c - I try to switch a specific struct by creating an int of it and can't find a fault -

i defined structure:

struct command_type {                        uint8_t a,b,command; }; // struct command_type  

and constants like:

#define _heartbeat ((struct command_type) {0x10,0x01,0}) 

i did use in funktions , use how wanted want switch funtion case should use labels. there defined int_of_command_type this:

#define int_of_command_type(a) ((unsigned int) (((int) ((a).a)) << 16) |\                                 (unsigned int) (((int) ((a).b)) << 8) | \                                 (unsigned int) (((int) ((a).command)))) 

when use function print out %u value variable of type struct command_type works perfect.

now did want this:

struct command_type command_type_var=_hearbeat;   ....some code ....  switch(command_type_var) {   case inf_of_command_type(_heartbeat):  .....                                         break;   default:  }; 

and following fault message: case label not reduce integer constant. know, how can round problem easy way, because not understand @ wrong when works testprints , not in case label position. i've got lots of defined commands , don't want change of them anyway don't understand how handle it. maybe (const int) or ever needed?

hope know quick fault , if there way round.

the constraint of switch statement requires according c11 standard integer expression::

6.8.4.2/1 controlling expression of switch statement shall have integer type.

command_type_var doesn't match constraint, struct (even if struct holds in integer, , if hold integer bit fields).

how solve it:

it's quite simple: convert struct int in switch, using macro:

switch(int_of_command_type(command_type_var))  // here use int { case int_of_command_type(_heartbeat):  // fortunately constant       break; default:        break;  }; 

of course, works if macro defined correctly: parameter can't take name of 1 of struct member, or otherwhise you'll end weird errors due unexpected substitutions (the parameter should renamed x):

#define int_of_command_type(x) ((unsigned int) (((int) ((x).a)) << 16) |\                                (unsigned int) (((int) ((x).b)) << 8) | \                                (unsigned int) (((int) ((x).command)))) 

live demo

edit: if, despite fact int_of_command_type(_heartbeat) calculated @ compile time, compiler doesn't recognize constant integer expression, should replace switch if/else sequences. case msvc 2015, seems use stricter rules of 6.6/6 not require implementation provide possibility use . operator on structure constants.