare a void variable because it occupies no space. It would be like building a bridge out of a vacuum.

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oh come on, the analogy wasn't THAT bad :)

Quote:

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Or your building an API (no templates for APIs).

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And/or you are using DLLs which generates warning C4251 when you attempt to export a template from a DLL. Templates are fine as long as they are internalized but exposing them to the outside world from an API introduces an entire plethora of issues.