I'm writing some code that looks like this:
while(true) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
break; // **HERE, I want to break out of the loop itself**
}
}
Is there any direct way to do that?
I know I can use a flag, and break from the loop by putting a conditional break just after the switch. I just want to know if C++ has some construct for this already.
You can use goto.
while ( ... ) {
switch( ... ) {
case ...:
goto exit_loop;
}
}
exit_loop: ;
An alternate solution is to use the keyword continue in combination with break, i.e.:
for (;;) {
switch(msg->state) {
case MSGTYPE:
// code
continue; // continue with loop
case DONE:
break;
}
break;
}
Use the continue statement to finish each case label where you want the loop to continue and use the break statement to finish case labels that should terminate the loop.
Of course this solution only works if there is no additional code to execute after the switch statement.
Premise
The following code should be considered bad form, regardless of language or desired functionality:
while( true ) {
}
Supporting Arguments
The while( true ) loop is poor form because it:
Breaks the implied contract of a while loop.
The while loop declaration should explicitly state the only exit condition.
Implies that it loops forever.
Code within the loop must be read to understand the terminating clause.
Loops that repeat forever prevent the user from terminating the program from within the program.
Is inefficient.
There are multiple loop termination conditions, including checking for "true".
Is prone to bugs.
Cannot easily determine where to put code that will always execute for each iteration.
Leads to unnecessarily complex code.
Automatic source code analysis.
To find bugs, program complexity analysis, security checks, or automatically derive any other source code behaviour without code execution, specifying the initial breaking condition(s) allows algorithms to determine useful invariants, thereby improving automatic source code analysis metrics.
Infinite loops.
If everyone always uses while(true) for loops that are not infinite, we lose the ability to concisely communicate when loops actually have no terminating condition. (Arguably, this has already happened, so the point is moot.)
Alternative to "Go To"
The following code is better form:
while( isValidState() ) {
execute();
}
bool isValidState() {
return msg->state != DONE;
}
Advantages
No flag. No goto. No exception. Easy to change. Easy to read. Easy to fix. Additionally the code:
Isolates the knowledge of the loop's workload from the loop itself.
Allows someone maintaining the code to easily extend the functionality.
Allows multiple terminating conditions to be assigned in one place.
Separates the terminating clause from the code to execute.
Is safer for Nuclear Power plants. ;-)
The second point is important. Without knowing how the code works, if someone asked me to make the main loop let other threads (or processes) have some CPU time, two solutions come to mind:
Option #1
Readily insert the pause:
while( isValidState() ) {
execute();
sleep();
}
Option #2
Override execute:
void execute() {
super->execute();
sleep();
}
This code is simpler (thus easier to read) than a loop with an embedded switch. The isValidState method should only determine if the loop should continue. The workhorse of the method should be abstracted into the execute method, which allows subclasses to override the default behaviour (a difficult task using an embedded switch and goto).
Python Example
Contrast the following answer (to a Python question) that was posted on StackOverflow:
Loop forever.
Ask the user to input their choice.
If the user's input is 'restart', continue looping forever.
Otherwise, stop looping forever.
End.
Code
while True:
choice = raw_input('What do you want? ')
if choice == 'restart':
continue
else:
break
print 'Break!'
Versus:
Initialize the user's choice.
Loop while the user's choice is the word 'restart'.
Ask the user to input their choice.
End.
Code
choice = 'restart';
while choice == 'restart':
choice = raw_input('What do you want? ')
print 'Break!'
Here, while True results in misleading and overly complex code.
A neatish way to do this would be to put this into a function:
int yourfunc() {
while(true) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
return;
}
}
}
Optionally (but 'bad practices'): as already suggested you could use a goto, or throw an exception inside the switch.
AFAIK there is no "double break" or similar construct in C++. The closest would be a goto - which, while it has a bad connotation to its name, exists in the language for a reason - as long as it's used carefully and sparingly, it's a viable option.
You could put your switch into a separate function like this:
bool myswitchfunction()
{
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
return false; // **HERE, I want to break out of the loop itself**
}
return true;
}
while(myswitchfunction())
;
Even if you don't like goto, do not use an exception to exit a loop. The following sample shows how ugly it could be:
try {
while ( ... ) {
switch( ... ) {
case ...:
throw 777; // I'm afraid of goto
}
}
}
catch ( int )
{
}
I would use goto as in this answer. In this case goto will make code more clear then any other option. I hope that this question will be helpful.
But I think that using goto is the only option here because of the string while(true). You should consider refactoring of your loop. I'd suppose the following solution:
bool end_loop = false;
while ( !end_loop ) {
switch( msg->state ) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
end_loop = true; break;
}
}
Or even the following:
while ( msg->state != DONE ) {
switch( msg->state ) {
case MSGTYPE: // ...
break;
// ... more stuff ...
}
There's no C++ construct for breaking out of the loop in this case.
Either use a flag to interrupt the loop or (if appropriate) extract your code into a function and use return.
No, C++ does not have a construct for this, given that the keyword "break" is already reserved for exiting the switch block. Alternatively a do..while() with an exit flag could suffice.
do {
switch(option){
case 1: ..; break;
...
case n: .. ;break;
default: flag = false; break;
}
} while(flag);
You could potentially use goto, but I would prefer to set a flag that stops the loop. Then break out of the switch.
Why not just fix the condition in your while loop, causing the problem to disappear?
while(msg->state != DONE)
{
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
// We can't get here, but for completeness we list it.
break; // **HERE, I want to break out of the loop itself**
}
}
It amazes me how simple this is considering the depth of explanations... Here's all you need...
bool imLoopin = true;
while(imLoopin) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
imLoopin = false;
break;
}
}
LOL!! Really! That's all you need! One extra variable!
I think;
while(msg->state != mExit)
{
switch(msg->state)
{
case MSGTYPE: // ...
break;
case DONE:
// ..
// ..
msg->state =mExit;
break;
}
}
if (msg->state ==mExit)
msg->state =DONE;
I got same problem and solved using a flag.
bool flag = false;
while(true) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
flag = true; // **HERE, I want to break out of the loop itself**
}
if(flag) break;
}
Because the switch uses the break to break out from the switch (not from the while(1)), it needs the goto-statement:
while(1) {
switch (*p) {
case ' ':
p++;
break;
case '\n':
p++; *s=p; return(i);
case '\0':
*s=p; return(i);
default:
token[i]=p;
i++;
p++;
goto ex1;
};
};
ex1:
I can't add multiple case to same line like:
case ' ','\t':
it would be
case ' ': case '\t':
That's why maybe the break used here...
It looks the most frequent cases should be placed at the top of the list to make the program run faster.
It may not have parallel execution for searching the different cases.
It is possible that the standard c then has missing some methods about this switching:
https://blog.hackajob.co/better-c-switch-statements-for-a-range-of-values/
=> allows you to use the <, >, <=, and >= operators in a switch expression
I was thinking that it should be (if c-language syntax changed) like:
switch (c) {
case >= 5:
... op1
endcase;
case == 1:
case == 3:
... op2
endcase;
default:
...
};
where op2 is executed when c is equal to 1 or 3 and when c is larger than or equal to 5 op1 is executed.
Because comparison for equal or larger/smaller than would occur easily in similar manner.
while(1) {
switch (c) {
case >= 2:
... op1
case <= 5:
... op2
break;
default:
...
};
};
this case op1 is executed for c larger than 2 and op2 executed for 2<=c<=5 and break exits it from while-loop.
The simplest way to do it is to put a simple IF before you do the SWITCH , and that IF test your condition for exiting the loop .......... as simple as it can be
The break keyword in C++ only terminates the most-nested enclosing iteration or switch statement. Thus, you couldn't break out of the while (true) loop directly within the switch statement; however you could use the following code, which I think is an excellent pattern for this type of problem:
for (; msg->state != DONE; msg = next_message()) {
switch (msg->state) {
case MSGTYPE:
//...
break;
//...
}
}
If you needed to do something when msg->state equals DONE (such as run a cleanup routine), then place that code immediately after the for loop; i.e. if you currently have:
while (true) {
switch (msg->state) {
case MSGTYPE:
//...
break;
//...
case DONE:
do_cleanup();
break;
}
if (msg->state == DONE)
break;
msg = next_message();
}
Then use instead:
for (; msg->state != DONE; msg = next_message()) {
switch (msg->state) {
case MSGTYPE:
//...
break;
//...
}
}
assert(msg->state == DONE);
do_cleanup();
while(MyCondition) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
MyCondition=false; // just add this code and you will be out of loop.
break; // **HERE, you want to break out of the loop itself**
}
}
If I remember C++ syntax well, you can add a label to break statements, just like for goto. So what you want would be easily written:
while(true) {
switch(msg->state) {
case MSGTYPE: // ...
break;
// ... more stuff ...
case DONE:
break outofloop; // **HERE, I want to break out of the loop itself**
}
}
outofloop:
// rest of your code here
while(true)
{
switch(x)
{
case 1:
{
break;
}
break;
case 2:
//some code here
break;
default:
//some code here
}
}
I'm working on an embedded system where a register hast to be accessed and after that to be incremented to achieve the result I'm looking for, since the machine is notifying and configured to react on my access and changing or not changing the flag. So the switch's argument has to be kept as it is, since it would otherwise change the behaving of the embedded system.
But there may occur a situation where I don't want to get any of the cases get invoked. But I still need to acces and increment the argument of the switch.
(More indepth I'm converting a sequence of analog values to digital values conversions step by step. The index is used to stay synchronized with the current conversion and relating it with the corresponding case to handle the figure correct. There may occur a state in which the index desynchronisizes to the current conversion so the sequence of conversions must be run through without any of the cases getting invoked (to prevent setting wrong data) untill the sequence is finished and the resynchroinisation can get performed)
The way I'm currently doing this is this:
switch (RunIndex++)/*RunIndex may only be accessed one time per execution
of this construct and has to be incremented in the same step. thats given.*/
{
if (RunIndexSyncedWithADCensured == false)
{
break;
}
case 0:
Case0RelatedData = SomeOperationsForCase0(RawData);
break;
case 1:
Case1RelatedData = SomeOperationsForCase1(RawData);
break;
case 2:
Case2RelatedData = SomeOperationsForCase2(RawData);
break;
default:
RunIndex = 0;
break;
}
This construct does the job but it looks like it is a bit controversial and I don't feel well by considering about committing this into productinal code.
So is there a better looking way to achieve the same, without the need of additional variables or assignements?
note:
Also it may be relevant, that this is in the first part of a interupt function consisting of 2 parts.
The first part handles what has to happen if() a conversion is finished. The second part, what has additional to be done if() this conversion also ended the sequence. So it is no option to simply return from the function without getting into the second part. and there is currently no loop structure where an if(...)break; may break out. (What is also the reason why I'm putting the if inside the switch scope, as it is at least by standard a valid way to break out.)
Firstly, the if() inside switch() will never be executed.
Consider the below code snippet:
#include <stdio.h>
int main(int argc, char *argv[])
{
int i = 2;
switch(i) {
if (i == 2) {
printf("I M HERE\n");
}
case 1:
printf("1\n");
break;
case 2:
printf("2\n");
break;
default:
printf("default\n");
break;
}
return 0;
}
For your code: you expect the string I M HERE to be printed. But that is not the case.
The output for the above code snippet is:
2
No statements before case/default(switch constructs): is executed inside switch
Now to answer for
I don't want to get any of the cases get invoked. But I still need to acces and increment the argument of the switch
Just move the if() outside to the switch().
if (RunIndexSyncedWithADCensured) {
switch (RunIndex++) {
case 0:
Case0RelatedData = SomeOperationsForCase0(RawData);
break;
/* Other cases here */
default:
RunIndex = 0;
break;
}
} else
RunIndex++;
Why not save the value first and then increment it and use the saved value in the switch? By the way this also includes two accesses, first to read the value from RunIndex and the second to increment it.
int runIndex = (RunIndex++);
if (RunIndexSyncedWithADCensured )
{
switch (runIndex)/*RunIndex may only be accessed one time per execution
of this construct and has to be incremented in the same step. thats given.*/
{
case 0:
Case0RelatedData = SomeOperationsForCase0(RawData);
break;
case 1:
Case1RelatedData = SomeOperationsForCase1(RawData);
break;
case 2:
Case2RelatedData = SomeOperationsForCase2(RawData);
break;
default:
RunIndex = 0;
break;
}
}
Since you are using adjacent index numbers, you could make an array of function pointers to replace the switch. That's what the optimizer will turn the switch into anyhow. So instead of your obscure switch, you get this:
if (RunIndexSyncedWithADCensured)
{
SomeOperationsForCase[RunIndex](RawData);
}
RunIndex++;
if (RunIndex > MAX)
{
RunIndex = 0;
}
Completely unrelated to the switch statement design: in case RunIndex is a sensitive volatile variable such as some hardware register, then you shouldn't use it directly in any form of computations. Make a copy of it:
volatile int RunIndex;
...
int index = RunIndex; // read from RunIndex
if (RunIndexSyncedWithADCensured)
{
SomeOperationsForCase[index](RawData);
}
index++;
if (index > MAX)
{
index = 0;
}
RunIndex = index; // write to RunIndex
This is standard practice for all such volatile variables.
So I am programming with the mbed compiler and this is currently my code.
pc.baud(9600);
dev1.baud(19200);
char command[5];
//pc.puts("Initialized1X\n");
//while(1) {
if (1) {
dev1.gets(command, 6); //this is causing my error...just waiting for command, must fix
if (command[0] != 0x68){
for (int x=0; x<4; x++){
command[x] = command[x+1];
}
command[4] = 0x16;
}
for (int i=0; i<5; i++) {
pc.printf("%x ", command[i]);
}
pc.printf("\n");
sampleRates = command[2];
if (command[1] == 0x01){
switch (sampleRates){
case 0x01: pc.printf("44.1k\n");
return;
case 0x02: pc.printf("48k\n");
return;
case 0x03: pc.printf("88.2k\n");
return;
case 0x04: pc.printf("96k\n");
return;
case 0x05: pc.printf("176.4k\n");
return;
case 0x06: pc.printf("192k\n");
return;
case 0x07: pc.printf("352k\n");
return;
case 0x08: pc.printf("384k\n");
return;
case 0x09: pc.printf("705.6k\n");
return;
case 0x10: pc.printf("768k\n");
return;
}
}
}
//}
With the dev1.gets(command, 6); I am getting the input from another device. The code works fine except when I need to add other functions to my program. The other functions wont run because it just ends up waiting for input on the dev.1gets() command. Is there any way I could just make it check real quick, accept the input and move on? Ive used an if statement with dev1.readable(); but then it doesnt take the commands into the array until I do it again since it only entered the loop. Maybe is there a way to check and save at the same time?
input data via an interrupt into a buffer.
Probably a good idea to use double buffering and keep a flag that indicates if a particular buffer contains a 'new' 'complete' command
that is waiting to be processed.
The interrupt handler maintains the circular buffer and handles the actual I/O.
have the interrupt handler smart enough to recognize when a complete 'command' has been entered.
When a complete 'command' has been entered, the interrupt sets a flag for the specific buffer, that the main program can look at/reset to determine if a 'command' is available to process.
The above flag needs to be protected so use a mutex or have the main function disable interrupts while reading/resetting the flag.
I am looking through someone's C code, and discovered something I didn't even know was possible. Inside some of the cases, the switch variable is modified, with the intention that another case is executed after the break.
The switch is inside an interrupt handler, so it will get called repeatedly.
switch (variable)
{
case 1:
some_code();
variable = 3;
break;
case 2:
more_code();
variable = 5;
break;
case 3:
more_code();
variable = 5;
break;
case 4:
my_code();
break;
case 5:
final_code();
break;
}
The code seems to be working as the author intended.
Is this guaranteed behaviour in C? I always assumed that the break statement would cause execution to jump to directly after the switch statement. I wouldn't have expected it to continue testing every other case.
This is a common technique for state machine type code. But it doesn't jump automatically like you imagine. The switch statement has to be put inside a while loop. In this case, I imagine the loop would look something like:
while (!done) {
done = (variable == 5);
switch (variable) {
/*...*/
}
}
Alternatively, the switch statement could be inside a function body, and that function is called from a loop until the done condition is met.
Is it possible to initialise CRITICAL_SECTION statically, as in pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER?
In other words, is it possible in C to initialize a global CRITICAL_SECTION inside a library without having to mess with DllMain etc.?
Yes, simply initialize in DLL_PROCESS_ATTACH and delete in DLL_PROCESS_DETACH
CRITICAL_SECTION g_cs = {0};
BOOL WINAPI DllMain(
HINSTANCE hinstDLL, // handle to DLL module
DWORD fdwReason, // reason for calling function
LPVOID lpReserved ) // reserved
{
// Perform actions based on the reason for calling.
switch( fdwReason )
{
case DLL_PROCESS_ATTACH:
// Initialize once for each new process.
// Return FALSE to fail DLL load.
InitializeCriticalSection(&g_cs);
break;
case DLL_THREAD_ATTACH:
// Do thread-specific initialization.
break;
case DLL_THREAD_DETACH:
// Do thread-specific cleanup.
break;
case DLL_PROCESS_DETACH:
// Perform any necessary cleanup.
DeleteCriticalSection(&g_cs);
break;
}
return TRUE; // Successful DLL_PROCESS_ATTACH.
}
References:
InitializeCriticalSection
DeleteCriticalSection
Yes! But you have to make sure that it's only done once per process.
But this is typically easiest to achieve by using the DLL_PROCESS_ATTACH case of the DLLMain
switch( fdwReason ) statement.
Another possibility in earlier versions is to instruct the linker to set a pointer to your initialization function as a user-defined global initializer. There is some discussion of this here:
http://msdn.microsoft.com/en-us/library/bb918180.aspx
Here is an example:
CRITICAL_SECTION criticalSection;
static void __cdecl Initialize(void) {
InitializeCriticalSection(&criticalSection);
}
#pragma section(".CRT$XCU", read)
__declspec(allocate(".CRT$XCU"))
const void (__cdecl *pInitialize)(void) = Initialize;
The answer above from Raymond Chen solves the problem: "You can use InitOnceExecuteOnce to initialize the critical section on first use. That's what PTHREAD_MUTEX_INITIALIZER does under the covers."
note that this will work on Vista and above only --- #rkosegi
You can do it for older versions of Windows by writing your own InitOnceExecuteOnce function using InterlockedCompareExchange. --- #RaymondChen