Is there a principle for choosing between embedding a struct itself or the pointer to a struct inside a struct?

Question

This is a code snippet from qemu.(qemu-5.1.0 include/hw/arm/smmu-common.h)

typedef struct SMMUDevice {
    void               *smmu;
    PCIBus             *bus;
    int                devfn;
    IOMMUMemoryRegion  iommu;
    AddressSpace       as;
    uint32_t           cfg_cache_hits;
    uint32_t           cfg_cache_misses;
    QLIST_ENTRY(SMMUDevice) next;
} SMMUDevice;

I've seen many such codes until now but I am now curious if there is any principle/rule in choosing between

1. embedding a struct A inside a struct B
2. embedding a pointer to the struct A inside a struct B

Two things that come to my mind right away is that if a struct A is to be shared by many structs, it is better to use pointer. or if the struct containing the struct(that is, struct B) is to be frequently passed as a function argument, it would be better to use pointer(pointer to struct B as argument, or pointer to A inside struct B and struct B is the argument) because copying the struct to stack would take long time.

I am curious if there are other important rules.

Answer 1

There's no correct answer because it depends on what you want to use them for. Storing a struct inside another struct is generally more efficient, since it gives faster access and better data cache use.

However, it isn't as flexible. If you wish to swap out the whole contents of a big struct for something else, it goes much faster to just swap two pointers than doing a hard copy of all the data. Pointers also enable different forms of allocation - you could have a static storage struct with a pointer at dynamically allocated memory for example.

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if a struct A is to be shared by many structs, it is better to use pointer

I don't see how that matters at all. It's just a . vs -> notation by the code using it.

or if the struct containing the struct(that is, struct B) is to be frequently passed as a function argument, it would be better to use pointer

No that's nonsense, you'd always pass the outer struct through a pointer no matter what members it got. Passing it by value doesn't make any sense in either scenario.