CODEWITHSUNDEEP
c++c++14
Mihai Galos
Mihai Galos

Reputation: 1897

Restrict enum class number of bits

In C++14, I would like to restrict the total number of bits held by an enum class:

enum class InstalledCapacity : uint8_t{
  None,
  One_1000uF,
  Two_1000uF,
  Three_1000uF,
  One_1F,
  One_3_3F,
  Reserved2,
  Invalid
};

using HarvestingCapability = uint8_t;

typedef struct {
  InstalledCapacity installed_capacity : 3;
  HarvestingCapability harversting_capability_x_15mW : 5;
}EnergyInfo;

This does not seem to work, and I get the following warning:

eeprom_metadata.h:51:42: warning: '<anonymous struct>::installed_capacity' is too small to hold all values of 'enum class InstalledCapacity'
   InstalledCapacity installed_capacity : 3;
                                          ^

Since I only have 7 values in my InstalledCapacity enum class, I would expect to be able to only use 3 bits for it.

What am I doing wrong, is this even possible? Many thanks in advance!

Upvotes: 4

Views: 764

Answers (1)

user975989
user975989

Reputation: 2648

Nothing is wrong, the compiler is just commenting that 3 bits is too small to hold all possible values of the enum, which can be as large as 8 bits. Just because all of your named enumeration values can fit into 3 bits doesn't mean that all the possible values of InstalledCapacity can fit into the bitset. A value of 255 is perfectly valid for the enum, but cannot fit into your bitset.

Upvotes: 5

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