Non-traditional uses of switch statement

Question

Recently I found out that the body of a switch can be any statement (C99 6.8.4). The idea was first suggested to me by this: https://stackoverflow.com/a/9220598/515212

So one can have switch statements like

void f(int n)
{
    switch (n)
    case 0:
        printf("zero\n");
}

or even put ifs, whiles, etc.

void f(int n)
{
    switch (n) 
    if (1)
    {
        case 0:
            printf("zero\n");
    }
    else
        while (--n)
        {
            default:
                printf("non-zero\n");
        }
}

Just out of interest, I was wondering whether this syntax has some usage or is just an artifact of how the switch statement is defined in the standard?

Answer 1

You can consider the switch statement as a code block with labels (case(s) are indeed labels) where the control is passed with a goto statement.

Something like

void f(int n)
{
    if ( n == 0 ) goto Label_case_0;
    else goto Label_default;

    {
        if ( 1 )
        {
            Label_case_0:
            printf("zero\n");
        }
        else 
            while (--n)
            {
                Label_default:
                printf("non-zero\n");
            }
    }
}

In my opinion it is not a good idea to place case labels inside some other control structures because this makes the code difficult to read and can lead to errors.

Answer 2

This is valid C code. It origins from assembly where every conditional statement uses if, goto and label jumps.

Implementation of array copy using this feature is called Duff's Device

void copy(char *from, char *to, int count)
{
    int n = (count + 7) / 8;

    switch(count % 8) {
        case 0: do { *to++ = *from++;
        case 7:      *to++ = *from++;
        case 6:      *to++ = *from++;
        case 5:      *to++ = *from++;
        case 4:      *to++ = *from++;
        case 3:      *to++ = *from++;
        case 2:      *to++ = *from++;
        case 1:      *to++ = *from++;
                   } while(--n > 0);
    }
}

When you replace while with if and goto

void copy(char *from, char *to, int count)
{
    int n = (count + 7) / 8;

    switch(count % 8) {
        case 0: 
        loop:        *to++ = *from++;
        case 7:      *to++ = *from++;
        case 6:      *to++ = *from++;
        case 5:      *to++ = *from++;
        case 4:      *to++ = *from++;
        case 3:      *to++ = *from++;
        case 2:      *to++ = *from++;
        case 1:      *to++ = *from++;
        if(--n > 0) goto loop;
    }
}

Then replace switch with if and goto

void copy(char *from, char *to, int count)
{
    int n = (count + 7) / 8;

    if(count%8==7)goto case7;
    if(count%8==6)goto case6;
    if(count%8==5)goto case5;
    if(count%8==4)goto case4;
    if(count%8==3)goto case3;
    if(count%8==2)goto case2;
    if(count%8==1)goto case1;
    if(count%8==0)goto case0; // this can be omitted

    case0:                    // this can be omitted
    loop:        *to++ = *from++;
    case7:       *to++ = *from++;
    case6:       *to++ = *from++;
    case5:       *to++ = *from++;
    case4:       *to++ = *from++;
    case3:       *to++ = *from++;
    case2:       *to++ = *from++;
    case1:       *to++ = *from++;

    if(--n > 0) goto loop;
}

Which is functionally (almost) equivalent to

void copy(char *from, char *to, int count)
{
    while(--n > 0) { 
        *to++ = *from++;
    }
}

It's almost equivalent because in the last implementation loop check is performed 8 times more often, what have impact on performance.