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I saw such union inside struct definition in pure c code in Linux kernel sources struct cma_multicast (it's not the only one place. Seems that it is some common practice):
struct cma_multicast {
struct rdma_id_private *id_priv;
union {
struct ib_sa_multicast *ib;
} multicast;
struct list_head list;
void *context;
struct sockaddr_storage addr;
struct kref mcref;
};
But I can't figure out what is the purpose of union with only one member inside the struct? Why can't we just type struct ib_sa_multicast *ib; ?
I read this post but it has no usage explanation and has C++ specificity only.
UPD:
Posted example from Linux kernel instead of proprietary code.
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A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.
You use a union when your "thing" can be one of many different things but only one at a time. You use a structure when your "thing" should be a group of other things.
A structure can be nested inside a union and it is called union of structures. It is possible to create a union inside a structure.
A union is a user-defined type similar to structs in C except for one key difference. Structures allocate enough space to store all their members, whereas unions can only hold one member value at a time.
This says in this case multicast can have only one polymorphic dispatcher.
It is object oriented programming made in C. The union is kept only for uniformity of naming.
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