Is there a difference between these two stuct-related declarations?

Question

I have the following structure

typedef struct _LSHFunctionT 
{
    double *a;
    double b;
} LSHFunctionT, *PLSHFunctionT;

My question is; is there a difference between these two declarations

PLSHFunctionT myPointer1;

and

LSHFunctionT *myPointer2;

and if not, then why do people explicitly use two of them (LSHFunctionT and *PLSHFunctionT). Why not just use LSHFunctionT.

Does it go the same way for the following two declarations

PLSHFunctionT *myPointer3;

and

LSHFunctionT **myPointer3;

And to your question "why do people explicitly use two of them (LSHFunctionT and *PLSHFunctionT)", I think some people just find it "cleaner" to write `Pwhatever ptr` than `whatever* ptr`, because they don't like asterisks or something. ::shrug:: — Mr Lister, Feb 06 '12 at 18:30

score 2 · Accepted Answer · edited May 23 '17 at 12:27

typedef struct _LSHFunctionT 
{
    double *a;
    double b;
} LSHFunctionT, *PLSHFunctionT;

Yes, PLSHFunctionT x; is equal to LSHFunctionT* x;

And yes, PLSHFunctionT* x; is equal to LSHFunctionT** x;

The purpose of typedef is to assign new names to existing types. You can define typedef int lol; and declare variable lol i;, but compiler will consider it int anyway.

You should also check these questions:
When should I use typedef in C++?
Why should structure names have a typedef?

Hope this helps.

score 2 · Answer 2 · answered Feb 06 '12 at 18:29

2

Yes, they are identical.

One good reason to define a pointer type is for complicated expressions. If for example you have a function that takes a reference to a pointer, which do you find easier to understand?

void foo(PLSHFunctionT & ref);

void foo(LSHFunctionT * (&ref));

I'm not even sure I got the syntax correct for the second one!

answered Feb 06 '12 at 18:29

Mark Ransom

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`void foo(LSHFunctionT * & ref)` is fine. No need to use extra-braces. – Nawaz Feb 06 '12 at 18:31
@Nawaz, thanks. But you see my point - it's hard to remember if it's not something you do every day. And I did have it incorrect at first but I did a quick edit. – Mark Ransom Feb 06 '12 at 18:32
Yup. Its easy to make mistake. One may try `void foo(LSHFunctionT & * ref)` which is a compilation error. But it is equally possible when you use `typedef`. e.g `typedef T& RT;void foo(RT * ptr)` which is compilation error. – Nawaz Feb 06 '12 at 18:36

perreal · Answer 3 · 2012-02-06T18:51:34.467

1

The difference is in emphasis. Generally not explicitly writing the * may indicate that the PLSHFunctionT is designed to be used as a handle (without knowing/accessing structure elements ). If * is explicitly written, as in LSHFunctionT *myPointer, it might indicate an array or a structure that is to be used to access the values.

edited Feb 06 '12 at 18:51

answered Feb 06 '12 at 18:28

perreal

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score 1 · Answer 4 · answered Feb 06 '12 at 18:32

There's no difference between the two on the surface. However, with the pointer typedef there's no way to declare it as pointer-to-const, just const-pointer-to-non-const.

For example you can say

const LSHFunctionT* const_ptr; but const PLSHFunctionT const_ptr2; makes the pointer const, NOT the pointee.

Finally note that in C++ the whole thing is of questionable legality because names starting with _<capital> are reserved for the implementation, and that typedefs are almost never used in such a way.

score 0 · Answer 5 · answered Feb 06 '12 at 18:24

0

I seem there is no difference. There are different programming styles.

answered Feb 06 '12 at 18:24

mikithskegg

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Nawaz · Answer 6 · 2012-02-06T18:30:23.010

0

Yes. They are exactly same, even in the second case.

I personally prefer using * explicitly if I want to declare a pointer. It makes the code readable, in most cases. Usage of typedef of pointer-type reduces readability usually, though sometimes it may increases readability especially when you work with, say, Windows API.

edited Feb 06 '12 at 18:30

answered Feb 06 '12 at 18:25

Nawaz

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Ah yes, the Windows API, with its PSTR, LPSTR, PCSTR, LPCSTR, LPCTSTR, LPUTSTR, LPCUTSTR etcetera. Lovely. – Mr Lister Feb 06 '12 at 18:41

score 0 · Answer 7 · answered Feb 06 '12 at 18:32

0

Those declarations are the same. The pointer-typedef approach improves code readability in some cases. One could argue that this:

PLSHFunctionT calls[];

is easier to read than this:

LSHFunctionT *calls[];

answered Feb 06 '12 at 18:32

Krzysztof Kozielczyk

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Is there a difference between these two stuct-related declarations?

7 Answers7