CODEWITHSUNDEEP
c++11
Bulletmagnet
Bulletmagnet

Reputation: 6010

How many bits does an enum value need?

#include <stdint.h>

enum state : uint8_t {
    NONE,
    USA,
    CAN,
    MEX
};

struct X {
    state st : 2;  // compiles with uint8_t st : 2
};

Clang 3.9.0 compiles successfully.

GCC 4.8.4 and 5.3.0 complain with:

warning: ‘X::st’ is too small to hold all values of ‘enum state’

Who is right?

Upvotes: 4

Views: 519

Answers (1)

DevSolar
DevSolar

Reputation: 70263

TL;DR

Both are correct.

The value of an enumeration is limited by the underlying type, not by the enumerators!

C++14, 7.2 Enumeration declarations, paragraph 8:

It is possible to define an enumeration that has values not defined by any of its enumerators.

Which means it is possible to:

state x = static_cast< state >(5);

That is what GCC is warning you about: enum state could have values that do not fit into 2 bits.

However, as long as you don't try to actually do that to X::st, everything is shiny.

That's (probably) why Clang is not warning you about it.

Since the standard does not demand a diagnostic either way, there's nothing wrong about warning, or not warning you.

Upvotes: 1

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