C/C++ Programming Questions Not Worth Their Own Thread

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Dijkstracula: Mar 18, 2003; You can't spell 'vector field' without me, Professor!

Zopotantor posted:

This has the same problem as including a semicolon at the end of a statement-like macro: users can leave off the semicolon when calling it. AND THEY WILL

Thanks for playing along :3:

In this particular case, despite TRACE() appearing like a function call, users would have to leave off the semicolon for the macro-expanded code to be syntactically valid! With the user's "statement"-terminating semicolon so reasonably added, you'd end up with something that looks like:

C code:

if (1 < 2)
  { ... } ;
else 
  { ... } ;

This is why the idiom, weird as it is, has to be this way: we need a way to splat out the contents of the macro in a new block scope but also wrapped in something that looks like an expression statement. And the do-while construct is the only way that we can do it!

The definition of an iteration statement in the C language standard posted:

* while ( expression ) statement * do statement while ( expression ) ; <-- hay guys * for ( expressionopt ; expressionopt ; expressionopt ) statement * for ( declaration expressionopt ; expressionopt ) statement

Notice that a do-while loop is terminated with the semicolon but no others are - this is why, if anyone was wondering if we could write the macro with a nominally-equivalent single-iteration for-loop wrapping the new scope, as in:

C code:

    5 #define WARN(msg)  for (int i = 0; i < 1; i++) { if (VERBOSITY > 10)  fprintf(stderr, "WARN:  " msg "\n"); }
    6 #define DEBUG(msg) for (int i = 0; i < 1; i++) { if (VERBOSITY > 30)  fprintf(stderr, "DEBUG: " msg "\n"); }
    7 #define TRACE(msg) for (int i = 0; i < 1; i++) { if (VERBOSITY > 50)  fprintf(stderr, "TRACE: " msg "\n"); }

You would expect from the language grammar that this would have the same terminating semicolon issue!

hoo boy language lawyering hyperfocus is a trip lemeetellya

# ? Nov 1, 2023 14:20

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# ? May 26, 2024 23:02

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

Good thing empty statements are valid.

# ? Nov 1, 2023 15:30

PDP-1: Oct 12, 2004; It's a beautiful day in the neighborhood.

Phobeste posted:

Dijkstracula posted:

Thanks much for the excellent and detailed explanation! :tipshat:

You wrap your code in do{}while(0) so that it works syntactically as you want with semicolons, but since that structure doesn't do anything useful it gets optimized out to nothing in the actual output. Other constructs like if statements don't work the same way with semicolons so it has to be do-while.

pseudorandom name posted:

Well, you screwed up your examples, it should be do { } while (0)

True, I was typing it out from memory and auto-included the semicolon by force of habit. I didn't understand the importance of it not being there so it didn't jump out at me. I get why you don't want it there now.

# ? Nov 1, 2023 16:23

rjmccall: Sep 7, 2007; no worries friend; Fun Shoe

You could also use if (1) { statements; } else (void) 0, but do/while is simpler.

# ? Nov 1, 2023 17:46

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

PDP-1 posted:

Thanks much for the excellent and detailed explanation!

You wrap your code in do{}while(0) so that it works syntactically as you want with semicolons, but since that structure doesn't do anything useful it gets optimized out to nothing in the actual output. Other constructs like if statements don't work the same way with semicolons so it has to be do-while.

True, I was typing it out from memory and auto-included the semicolon by force of habit. I didn't understand the importance of it not being there so it didn't jump out at me. I get why you don't want it there now.

If you add it then the semicolon is just optional for the user it's not really a big deal

# ? Nov 1, 2023 18:03

Zopotantor: Feb 24, 2013; ...und ist er drin dann lassen wir ihn niemals wieder raus...

leper khan posted:

If you add it then the semicolon is just optional for the user it's not really a big deal

Missing semicolons can mess up auto indentation in editors.

# ? Nov 1, 2023 18:23

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

Zopotantor posted:

Missing semicolons can mess up auto indentation in editors.

Extra semicolons cause few to no issues. Typically a generated warning about a null statement.

# ? Nov 1, 2023 18:29

StumblyWumbly: Sep 12, 2007; Batmanticore!

Very inspired by the planning committee that designed the C language that they included this intuitive syntax for such common situations.

# ? Nov 1, 2023 20:27

Zopotantor: Feb 24, 2013; ...und ist er drin dann lassen wir ihn niemals wieder raus...

StumblyWumbly posted:

Very inspired by the planning committee that designed the C language that they included this intuitive syntax for such common situations.

"Planning committee"? "Designed"? What do you think this is, f*cking Ada?

# ? Nov 1, 2023 21:03

Plorkyeran: Mar 22, 2007; To Escape The Shackles Of The Old Forums, We Must Reject The Tribal Negativity He Endorsed

Are you suggesting that Kernigham and Ritchie made some bad decisions when creating C?

# ? Nov 1, 2023 21:45

pseudorandom name: May 6, 2007

leper khan posted:

If you add it then the semicolon is just optional for the user it's not really a big deal

leper khan posted:

Extra semicolons cause few to no issues. Typically a generated warning about a null statement.

Extra semicolons cause compilation errors, e.g.

code:

#define MACRO(x) do { (void) x; } while (0);

void test(int a, int b)
{
        if (a)
                MACRO(b);
        else
                MACRO(a);
}

# ? Nov 1, 2023 21:57

StumblyWumbly: Sep 12, 2007; Batmanticore!

Plorkyeran posted:

Are you suggesting that Kernigham and Ritchie made some bad decisions when creating C?

The post above you did not end in a semicolon, so I am reporting this post;
I am a C compiler;

# ? Nov 1, 2023 22:09

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

pseudorandom name posted:

Extra semicolons cause compilation errors, e.g.

code:

#define MACRO(x) do { (void) x; } while (0);

void test(int a, int b)
{
        if (a) {
                MACRO(b);
        }
        else {
                MACRO(a);
        }
}

What's the problem?

# ? Nov 1, 2023 23:24

b0lt: Apr 29, 2005

leper khan posted:

What's the problem?

the bug happens if the if doesn't use braces, the following is invalid:

code:

if (true) (void)0;; else {}

# ? Nov 1, 2023 23:35

Dijkstracula: Mar 18, 2003; You can't spell 'vector field' without me, Professor!

What I�m hearing is we should move to m4 for all our syntax transforming needs

# ? Nov 2, 2023 00:00

pseudorandom name: May 6, 2007

All good C/C++ compilers support statement expressions already anyway, you can just do #define TRACE(x) ({ blah blah blah })

# ? Nov 2, 2023 00:15

Jabor: Jul 16, 2010; #1 Loser at SpaceChem

pseudorandom name posted:

All good C/C++ compilers support statement expressions already anyway, you can just do #define TRACE(x) ({ blah blah blah })

Cool, how does that evaluate if I put it in an unbraced if-else block with a semicolon after it?

# ? Nov 2, 2023 03:26

more falafel please: Feb 26, 2005; forums poster

Jabor posted:

Cool, how does that evaluate if I put it in an unbraced if-else block with a semicolon after it?

unbraced if blocks should be a warning

# ? Nov 2, 2023 07:44

Beef: Jul 26, 2004

pseudorandom name posted:

All good C/C++ compilers support statement expressions already anyway, you can just do #define TRACE(x) ({ blah blah blah })

This is the biggest reason why I hate that MSVC exists. Make statement expressions part of the language already ffs.

# ? Nov 2, 2023 10:58

Qwertycoatl: Dec 31, 2008

Beef posted:

This is the biggest reason why I hate that MSVC exists. Make statement expressions part of the language already ffs.

It took 15 years between designated initialisers being in the standard and MSVC supporting them, I wouldn't hold my breath

# ? Nov 2, 2023 11:18

qsvui: Aug 23, 2003; some crazy thing

qsvui posted:

You're right. It should be an error.

# ? Nov 3, 2023 02:49

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

Presto posted:

You're right. It should be an error.

# ? Nov 3, 2023 03:03

Zopotantor: Feb 24, 2013; ...und ist er drin dann lassen wir ihn niemals wieder raus...

qsvui posted:

goto fail;

# ? Nov 3, 2023 04:20

Dijkstracula: Mar 18, 2003; You can't spell 'vector field' without me, Professor!

Zopotantor posted:

goto fail;

I would simply make whitespace indentation significant

# ? Nov 3, 2023 15:51

chglcu: May 17, 2007; I'm so bored with the USA.

While I agree with requiring braces on a practical level, aesthetically it just irks me every time. Especially when sickos put the opening brace on its own line.

# ? Nov 3, 2023 16:06

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

chglcu posted:

While I agree with requiring braces on a practical level, aesthetically it just irks me every time. Especially when sickos put the opening brace on its own line.

Opening on same, closing on its own line. Don't curry

# ? Nov 3, 2023 16:10

Presto: Nov 22, 2002; Keep calm and Harry on.

chglcu posted:

Especially when sickos put the opening brace on its own line.

Our style guide at my job mandates Allman style braces. :negative:

# ? Nov 3, 2023 20:04

Zopotantor: Feb 24, 2013; ...und ist er drin dann lassen wir ihn niemals wieder raus...

Dijkstracula posted:

I would simply make whitespace indentation significant

# ? Nov 3, 2023 20:50

StumblyWumbly: Sep 12, 2007; Batmanticore!

There is a non zero chance I am getting ghosted because I added brackets to an if statement in a coding exercise.
The point here is people who don't use brackets are cowards at heart.

# ? Nov 3, 2023 21:41

more falafel please: Feb 26, 2005; forums poster

chglcu posted:

While I agree with requiring braces on a practical level, aesthetically it just irks me every time. Especially when sickos put the opening brace on its own line.

I also program on an 80x24 VT100 and not a vertical monitor that can show around 100 lines on a screen at a time

# ? Nov 3, 2023 21:45

Sweeper: Nov 29, 2007; The Joe Buck of Posting; Dinosaur Gum

StumblyWumbly posted:

There is a non zero chance I am getting ghosted because I added brackets to an if statement in a coding exercise.
The point here is people who don't use brackets are cowards at heart.

If true you wouldn�t want to work there anyway

# ? Nov 3, 2023 22:46

chglcu: May 17, 2007; I'm so bored with the USA.

Sweeper posted:

If true you wouldn�t want to work there anyway

Very true

# ? Nov 3, 2023 23:00

roomforthetuna: Mar 22, 2005; I don't need to know anything about virii! My CUSTOM PROGRAM keeps me protected! It's not like they'll try to come in through the Internet or something!

leper khan posted:

Opening on same, closing on its own line. Don't curry

Opening on same, closing on *the start* of a line. Sometimes you close and it's an else or the while of a do-while.
Also, sometimes opening and closing both on the same line if it's short.

# ? Nov 3, 2023 23:06

Jabor: Jul 16, 2010; #1 Loser at SpaceChem

Having a blank line before a conditional block is way better for at-a-glance understanding than having a nearly-blank line after the if statement but before the actual code, which is what putting the opening brace on its own line forces.

# ? Nov 4, 2023 00:05

Presto: Nov 22, 2002; Keep calm and Harry on.

roomforthetuna posted:

Sometimes you close and it's an else or the while of a do-while.

No it isn't because those go on the line after the brace. :colbert:

# ? Nov 4, 2023 06:36

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

Presto posted:

No it isn't because those go on the line after the brace.

This one gets it

# ? Nov 4, 2023 09:59

roomforthetuna: Mar 22, 2005; I don't need to know anything about virii! My CUSTOM PROGRAM keeps me protected! It's not like they'll try to come in through the Internet or something!

Presto posted:

No it isn't because those go on the line after the brace.

Yes it still is sometimes an else after a brace, you're just saying you should format that like a maniac. You appear to be suggesting this monstrosity?

code:

if (banana) {
  doStuff();
}
else {
  doOtherStuff();
}

# ? Nov 4, 2023 14:40

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# ? May 26, 2024 23:02

leper khan: Dec 28, 2010; Honest to god thinks Half Life 2 is a bad game. But at least he likes Monster Hunter.

roomforthetuna posted:

Yes it still is sometimes an else after a brace, you're just saying you should format that like a maniac. You appear to be suggesting this monstrosity?
code:
if (banana) {
  doStuff();
}
else {
  doOtherStuff();
}

That looks correct.

# ? Nov 4, 2023 14:56

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > C/C++ Programming Questions Not Worth Their Own Thread

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