About the size of structure

[Albert Yan Qing Ge]

typedef struct AA {
} A;

when use gcc compiles, sizeof(A) is 0
when use g++ compiles, sizeof(A) is 1

So I can known the length of this pointer is 1

but when I add one member to the structure,
typedef struct AA {
int i;
} A;

Now if use gcc, sizeof(A) is 4
if use g++, sizeof(A) is still 4.

where is this pointer?

Thanks

 

[Rob Williscroft]

Albert Yan Qing Ge wrote in

typedef struct AA {
} A;

when use gcc compiles, sizeof(A) is 0
when use g++ compiles, sizeof(A) is 1

Yup C++ insists every type has a minimum size of 1, I don't know
about C so whether gcc output is correct I can't say.

So I can known the length of this pointer is 1

What pointer ?. There is non *special* pointer stored in every C++
user defined type.

but when I add one member to the structure,
typedef struct AA {
int i;
} A;

Now if use gcc, sizeof(A) is 4
if use g++, sizeof(A) is still 4.

The extra byte in the first example was just padding (or a filler byte
if you like), it'e not some special value (i.e. 'this').

where is this pointer?

Again what pointer !

HTH.

Rob.

 

[Jack Klein]

typedef struct AA {
} A;

when use gcc compiles, sizeof(A) is 0

GCC is a C compiler. The structure definition is illegal in C, where
structures with no members are not allowed. So you are not using GCC
as a conforming C compiler, consult your documentation or a gcc
newsgroup for how to do this.

when use g++ compiles, sizeof(A) is 1

Structs or classes with no members are allowed in C++, but 0 length
objects are forbidden. So a class or struct with no members must have
sizeof of at least 1. Otherwise it would be impossible to have an
array of such objects, as each array element must have a unique
address.

So I can known the length of this pointer is 1

Do you mean the "this" pointer that is passed to non-static member
functions? The sizeof value of a class never includes room for that,
it is not a member of the class or struct.

but when I add one member to the structure,
typedef struct AA {
int i;
} A;

Now if use gcc, sizeof(A) is 4
if use g++, sizeof(A) is still 4.

where is this pointer?

The "this" pointer is not EVER in the class or struct. It has no
effect on the size of a class or struct.

Thanks

You need to get a good book on C++.

--
Jack Klein
Home: http://JK-Technology.Com
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[JustSomeGuy]

Try #pragma pack(1)
before the structure definition... then check your sizeof(...)

 

[Ron Natalie]

Albert Yan Qing Ge wrote in

Yup C++ insists every type has a minimum size of 1, I don't know
about C so whether gcc output is correct I can't say.

The above struct is illegal in C. You aren't allowed to define
empty structs. GCC is wrong.