C structs don't define types?

Question

I\'ve just started learning C with a professional Java background and some (if no too much) C++ knowledge, and I was astonished that this doesn\'t work in C:

str         


        

Answer 1

So...

Point           a tag
struct Point    a type

typedef struct {
    . . .
} Point_t;

Point_t         a type

I often see a why? written between the lines. After all, it does seem perfectly reasonable to just say Point x;, so why can't you?

As it happens, early implementations of C established a separate name space for tags vs other identifiers.There are actually 4 name spaces¹. Once the language was defined this way, it was then not possible to allow the struct tag to be used as a type because then all existing code with name collisions between ordinary identifiers and tags would be suddenly in error.

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^{1. The 4 name spaces are:
-- label names (disambiguated by the syntax of the label declaration and use);
-- the tags of structures, unions, and enumerations (disambiguated by following any) of the keywords struct, union, or enum);
-- the members of structures or unions; each structure or union has a separate name space for its members (disambiguated by the type of the expression used to access the member via the . or -> operator);
-- all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumeration constants)}