Structure without a tag

问题

If I declare a struct like this:

struct
{
   int a;
   char b;
} ident;

does that structure has a type? (i.e. an unspecified type, a default type, etc.).

Instead If I declare a struct like:

struct J
{
  int a;
  char b;
} ident; 

we can say that ident is a stucture variabile of type struct J.

回答1:

After

struct { int a; char b; } ident;

ident has an “anonymous structure type” and you won’t be able to declare another variable of the same type^*). That is, two anonymous structure types are never compatible. If you did, for example,

struct { int a; char b; } ident2;

afterwards, ident and ident2 would be of different (though identical) type and an assignment like

ident2 = ident;

would be a constraint violation (a compiler must give a diagnostic message and may refuse to compile, what most compilers probably will).

---

^*) Some compilers have extensions to allow it. For example, with Gcc, you can do:

typeof(ident) ident2 = ident;

来源：https://stackoverflow.com/questions/25871851/structure-without-a-tag