In C/C++ why does the do while(expression); need a semi colon?

Question

My guess is it just made parsing easier, but I can't see exactly why.

So what does this have ...

do {   some stuff } while(test);  more stuff 

that's better than ...

do {   some stuff } while(test)  more stuff 

Answer 1

Because you're ending the statement. A statement ends either with a block (delimited by curly braces), or with a semicolon. "do this while this" is a single statement, and can't end with a block (because it ends with the "while"), so it needs a semicolon just like any other statement.

Answer 2

If you take a look at C++ grammar, you'll see that the iteration statements are defined as

while ( condition ) statement

for ( for-init-statement condition-opt ; expression-opt ) statement

do statement while ( expression ) ;

Note that only do-while statement has an ; at the end. So, the question is why the do-while is so different from the rest that it needs that extra ;.

Let's take a closer look: both for and regular while end with a statement. But do-while ends with a controlling expression enclosed in (). The presence of that enclosing () already allows the compiler to unambiguously find the end of the controlling expression: the outer closing ) designates where the expression ends and, therefore, where the entire do-while statement ends. In other words, the terminating ; is indeed redundant.

However, in practice that would mean that, for example, the following code

do {   /* whatever */ } while (i + 2) * j > 0; 

while valid from the grammar point of view, would really be parsed as

do {   /* whatever */ } while (i + 2)  *j > 0; 

This is formally sound, but it is not really intuitive. I'd guess that for such reasons it was decided to add a more explicit terminator to the do-while statement - a semicolon. Of course, per @Joe White's answer there are also considerations of plain and simple consistency: all ordinary (non-compound) statements in C end with a ;.