void in C++

[GywGod133]

Hello Hivers

I have a project in C++ (Collage)

can you tell me what use of this void

I dunno how to use this..

can you make me a example for this

Thank You!

 

[IcemanBo]

Where exactly?

void myFunc(int i) --> no returnType for your function, int i is parameter

int myFunc(void) --> int as returnType, no parameters needed ... but "(void)" can also just become "()"

void myFunc(void) / void myFunc() --> no returnType, no parameters

edit:

When you declare a pointer you usually define the type of the pointer. If you just write "void *pt;" the type will be generic.

 

[edo494]

void in C++ and in C means "nothing". It can be used as return type, or as unnamed parameter.

void myFunc(void)
{
...
}

basically this function tells you that it doesnt accept anything as argument and does not return anything.

You can replace void with nothing in argument signature, but this only works in C++, because in C that means the function takes any number of any arguments. So this is the same as above(in C++)

void myFunc()
{
...
}

 

[GywGod133]

Do nothing?

so, void main() is nothing?

 

[chobibo]

no, it means function main returns no value.

int main(){
 // do something here
 return 0;
}

void main(){
 // do something here
}

 

[GywGod133]

so void is a variable,

 

[chobibo]

it's a keyword, just like int, float, bool etc.

 

[IcemanBo]

so void is a variable,

What makes you think now it is a variable?

It's not a variable. It's like an indicator for a non-type.

int myFunc()	-->	returns an int type
void myFunc()	-->	returns nothing (void just indicates here that no data type will be returned in the function)

Like in chobibo's example before.

 

[GywGod133]

oh okay....

It means you can do the function int main() same in void main()

for example

int main()
{
cout<<"Hello";
}


void main()
{
cout<<"Hello";
}

 

[edo494]

void main is not legal C nor C++, and if your compiler allows compilation of void main, you should considering upgrading to something that is at least standard compilant

 

[LordDz]

void main(void) //All C++ projects need a main function
{
function1();
}

void function1() //A function that doesn't return anything when called
{
cout<<"Hello!\n";
cout<<function2();
}

int function2() //Returns an int when called
{
return 2;
}

Output:
Hello!
2

 

[Dr Super Good]

void means an absence of type.

returning a void type means the function does not return anything.

a void pointer means a pointer to no defined type. Although void pointers themselves are valid types (they correspond to a memory address) they cannot be dereferenced due to void having no type information. They also cannot be manipulated like normal pointers as the compiler has no size information for the void type (cannot commute byte offset).

no, it means function main returns no value.

Incorrect. The main function of a program should return an integer which corresponds to the application exit status. It should also take an array of character pointers and a argument count. The arc count and argument array can optionally be left out, which is usually the case for embedded software since it becomes the entry point of the processor so those arguments are not useful.

int main(int argc, char *argv[]);

Next to as a return type, void is mostly used for generic pointers. This is best seen in standard C for generic data structures like lists where void pointers to elements are commonly used. When getting the element you get a void pointer out of the data structure interface and then type cast it to the appropriate type (which you should know based on what you put into the data structure). In C++ void pointers see considerably less use as virtual methods and templates should be used instead as they are considerably more type safe and flexible.

 

[edo494]

void main(void) //All C++ projects need a main function
{
function1();
}

void function1() //A function that doesn't return anything when called
{
cout<<"Hello!\n";
cout<<function2();
}

int function2() //Returns an int when called
{
return 2;
}

Output:
Hello!
2

http://ideone.com/6StORy

that summed it up.

And I dont give a shit about the order of declaration in your code, but the first word is already wrong

 

[-Kobas-]

void main(void) //All C++ projects need a main function
{
function1();
}

void function1() //A function that doesn't return anything when called
{
cout<<"Hello!\n";
cout<<function2();
}

int function2() //Returns an int when called
{
return 2;
}

Output:
Hello!
2

You should always stick with int main(){return 0;}.
Some compilers require that you explicitly define main as such (GNU GCC Compiler - Code::Blocks IDE).
At the other hand some compilers will allow you to define main as void function, but still in background use it as int (Visual Studio Compiler - Visual Studio 2013 IDE, for example).

Another reason why I mention those two is that for example, Visual Studio will terminate your program asap main return value, because, you actually write that, right. At the other hand Code::Blocks will insert system("pause") command just before return, as well as do few other things, but that's story for itself.

Finally to use those functions (function1, function2) you need to declare them above main, and ofc. #include <iostream> using namespace std;.

EDIT:

Oh, another thing, it's not true that all C++ projects require a main function.
If you want to create Dynamic-Link Library (well DLL file) and link your core C++ code with C# project it won't have main at all.

 

[edo494]

to quote standard:

An implementation shall not predefine the main function. This function shall not be overloaded. It shall
have a return type of type int, but otherwise its type is implementation-defined. All implementations shall
allow both
— a function of () returning int and
— a function of (int, pointer to pointer to char) returning int

source: N3690 standard draft §3.6.1.2

it is required that main returns int

 

[Dr Super Good]

All Windows, MAC or Linux programs should take command line arguments.

 

[edo494]

you cant find the standard and no, it is fully optional

 

[LordDz]

http://ideone.com/6StORy

that summed it up.

And I dont give a shit about the order of declaration in your code, but the first word is already wrong

Yeah, it was a long time since I did C++ sorry

 

[gorillabull]

i think for the purpose that you are asking its just a function which does not require you to write
return something;
at any point in it.

return type of a function is always specified at the start: int main, float g() etc..
these two have to have a return type int and float whereas void does not void functions are usually used to just perform an action (like displaying something) and not to modify data.

 

[edo494]

not void functions are usually used to just perform an action (like displaying something) and not to modify data.

This is fully opinion and design based. In fact, most functions that change state of instance of given member usually dont return anything, why would they after all

 

[Kanadaj]

Well... If you guys haven't noticed yet, the OP is more or less clueless in programming - why do you start throwing all the unnecessary stuff at him?

To answer the OP, think of a function as a mathematical function machine:

Now take a function in C++:

int Add(int i, int j)

The input of the machine thus are two integers: i and j, both of them being integers.

The machine will do something - here it's probably:

return i + j;

However, you need to specify beforehand what comes out of the machine. In this case, you set the type of the output as an integer. Why do you need this?

Because in case of a more complicated function that's not even written by you, you won't know what's inside the machine. It may return an integer, or maybe a vector? The point is, to use such a function, you need to know what you'll receive when calling it.

Now after this, void means that there is no output. You just call the function, give it some input values; it does something with it, and that's it. That something might be updating a database or uploading data, or maybe just increasing your input values.

Now having said this, there are functions where you have to use what you're told: i.e. Main() should always return an integer.