What are some interesting uses of 'switch' in C/C++? [closed] - c++

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The switch statement in C/C++ has an interesing feature that all subsequent blocks will be executed if a condition is met
For example,
int a = 2;
int b = a;
switch(b)
{
case 1:cout<<1;
case 2:cout<<2;
case 3:cout<<3;
case 4:cout<<4;
};
The above code will output 234 unless I put a break statement in case 2.
In 3 years(quite small,yeah) of my C/C++ programming experience, I have never encountered a problem where I had to use switch without putting break statments in every case. But judging by the fact that this feature has been stuck for so long, there might be some utility of it.
Question: What are some clever uses of switch statement as to utilize the above mentioned feature in C/C++?

Answer 1

Probably one of the most interesting use cases I have seen would be Duff's Device the case where you extend a scope within a switch over multiple cases which would look something like this:
void send( int *to, const int *from, int count)
{
int n = (count + 7) / 8;
switch(count % 8)
{
case 0: do { *to = *from++; // <- Scope start
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); // <- Scope end
}
}

Answer 2

This is usually used when you want to apply a similar action to a set of values. For instance, the following :
switch (event) {
case DEVICE_DISCONNECTED:
case CONNECTION_ERROR:
case CONNECTION_TIMEOUT:
transitionTo(disconnectedState);
break;
case CONNECTION_SUCCESS:
transitionTo(connectedState);
break;
}
is much more concise and readable in my opinion than :
switch (event) {
case DEVICE_DISCONNECTED:
transitionTo(disconnectedState);
break;
case CONNECTION_ERROR:
transitionTo(disconnectedState);
break;
case CONNECTION_TIMEOUT:
transitionTo(disconnectedState);
break;
// ...
}

Answer 3

In my current project, I have the following enumeration:
enum NodeType
{
SCALAR, COMPOSITE, ARRAY, RESTRICTED_ARRAY
};
Thus, quite a few node-processing routines use this pattern:
switch (nodeType)
{
case SCALAR:
processScalar();
break;
case COMPOSITE:
processComposite();
break;
case RESTRICTED_ARRAY:
if (!handleRestrictions())
return false;
// continue to next case
case ARRAY:
processArray();
break;
}
Note that it's almost necessary to always mark the lack-of-break as explicitly intended with a comment (like I did above) - future maintainers (including yourself in 3 months) will thank you.

Answer 4

I've often used a construct to parse command line arguments like this:
switch (argument) {
case arg_h:
case arg__help:
case arg_questionmark:
printf("Help\n");
break;
case arg_f:
case arg__file:
//...
}
where argument is an enum type.