Constexpr constructor for a struct with bitfields

Question

struct data { uint8_t nibble1 : 4, nibble2 : 4; constexpr data() { nibble1 = 2; nibble2 = 4; } };

This gives me the following two compilation errors on GCC 9.2.0:

error: member 'data::nibble1' must be initialized by mem-initializer in 'constexpr' constructor error: member 'data::nibble2' must be initialized by mem-initializer in 'constexpr' constructor

But I'm pretty sure my constructor intialises both of them. I've taken a look at https://en.cppreference.com/w/cpp/language/constexpr and I don't see any requirements which my constexpr constructor doesn't satisfy.

How can I get rid of this error?

You are assigning to both of them. You are not initializing them with the values. — walnut
– walnut, Commented Dec 30, 2019 at 15:20

Holt · Accepted Answer · 2019-12-30 15:24:30Z

You need to initialize the members with a member initializer list:

constexpr data() : nibble1(2), nibble2(4) { }

The page you linked show the following requirements for constexpr constructors:

for the constructor of a class or struct, every base class sub-object and every non-variant non-static data member must be initialized.

But this:

nibble1 = 2;

...is not an initialization for nibble1, it's an assignment. The only way to initialize member variables is to use a member initializer list or to default them (which is not possible for bit-fields until C++20 I think).

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