I don't think that is possible in standard C (PIC & XC8).
I was trying to use an enum so that any out of range item would generate a compile time error but in another part of the program I want to be able to iterate through through all the values in the enum so I can't do that. I could put the values in a constant array which would be easy to iterate but then using an incorrect value would not generate a compile time error.

Is there some way of achieving both requirements?

 

Not sure what you want to do but adding a extra label at the end gives you the range of enum values. (if they are sequential from zero)
Software timer example:
https://raw.githubusercontent.com/nsaspook/vtouch_v2/mbmc/touch_v2.X/timers.h

ifndef TIMERS_H
#define TIMERS_H

#include <stdint.h>
#include <stdbool.h>

//Software timers - use these to refer to timers rather than
//integers.  Add more as needed.
//NOTE: 32767 ms max delay

enum APP_TIMERS {
    TMR_INTERNAL = 0, //Used in timers.c - do not remove or use elsewhere
    TMR_ADC,
    TMR_T2,
    TMR_T3,
    TMR_T4,
    TMR_MC_TX,
    TMR_HBIO,
    TMR_INFO,
    TMR_HELP,
    TMR_HELPDIS,
    TMR_DISPLAY,
    TMR_FLIPPER,
    TMR_ESR,
    //
    //(Add timers here as needed)
    //
    TMR_COUNT //number of timers - always keep at end of enum!
};

void StartTimer(const uint8_t timer, const uint16_t count);
bool TimerDone(const uint8_t timer);
void WaitMs(const uint16_t numMilliseconds);

#endif //TIMERS_H

https://raw.githubusercontent.com/nsaspook/vtouch_v2/mbmc/touch_v2.X/timers.c
https://github.com/nsaspook/vtouch_v2/blob/mbmc/touch_v2.X/mcc_generated_files/tmr6.c

void TMR6_DefaultInterruptHandler(void)
{
    uint8_t i;
    // add your TMR6 interrupt custom code
    // or set custom function using TMR6_SetInterruptHandler()

    //Decrement each software timer
    for (i = 0; i < TMR_COUNT; i++) {
        if (tickCount[i] != 0) {
            tickCount[i]--;
}
    }
}

 

Nice!
But the values may not be consecutive.
I have an idea to use an array but then when a function is passed a value which is not one of the values in the array the function would search for the nearest value and use that.

 

It cannot be done with a simple enum. C enums are just a list of identifiers, basically just a list of integer constant defines useful at compile time. The list is gone at run time.

Several recent languages such as Java, C#, Python (VB?) and the like have enumeration objects with multiple data items and methods so constructs such as "for each X in Y" will work, and work nicely too. This was not the intent of C.

 

Nice!
But the values may not be consecutive.
I have an idea to use an array but then when a function is passed a value which is not one of the values in the array the function would search for the nearest value and use that.

In that case a level of indirection with range validation in a lookup table (array) might be useful. Consecutive enums as the key to the actual values in a array.

 

I came across this which gives some ways of doing this. Answer 5 gets into stuff that I didn't know you could do.
https://stackoverflow.com/questions/11040964/check-if-a-value-is-defined-in-an-c-enum