Can somebody explain this weird loop for me?

Question

I'm newer to c++ and today I looked at a game engine main render loop which looked like this:

for (register unsigned long long i = 0xaull; engine_tick(); kRuntimeStatistics::INSTANCE.FramesSinceStart++, ++ i); 

Why did it have any body and when it is exiting? I thought for loops are for numbers only? And what does 0xaull mean behind the long i?

Answer 1

General case

No the for loop is for any kind of iterative processing. Not only for numbers.

The statement

for (a;b;c) 
   d; 

is (almost) the same as

{
    a;
    while (b) {
        d; 
        c; 
    }
}

The empty body means that the main benefit is already in the a,b and c.

Application to your statement

So for your statement:

for (register unsigned long long i = 0xaull; engine_tick(); kRuntimeStatistics::INSTANCE.FramesSinceStart++, ++ i); 

it's the same as

{
    register unsigned long long i = 0xaull;
    while (engine_tick()) {
        kRuntimeStatistics::INSTANCE.FramesSinceStart++; 
        ++ i;  
    }
}

Several remarks:

• the empty for body, with semicolon at the end of the line is extremely error prone (I spent a whole night debugging some code once, just to figure out that I didn't see the semicolumn, so that I was mislead by the indentation). I'd therefore recommend to put the semicolumn on the next line with proper indentation, just to draw attention on the potential trap.
• the register has no longer added value, since the compiler's optimizer do much better job than the manual selection of register variables. It is even deprecated now.
• the comma operator allows to combine two expression in one. I've translated this in a semi-column for the sake of clarity. Note that the ++i could have been moved to the for body in the original code for the same reason.
• 0xaull means that it is an hexadecimal literal ( 0x ) of value a (A in hexadecimal is 10) of type unsigned long long .

Answer 2

It will exit when engine_tick() returns false. A while statement doesn't need a body. The ull on the end of 0xa just specifies that its an unsigned long long, while the 0x specifies that its in hex.