confusion about void and what it means. - c++

Question

I am very new to programming and am confused about what void does, I know that when you put void in front of a function it means that "it returns nothing" but if the function returns nothing then what is the point of writing the function?? Anyway, I got this question on my homework and am trying to answer it but need some help with the general concept along with it. any help would be great, and please try to avoid technical lingo, I'm a serious newb here.
What does this function accomplish?
void add2numbers(double a, double b)
{
double sum;
sum = a + b;
}

Answer 1

void ReturnsNothing()
{
cout << "Hello!";
}
As you can see, this function returns nothing, but that doesn't mean the function does nothing.
A function is nothing more than a refactoring of the code to put commonly-used routines together. If I'm printing "Hello" often, I put the code that prints "Hello" in a function. If I'm calculating the sum of two numbers, I'll put the code to do that and return the result in a function. It's all about what you want.

Answer 2

There are loads of reasons to have void functions, some of these are having 'non pure' side effects:
int i=9;
void f() {
++i;
}
In this case i could be global or a class data member.
The other is observable effects
void f() {
std::cout <<"hello world" << std::endl;
}
A void function may act on a reference or pointer value.
void f(int& i) {
++i;
}
It could also throw, although don't do this for flow control.
void f() {
while(is_not_broke()) {
//...
}
throw std::exception(); //it broke
}

Answer 3

The purpose of a void function is to achieve a side effect (e.g., modify a reference parameter or a global variable, perform system calls such as I/O, etc.), not return a value.

Answer 4

The use of the term function in the context of C/C++ is rather confusing, because it disagrees wiht the mathematical concept of a function as "something returning a value". What C/C++ calls functions returning void corresponds to the concept of a procedure in other languages.
The major difference between a function and a procedure is that a function call is an expression, while a procedure call is a statement While functions are invoked for their return value, procedures are invoked for their side effects (such as producing output, changing state, and so on).

Answer 5

A function with void return value can be useful for its side effects. For example consider the standard library function exit:
void exit(int status)
This function doesn't return any value to you, but it's still useful for its side-effect of terminating the process.

Answer 6

You are on the right lines - the function doesn't accomplish anything, because it calculates something but that something then gets thrown away.
Functions returning void can be useful because they can have "side effects". This means something happens that isn't an input or output of the function. For example it could write to a file, or send an email.

Answer 7

Function is a bit of a missnomer in this case; perhaps calling it a method is better. You can call a method on an object to change its state, i.e. the values of it's fields (or properties). So you might have an object with properites for x and y coordinates and a method called Move which takes parameters xDelta and yDelta.
Calling Move with 2, 3 will cause 2 to be added to your X property and 3 to be added to your Y property. So the state of the object has changed and it wouldn't have made musch sense for Move to have returned a value.

Answer 8

That function achieves nothing - but if you had written
void add2numbers(double a, double b, double &sum)
{
sum = a + b;
}
It would give you the sum, whether it's easier to return a value or use a parameter depends on the function
Typically you would use a parameter if there are multiple results but suppose you had a maths routine where an answer might not be possible.
bool sqrt(double value, double &answer)
{
if value < 0.0 ) {
return false;
} else {
answer = real_sqrt_function(value);
return true;
}
}

Answer 9

I currently use a visualization library called VTK. I normally write void functions to update some part of the graphics that are displayed to the screen. I also use void functions to handle GUI interaction within Qt. For example, if you click a button, some text gets updated on the GUI.

Answer 10

You're completely right: calculating a function that returns nothing is meaningless – if you're talking about mathematical functions. But like with many mathematical concepts, "functions" are in many programming languages only related to mathematical functions, but behave more or less subtly different.
I believe it's good to explain it with a language that does not get it wrong: one such language is Haskell. That's a purely functional language which means a Haskell function is also a mathematical function. Indeed you can write Haskell functions much more mathematical-styled, e.g.
my_tan(x) = sin(x)/cos(x) -- or (preferred): tan' x = sin x / cos x
than in C++
double my_tan(double x) { return sin(x)/cos(x); }
However, in computer programs you don't just want to calculate functions, do you? You also want to get stuff done, like displaying something on your screen, sending data over the network, reading values from sensors etc.. In Haskell, things like these are well separated from pure functions, they all act in the so-called IO monad. For instance, the function putStrLn, which prints a line of characters, has type String -> IO(). Meaning, it takes a String as its argument and returns an IO action which prints out that string when invoked from the main function, and nothing else (the () parens are roughly what's void in C++).
This way of doing IO has many benefits, but most programming languages are more sloppy: they allow all functions to do IO, and also to change the internal state of your program. So in C++, you could simply have a function void putStrLn(std::string), which also "returns" an IO action that prints the string and nothing else, but does not explicitly tell you so. The benefit is that you don't need to tie multiple knots in your brain when thinking about what the IO monad actually is (it's rather roundabout). Also, many algorithms can be implemented to run faster if you have the ability to actually tell the machine "do this sequence of processes, now!" rather than just asking for the result of some computation in the IO monad.