Possible Duplicates:
Why should we typedef a struct so often in C?
Difference between ‘struct’ and ‘typedef struct’ in C++?
What is the difference between the following type declarations?
struct Person { int age; }; typedef struct { int age; }Person; I understand that
struct { int age; }Person; Creates and instance of an unnamed struct called person, where
struct Person { int age; }; declares a type called person, but not an instance. But I still dont get what the typedef does.
I think that's the same as in C, typedef creates an alias of a type... in your first case, the name of the type is "struct Person", while in the second case is just "Person".
Usually, when you have to declare self referencing structures (like lists), you use both, because the typedef has not effect until the structure is defined (unless you make a forward declaration), for example:
typedef struct node { void *data; struct node *next; } TNode, *PTNode; so now you can declare variables of the same type in the following ways:
struct node *node1; TNode *node2; PTNode node3; the three variables above are the same, pointers to the node structure.
struct X {} can be referred to as X (while in C, it must be referred to as struct X). So in C++, there is no need for the typedef at all. Except when you write headers that are supposed to be parsed by a C compiler, too, this is just a C-ism that people use without thinking (or knowing) about.In C, structs live in their own name space, so you have to write struct Person if you want to use the struct's type name. The typedef eliminates the need for that prefix.
In C++, structs live in the same name space as everything else, so there's no need to do this. It's usually seen as an unnecessary C-ism.
you can tell the difference when passing Person or struct Person to a function
If you defined
struct Person { ... }; You should pass it to a function explicitly stating that it is a struct
void foo(struct Person person) { ... } On the other hand, if you defined
typedef struct Person { ... }; You must not state that it is a struct, since the keyword typedef allows you to actually define a data type, in this case Person and you pass it this way
void foo(Person person) { ... } pretty much like any other data type
Hope this helps
In C++ typedefs are synonyms for other types rather than distinct types. They are generally used for two purposes:
Creating portable code where you abstract the platform specific away. On one platform you can have typedef int int32 and on another platform typedef long int32. You would use int32 throughout your code and when porting to the a new platform you would only have to modify the typedefs to match your requirements.
Creating aliases for very complicated types. E.g. typedef char const* const* cppchar. Instead of writing char const* const* in your code you can now write cppchar.
