using struct keyword in variable declaration in C++

Question

I have a feeling this may be related to C syntax, but I started my programming life with C++ so I am not sure.

Basically I have seen this:

struct tm          


        

Answer 1

In C, the struct tag names do not form identifiers on the global name space

struct not_a_global_identifier { /* ... */ };

To refer to that struct you have to use the keyword struct (to specify the name space)

struct not_a_global_identifer object;

or create a new identifier, in the global name space, with typedef

typedef struct not_a_global_identifer { /* ... */ } global_name_space_identifier;

There are 4 namespaces in C, see 6.2.3 in the C99 Standard:

• label names
• the tags of structures, unions, and enumerations
• the members of structures or unions (not a single name space ... as many as structures or unions are defined)
• global name space, for all other identifiers

This is a legal C program :-)

int main(void) {
  typedef struct foobar { int foobar; } foobar;
  foobar boo;
  boo.foobar = 42;
  if (boo.foobar) goto foobar;
foobar:
  return 0;
}