What does double semicolon ";;" in C++ programming langauge?

Question

I can not find any proper answer to my question on Internet, so I hope someone here would help me.

Answer 1

You know that a statement has to be ended with a semicolon, right?

A single semicolon is just an empty statement. You could string up as many as you want, and nothing would happen.

Two semicolons can also occur in for loops:

for(;;) {
...
}

In this case, all three 'slots' are empty, nothing is initialized, or incremented, and there's no condition for when the loop should run.

This is a way to write an eternal loop that can only be left from inside the block with a break statement.

Answer 2

Also works similarity in C, Java and C#.

Answer 3

I see, thank you! You made my day.

Answer 4

I would disagree about breaking for(;;)
You can also break it with "return" or "goto" (or any terminating function like exit)

Answer 5

Yeah, or by turning off your computer.

Answer 6

If you see two semicolons at the end of a statement then the second one is declring an empty statement. That's usually a mistake, but it's harmless.

Empty statements aren't always harmless though :

While (iter != svec.end()) ;
++iter;

Creates an endless loop, because the loop body is an empty statement and the ++iter is outside it.

Empty statements do have a use if the language requires a statement but your programs logic does not, like when you omit a parameter in a loop statement.