Why is a semicolon by itself allowed in C/C++?

Question

Why does the C/C++ parser not report an error on a semicolon when it is used alone? For example:

int a;
;
int b;

Is it a consequence of language grammar rules or is it a consequence of parsing algorithms? What do standards say about such a case? Whether ; or e.g. int; should be allowed?

`int;` is not allowed since the definition misses an identifier. — cadaniluk, Sep 07 '15 at 10:53
And also interesting answers in the very similar [Use of null statement in C](http://stackoverflow.com/questions/5599380/use-of-null-statement-in-c) — BrechtDeMan, Sep 07 '15 at 11:01

score 2 · Answer 1 · answered Sep 07 '15 at 11:13

2

Consider the classic string copy function from K&R:

void strcpy(char *src, char *dst)
{
    while (*src++ = *dst++);
}

As you can see the empty statement is not just legal, it's one of foundations of the short C syntax.

answered Sep 07 '15 at 11:13

Mark Shevchenko

7,937
1
25
29

Unfortunately, this is also _really terrible code_, and should have `{}` instead, so I'm not convinced this is a great example. – Lightness Races in Orbit Sep 07 '15 at 12:52
This is not the example, this is the citation from K&R. – Mark Shevchenko Sep 07 '15 at 12:55
Correct me if I'm wrong, but your answer is "empty statements are allowed in part because K&R wrote this crap code that uses them, which could and should have been written another way". Is that it? I just don't see how citing a use of `while (...);` is an answer here. – Lightness Races in Orbit Sep 07 '15 at 12:56
Without empty statement this code would have been impossible. But it's possible, and it's promoted as an example of a right C style by the authors of C. – Mark Shevchenko Sep 07 '15 at 13:01
"Impossible"? What's wrong with the superior `while (*src++ = *dst++) {}`? (I don't care whether the authors of C thought this was "a right C style" decades ago; it's inferior, and that's a common notion nowadays) – Lightness Races in Orbit Sep 07 '15 at 13:06
`{}` is not described in K&R. :) – Mark Shevchenko Sep 07 '15 at 13:16
Right, that's kind of my point. It's much better code, and K&R don't mention it, therefore referencing K&R doesn't really prove that the semicolon-terminated empty statement is needed! – Lightness Races in Orbit Sep 07 '15 at 16:10

score 1 · Answer 2 · answered Sep 07 '15 at 10:58

1

A semicolon terminates a statement.

If your statement doesn't do anything, that's your prerogative as a programmer.

An example of a completely valid use of an empty statment:

 for (int x = 0;; x++) {
     if (something(x)) {
         return 5;
     }
     if (bah(x)) {
         continue;
     }
     if (otherthing(x)) {
         return 3;
     }
 }

The middle statement in the for loop is empty. But it is still necessary, as the for loop takes three distinct statements, and the first and third are populated.

I don't see a good reason why a lone semicolon should be invalid; it does no harm. It is allowed by all relevant C/C++ standards, which is not a worse position than it not being allowed.

answered Sep 07 '15 at 10:58

Borealid

95,191
9
106
122

1. **Is it** allowed by all standards? (even pre-C++11?) IIRC it's not. – Luchian Grigore Sep 07 '15 at 11:05
@LuchianGrigore At the top of the loop. `for (int x = 0;; x++)`. Note the double semicolon. – Borealid Sep 07 '15 at 11:09
1

@LuchianGrigore - the empty statement is in `for (int x = 0;; x++)` - note the two semi-colons consecutively, this is an empty statement. As far as I know, the example given in the answer has been valid in all C and C++ standards, as well as K&R. – Tom Sep 07 '15 at 11:09
@LuchianGrigore If it not allowed by C98, then I've never seen a conforming compiler in my entire life. I have yet to have a program rejected because it contains an extra semicolon at the end of a line. – Borealid Sep 07 '15 at 11:10
Ah, missed the loop condition, my bad. – Luchian Grigore Sep 07 '15 at 11:15
empty statement within for loop is legal even in K&R. As a side note, inline `int` declaration is not. – Non-maskable Interrupt Sep 07 '15 at 11:26
2

I hate to say it, but your for-loop does not contain an empty (null) statement. It merely contains a missing condition. To see it, consider that you cannot put statements there (but only conditions which are expressions or declarations whose type can convert to booleans). What is similar is merely that both in the null statement and the missing condition, an expression that is optional is missing, but that's about it. – Johannes Schaub - litb Sep 07 '15 at 11:29
@JohannesSchaub-litb Perhaps I should have left the first one blank, then. That doesn't have to be a boolean... – Borealid Sep 07 '15 at 11:32
litb's right; this example is wrong. – Lightness Races in Orbit Sep 07 '15 at 12:53
for (int x = 0; true; x++) {} is more readable imho – Ben Alan Dec 06 '21 at 23:15

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

0

It's specified in the standard, §6/1 and §6.2/1 (revision N3376):

statement:

...

attribute-specifier-seq_opt expression-statement

...

and

expression-statement:

expression_opt;

So a statement can be an expression statement with a colon and optionally a expression and an attribute specifier sequence before.

edited Jun 20 '20 at 09:12

Community

1
1

answered Sep 07 '15 at 10:59

JBL

12,588
4
53
84

Why is a semicolon by itself allowed in C/C++?

3 Answers3