Gist to explain DecreaseMember

main.cpp

/**
 * Copy <cs_Math> from bluenet repository and if not present add to it the following function.
 
   template<class T, class M, class U=int>
   constexpr inline decltype(auto) DecreaseMember(T& obj, M member, U diff=1) {
	   obj.*member = SafeAdd(obj.*member, -diff);
	   return obj.*member;
   }

 * Compile with g++ -I. main.cpp -o main
 */

#include <cs_Math.h>
#include <iostream>

#define TRACKED_DEVICE_TOKEN_SIZE 3
#define NOT_SET 0

// Set to 1 for correct solution, set to 2 for incorrect solution
#define TEST_PACKED 1

struct
#if TEST_PACKED != 0
__attribute__((packed))
#endif
register_tracked_device_packet_t {
	uint16_t deviceId;
	uint8_t locationId = 0;
	uint8_t profileId;
	int8_t rssiOffset = 0;
	union __attribute__((packed)) {
		struct __attribute__((packed)) {
			bool reserved : 1;
			bool ignoreForBehaviour : 1;
			bool tapToToggle : 1;
		} flags;
		uint8_t asInt;
	} flags;
	uint8_t deviceToken[TRACKED_DEVICE_TOKEN_SIZE];
	uint16_t timeToLiveMinutes = 0;
};

struct
#if TEST_PACKED != 0
__attribute__((packed))
#endif
internal_register_tracked_device_packet_t {
	register_tracked_device_packet_t data;
	uint8_t accessLevel = NOT_SET;
};

struct
#if TEST_PACKED != 0
__attribute__((packed))
#endif
TrackedDevice {
	uint8_t fieldsSet = 0;
	uint8_t locationIdTTLMinutes;
	uint8_t heartbeatTTLMinutes;
	internal_register_tracked_device_packet_t data;
};

int main() {
	int t = 0;
	TrackedDevice device;
	device.data.data.timeToLiveMinutes = 5;
	while(t < 10) {
#if TEST_PACKED == 0
		if (CsMath::Decrease(device.data.data.timeToLiveMinutes) == 0) {
			std::cout << "Timeout " << std::endl;
		}
#elif TEST_PACKED == 1
		if (CsMath::DecreaseMember(device.data.data, &register_tracked_device_packet_t::timeToLiveMinutes) == 0) {
			std::cout << "Timeout " << std::endl;
		}
#elif TEST_PACKED == 2
		if (*CsMath::DecreaseByPointer(&device.data.data.timeToLiveMinutes) == 0) {
			std::cout << "Timeout " << std::endl;
		}
#else
#warning "Not implemented"
#endif
		std::cout << "Time to live: " << device.data.data.timeToLiveMinutes << std::endl;
		t++;
	}
	return 0;
}