Does the semicolon have a meaning beyond aesthetics

da peppester (53)
This article ( https://gcc.gnu.org/onlinedocs/cpp/Swallowing-the-Semicolon.html ) mentions, "semicolon is actually a null statement." What is meant by that, or if it is a typo, then is the semicolon actually something more than just syntaxical aesthetics?
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JLBorges (13770)
An expression such as x = 0 or i++ or printf(...) becomes a statement when it is followed by a semicolon ...
In C, the semicolon is a statement terminator, rather than a separator as it is in languages like Pascal. - K&R
Duthomhas (13290)
Every statement in C and C++ must be terminated with a semicolon.

x = 12;

However, a multi-statement block is not; it is delimited by curly braces.

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{
  x = 12;
  y = 9;
  std::cout << (x + y) << "\n";
}


You can place a multi-statement block anywhere you wish to place a single statement:

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if (x == 12)
  std::cout << "Yeah!\n";
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if (x == 12)
{
  x = 0;
  std::cout << "No twelves allowed.\n";
}

Notice the difference between the two: one has a semicolon at the end of the if-statement and one does not.

Now imagine a macro that expands to a multi-statement block, which you also terminate with a semicolon:

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#define MULTI_STATEMENT_MACRO( arg ) \
{ \
  foo; \
  bar; \
  baz; \
}

if (...)
  MULTI_STATEMENT_MACRO("hello") ;
 
if (...)
  {
    foo;
    bar;
    baz;
  } ;

Notice that extra semicolon after the multi-statement block? It is not part of the if statement. It is a new statement. (A "null statement", since there is nothing there but the semicolon.)

However, an else statement clause requires an if statement with no interleaving statements.

Clearly, the following is not legal:

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if (...)
  x = 12;
;
else x = 13;

Likewise, the following is not legal:

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if (...)
{
  x = 12;
  y = -7;
}
;
else x = 13;

That lone "null statement" between the if and else is a clear problem.


The macro trick you read about is to turn the multi-statement block into a single statement block that contains a multi-statement block. What better than a do..while statement: it is a single statement that contains a statement or statement block:

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if (...)
do {
  foo;
  bar;
  baz;
} while (0) ;
else x = 13;

In this case, the semicolon on line 6 terminates the do..while statement, and the else clause immediately follows, making a well-formed if..else statement.


Hope this helps.
}
cire (8284)
The term "null statement" is sometimes used to mean a statement consisting of only a semicolon, which is how the term is used in the article you refer to.

The semicolon in the article was a specific semicolon used in a specific example, not a statement about semicolons in general. Please consider the context of things you read.
da peppester (53)
@ JLBorges
@ Duoas
I know how to write in c++, my question was essentially do the required placements of the semicolons relate to the outputted machine code when you compile the program.

@ cire
Thank you so much, you have answered my question
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JLBorges (13770)
What the term "null statement" means in the context of C++ is precisely defined by the standard.
An expression statement with the expression missing is called a null statement. - IS
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