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

The user-defined types have their own identifier space, i.e. when the compiler parse the file it stores each identifier in its corresponding space.

When you refer to tm, the C compiler (as the C++ one) will search for this identifier in the global identifier space. The C++ compiler will then lookup in the user-defined types identifier space if it hadn't found the symbol before.

Basically, if you want to have the same behavior as in C++, add this line :

typedef struct tm tm;

You can combine struct declaration and typedef like that :

typedef struct tm { int field } tm;

Or using a anonymous struct :

typedef struct { int field } tm;

The same behavior applies for enum and union :

typedef enum { VALUE } myEnum;
typedef union { int integer; char charArray[4]; } myUnion;