Within a single source file, each structure or union definition creates a new type that is neither the same as nor compatible with any other structure or union type. However, a type specifier that is a reference to a previously defined structure or union type is the same type. The tag associates the reference with the definition, and effectively acts as the type name. To illustrate this, only the types of structures j and k are compatible in this example:
struct { int a; int b; } h;
struct { int a; int b; } i;
struct S { int a; int b; } j;
struct S k;Compatible structures may be assigned to each other.
Structures or unions with identical members but different tags are not compatible and cannot be assigned to each other. Structures and unions with identical members but using different alignments are not also compatible and cannot be assigned to each other.
Since the compiler treats enumeration variables and constants as integer types, you can freely mix the values of different enumerated types, regardless of type compatibility. Compatibility between an enumerated type and the integer type that represents it is controlled by compiler options and related pragmas. For a full discussion of the ENUM compiler option and related pragmas, see ENUM and #pragma enum in the ILE C/C++ Compiler Reference.
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