Hi

I find that some sample write some code like this

typedef struct _X
{
int a;
int b;
} X;


struct X A,B;

A.a = 1;
A.b = 2;
......

B=A;


I found this code works and I can change struct to class and test object
copy in
C++ sample.

I used to think we should use memcpy for structure and class object copy .
Can I use operator "=" handle object copy ?

Thank for your teaching.

Yes.

Default compiler-generated "operator =" is defined as calling "operator =" on
all fields and base class instances.

For a simple non-class type like int or pointer, "operator =" is a bitwise
copy.

So, in C++, if there is a C struct without "operator =" defined, then "=" will
mean bitwise copy field by field, which is the same meaning as in C.

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation
maxim@storagecraft.com
http://www.storagecraft.com

are you sure that it actually calls overloaded operator='s on all fields in
C++. IIRC, the default operator= just does a bitwise copy for the size of
the structure

d

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"Maxim S. Shatskih" <maxim@storagecraft.com> wrote in message
news:OMrcCvTwIHA.5832@TK2MSFTNGP02.phx.gbl...

Structure copy has always been legal and should be preferred over
memcpy since it is more clear, concise, and less error prone. You
sound very new to programming. Tip: simplify your code if using .cpp
as below by removing the unnecessary baggage. The less stuff you have
to read the quicker and easier the code is to understand.

struct X
{
int a;
int b;
};

X A;

A.a = 1;
A.b = 2;

X B(A); // an even better way to copy a struct

There is no requirement to change the word "struct" to "class" in C++.
"struct" can do everything "class" can--inheritance, private data,
functions inside, etc. When I learned the only subtle difference
between the two, I decided never to use "class" again.

Yes. And in C++ you can add intelligence to "=" by defining this
operator if appropriate. With C++ you even have the flexibility to
DENY structure copy if you don't want it such that A=B fails! Or you
can define structure equality "if (A==B)". The sky is the limit. Being
restricted to C means one must use this hack: if
(IsEqualGUID(a,b)) ... but if you use C++ you can do this: if (a ==
b) ... much better.

This is according to good old book of Stroustrup/Ellis of early 90ies.

Compiler-generated "operator =" is generated as a sequence of invocations of
"operator =" first for all base class instances in the declaration order of the
base classes, then to all fields in declaration order of the fields.

In this context, "invocation" is a call if the particular "operator =" function
really exists, i.e. it is present in the code or is compiler-generated. If the
function does not exist (i.e. simple type like int or pointer) - it is a
bitwise copy.

The book is old, but I do not think such a basic notions could change since
that times.

So, "operator =" turns to memcpy only if there are no manually written
"operator =" for any base classes or fields, or there are no base classes at
all.

In the other words: let's introduce the notion of "simple-copied" type. The
type is simple-copied if either:
a) it is a non-class type like int or pointer
b) all its fields and all its base classes are simple-copied

operator= is memcpy only for simple-copied classes.

--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation
maxim@storagecraft.com
http://www.storagecraft.com

440gtx@email.com wrote:
The main reason to use "class" vs. "struct" is a social and commenting
one. "class" communicates to other programmers that the object is
probably not a simple data structure, but has significant semantics.
Whereas "struct" communicates to other programmers that the object is
(almost entirely) just a data buffer.

People that override operator"=" (well... ever, but especially) for
structs deserve to be shot. Or to maintain their code for the rest of
their sorry lives, which will make them *wish* they'd been shot.
--
Ray