Why define a _struct and then typedef it without _?

Question

I was browsing the libgphoto2 repositories and saw this:

struct _PTPObjectHandles {
    uint32_t n;
    uint32_t *Handler;
};
typedef struct _PTPObjectHandles PTPObjectHandles;

Why would somebody want do that instead of just calling the struct PTPObjectHandles and omitting the whole typedef line?

EDIT: I should probably note that _PTPObjectHandles is never again used in the code.

Answer 1

The most common reason for doing this is simply to make it shorter to type and begin to look like a "real data type".

EG, this:

struct _PTPObjectHandles *ptr1;
int something;
struct _PTPObjectHandles *ptr2;

Simply looks "cooler" if you rewrite it as:

PTPObjectHandles *ptr1;
int something;
PTPObjectHandles *ptr2;

That's the main reason.

However, it also provides the library developer that is making you use it the ability to re-typedef it as something else in the future. I've seen cases (eg, OpenSSL) that changed a typedef from a real struct to a pointer to a struct (for example). (Granted, that didn't work, but you can imagine the case where a typedef does actually change and it would work.)

So, do you have to? No.

Do people do it to try and make code more readable? Yes.

Note that a C++ class is actually doing about the same thing. If you go read a good C++ from the ground up kind of book, you'll find it first starting with a struct and then changing the 'struct' word to 'class' and starting to instantiate it using the straight name.

Edit: And... make sure you read the comment from @Bathsheba below and check proper naming conventions within C. As he points out, it should be _pTPObjectHandles with a lower-case p. For my own style, I avoid using uppercase typedefs too simply because I like my upper-case types to be true classes in C++ and want to know when they're just typedefs. But, again, that's a style choice.

Answer 2

It's partly so you can forward reference the structure inside itself but also use a meaningful name in the rest of the code, this:

struct _mylist
{
    struct _mylist *next;
    /* etc ... */
};

typedef struct _mylist MyList;

Note that switching the order round makes things a little more readable, but you still need the two declarations, thus:

typedef struct _mylist MyList;

struct _mylist
{
    MyList *next;
    /* etc ... */
};

What you can't do is this:

typedef struct { MyList *next; } MyList;

so you have to have the 2 declarations.

I tend to use something similar to the first form, like this:

typedef struct _mylist
{
    struct _mylist *next;
    /* etc ... */
} MyList;

as it makes it fairly clear the two types are meant to be the same.

Answer 3

after that line of code, you can create struct by just using PTPObjectHandles. creating that struct

without typedef struct _PTPObjectHandles aStruct;
with typedef: PTPObjectHandles aStruct;

Answer 4

This pattern is used to say "_PTPObjectHandles is an internal (technical) name, don't use it. Use PTPObjectHandles instead."

Generally, in C, a leading underscore means "this is internal, go away". It's a cheap solution for a language that doesn't have proper name spaces (note: I'm talking about plain C here, not C++).