do {…} while(false)

Question

I was looking at some code by an individual and noticed he seems to have a pattern in his functions:

 function()
{
 

        

Answer 1

Many answerers gave the reason for do{(...)break;}while(false). I would like to complement the picture by yet another real-life example.

In the following code I had to set enumerator operation based on the address pointed to by data pointer. Because a switch-case can be used only on scalar types first I did it inefficiently this way

if (data == &array[o1])
    operation = O1;
else if (data == &array[o2])
    operation = O2;
else if (data == &array[on])
    operation = ON;

Log("operation:",operation);

But since Log() and the rest of code repeats for any chosen value of operation I was wandering how to skip the rest of comparisons when the address has been already discovered. And this is where do{(...)break;}while(false) comes in handy.

do {
    if (data == &array[o1]) {
        operation = O1;
        break;
    }
    if (data == &array[o2]) {
        operation = O2;
        break;
    }
    if (data == &array[on]) {
        operation = ON;
        break;
    }
} while (false);

Log("operation:",operation);

One may wonder why he couldn't do the same with break in an if statement, like:

if (data == &array[o1])
{
    operation = O1;
    break;
}
else if (...)

break interacts solely with the closest enclosing loop or switch, whether it be a for, while or do .. while type, so unfortunately that won't work.