struct and typedef in C versus C++

Question

I am currently using a C++ IDE for something that will need to work on C, and wanted to make sure that I won\'t have problems with this later on. After making the struct bel

Answer 1

typedef struct THIS_IS_A_TAG
{
    int a;
    int b;
} THIS_IS_A_TYPEDEF;

THIS_IS_A_TYPEDEF object1;     // declare an object.       C:Ok,     C++:Ok
struct THIS_IS_A_TAG object2;  // declare another object.  C:Ok,     C++:Ok
THIS_IS_A_TAG object3;         // declare another object.  C:Not Ok, C++:Ok

The reason for the typedef is because C programmers would like to be able to do that third thing, but they can't.

Answer 2

In both C and C++, the example construct is modestly pointless:

typedef struct test {
   int a;
   int b;
};

In C, this says there is a type struct test with the two integers as content. If there was a name between the close brace '}' and the semi-colon ';', then you would get some benefit from the keyword typedef; as it stands, the keyword typedef is redundant, and (if set fussy enough), GCC will warn you about it.

In C++, this says there is a type struct test; further, in C++, it creates a type test too (which does not happen in C). The keyword typedef can still be left out and the same result will be achieved.

The syntax is legal; it is not useful, that's all. The keyword typedef can be omitted without changing the program's meaning in the slightest.

You can do:

typedef struct test {
   int a;
   int b;
} test;

Now, in both C and C++, you have a type struct test and an alias for it test.

Answer 3

The difference between:

struct Name {};

And

typedef struct Name {} Name;

Is that, in C, you need to use:

struct Name instance_name;

With the former, whereas with the latter you may do:

Name instance_name;

In C++, it is not necessary to repeat the struct keyword in either case. Note that your example in which you create a typedef with no name (i.e. typedef struct Name{};) is non-standard AFAIK (if you use the keyword typedef, then you need to supply an alias to which to typedef that name).

As for the last variation:

typedef struct { /* ... */ } Name;

The code above creates an unnamed struct that is aliased to Name. You would use such a struct just the same way you would with typedef struct Name { /* ... */ } Name;, however compilers often emit the name of the struct (not the alias), and so you may get better error messages involving the struct if you give it a name and typedef that as opposed to typedef'ing an anonymous struct.