How to assign value to a struct with bit-fields?

bit-fields, c++

Solution

You should never rely on how the compiler lays out your structure in memory. There are ways to do what you want with a single assignment, but I will neither recommend nor tell you.

The best way to do the assignment would be the following:

static inline void to_id(struct CPUid *id, uint32_t value)
{
    id->Stepping         = value & 0xf;
    id->Model            = value >> 4 & 0xf;
    id->FamilyID         = value >> 8 & 0xf;
    id->Type             = value >> 12 & 0x3;
    id->Reserved1        = value >> 14 & 0x3;
    id->ExtendedModel    = value >> 16 & 0xf;
    id->ExtendedFamilyID = value >> 20 & 0xff;
    id->Reserved2        = value >> 28 & 0xf;
}

And the opposite

static inline uint32_t from_id(struct CPUid *id)
{
    return id->Stepping
         | id->Model << 4
         | id->FamilyID << 8
         | id->Type << 12
         | id->Reserved1 << 14
         | id->ExtendedModel << 16
         | id->ExtendedFamilyID << 20
         | id->Reserved2 << 28;
}

Problem

I have a struct with bit-fields (totally 32 bit width) and I have a 32-bit variable. When I try to assign the variable value to my struct, I got an error: error: conversion from ‘uint32_t {aka unsigned int}’ to non-scalar type ‘main()::CPUID’ requested. ``` struct CPUIDregs { uint32_t EAXBuf; }; CPUIDregs CPUIDregsoutput; int main () { struct CPUID { uint32_t Stepping : 4; uint32_t Model : 4; uint32_t FamilyID : 4; uint32_t Type : 2; uint32_t Reserved1 : 2; uint32_t ExtendedModel : 4; uint32_t ExtendedFamilyID : 8; uint32_t Reserved2 : 4; }; CPUID CPUIDoutput = CPUIDregsoutput.EAXBuf; ``` Do you have any idea how to do it in the shortest way? Thanks P.S. Of course I have more appropriate value of EAX in real code, but I guess it doesn't affect here.

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