Structure tag names share the same name space with union tags and enumeration tags (but enums within a structure are in a different name space in C++). This means that such tags must be uniquely named within the same scope. However, tag names need not differ from identifiers in the other three name spaces: the label name space, the member name space(s), and the single name space (which consists of variables, functions, typedef names, and enumerators). 

Member names within a given structure or union must be unique, but they can share the names of members in other structures or unions. For example

goto s;
    .
    .
    .
s:           // Label
struct s {   // OK: tag and label name spaces different
   int s;    // OK: label, tag and member name spaces different
   float s;  // ILLEGAL: member name duplicated
} s;         // OK: var name space different. In C++, this can only
             // be done if s does not have a constructor.
union s {    // ILLEGAL: tag space duplicate
   int s;    // OK: new member space
   float f;
} f;         // OK: var name space
struct t {
   int s;    // OK: different member space
    .
    .
    .
} s;         // ILLEGAL: var name duplicate