Does void have a value ?

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obdevel
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#1
Posted by obdevel: Wed. May 6, 2020 - 09:24 PM
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I'm discussing something elsewhere and looking for a reference to how void is resolved to a true/false value. e.g.

 

void func1(void) {
  ...
}

void main(void) {
  while (func1) {
    ...
  }
}

avr-g++ compiles it but warns:

/Users/xxx/Documents/Arduino/temp1/temp1.ino:24:10: warning: the address of 'void display()' will never be NULL [-Waddress]
   while (display) {
          ^~~~~~~

Running the program, the while loop never exits, so I assume void resolves to a non-zero value i.e. it's 'true' because it's 'not false'. But I can't find a reference to this, and my google skills are failing me.

 

This topic has a solution.

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Tools, Compilers and General Programming, solved
Last Edited: Wed. May 6, 2020 - 09:31 PM
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meolsen
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Posted by meolsen: Wed. May 6, 2020 - 09:28 PM
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That is not void, that's the address of the function display...

 

void would be the return of the function, i.e display()

 

Changing your code, I get this error:

error: void value not ignored as it ought to be

:: Morten

 

(yes, I work for Microchip, yes, I do this in my spare time, now stop sending PMs)

 

The postings on this site are my own and do not represent Microchip’s positions, strategies, or opinions.

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meolsen
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This reply has been marked as the solution. 
Posted by meolsen: Wed. May 6, 2020 - 09:29 PM
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And to answer your question, void does not have a value, so it does not have a boolean value...

:: Morten

 

(yes, I work for Microchip, yes, I do this in my spare time, now stop sending PMs)

 

The postings on this site are my own and do not represent Microchip’s positions, strategies, or opinions.

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obdevel
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Posted by obdevel: Wed. May 6, 2020 - 09:31 PM (Reply to #2)
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Thanks. My typo frown Or rather, blithely typing in someone else's code.

 

g++ now says:

/Users/xxx/Documents/Arduino/temp1/temp1.ino: In function 'void setup()':
temp1:24:17: error: could not convert 'display()' from 'void' to 'bool'
   while (display()) {
          ~~~~~~~^~
exit status 1
could not convert 'display()' from 'void' to 'bool'
 

Last Edited: Wed. May 6, 2020 - 09:37 PM
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Gold Markclawson
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Posted by clawson: Wed. May 6, 2020 - 09:31 PM
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As Morten says "func1" (or "display") is the address of the function so is not 0 (and not 'void'). I guess you mean while(display())? But now you are testing the return from a function that is defined as not returning a value which does not make much sense.

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obdevel
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Posted by obdevel: Wed. May 6, 2020 - 09:38 PM (Reply to #5)
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Yes, I'm guilty of blithely pasting in someone else's code, rather than reading it properly.

 

Thanks.

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El Tangas
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Posted by El Tangas: Wed. May 6, 2020 - 11:28 PM
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Next you're gonna ask what is sizeof(void) cheeky

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barnacle
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Posted by barnacle: Thu. May 7, 2020 - 06:59 AM
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Well according to King James, it's large enough to contain waters...

 

<g,d,r>

 

Neil

Neil Barnes
www.nailed-barnacle.co.uk

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N.Winterbottom
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Posted by N.Winterbottom: Thu. May 7, 2020 - 09:17 AM
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obdevel wrote:

Yes, I'm guilty of blithely pasting in someone else's code, rather than reading it properly.

I wouldn't listen to anything this "someone else" has to say about programming if I were you. Two pretty serious errors in as many lines, that's quite an achievement.

 

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Gold Markclawson
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Posted by clawson: Thu. May 7, 2020 - 09:29 AM
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BTW when I try:

void foo() {}

int main(void) {
	while(foo());
}

then you cannot even build this:

		.././main.c: In function 'main':
D:\test\test\main.c(6,8): error: void value not ignored as it ought to be
		  while(foo());
		        ^
		make: *** [main.o] Error 1

So how a void return is interpreted is kind of a moot question as you just cannot do it anyway.

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