Re: Default ctor, etc generated by compiler for structs?

"JohnQ" <>
Sat, 16 Jun 2007 16:26:59 -0500
"Victor Bazarov" <> wrote in message

JohnQ wrote:

Are a default constructor, destructor, copy constructor and assignment
operator generated by the compiler for a struct if they are not
explicitely defined?

If by a struct you mean POD, then yes, but only conceptually.

Different from an identical class? Conceptually how?

If by a struct you mean any class where you only specify data
members, then it depends on the data members.

Lately, I've been pondering, "when does a struct stop being a struct?". To
me, that's when its not the same size as a C struct or not layed out the
same. So obviously, when a virtual function gets added, then "a struct stops
being a struct". I think I can add non-virtual member functions, non-virtual
destructor and operators ad infinitum and the "struct will still be a
struct" (right?). Even though some explicit syntax would make things more

struct A
    int data;

struct B
     int data;

    B(int val){ data = val; } // OK, not virtual

class StructWrapper
    A a_struct;

    A(int val){ = val; }
    operator A&(){ return a_struct; }

Where struct B can be used identically to struct A or StructWrapper (I
hope!) as the layouts and sizes are the same (assume the operator is being
used for StructWrapper when I say "can be used identically").


But a_struct in StructWrapper would have to be made public to guarantee the
same layout as A and B?

I think the answer is yes, because "there is no difference between a
struct and a class except the public/private access specification"
(and a few minor other things). When I create a class, I always start
by declaring the default constructor, copy constructor and assignment
operator private with no implementation. I don't do that for structs
though (I consider structs like they were in C, but they really are
not in the implementation as far as I know). I'm thinking that I
don't handle structs the same because I trust the compiler to do the
right thing in the case of structs whereas bitwise copy for a class
may not be what is desired, for example.

What's "bitwise copy"? I don't know of that concept in C++.

The equivalent of doing a memcpy of the class or struct (whether I used the
right terminology or not I don't know... "bytewise" seems more appropriate).

Is the above reasoning OK? Should I continue NOT declaring/defining
the default stuff for structs?

It's up to you. And the ability of the compiler to generate the
functions you don't declare/define depends on the data members.

OK, so from what I gather, the compiler may or may not generate the
functions/operators depending on the data members. Is it that if there is
any non-POD data member, then the functions/operators get generated? Is it
always the full set (?): default constructor, copy constructor, assignment
operator, (destructor?).


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