Re: Copy Constructors and Assignment Operator, When should I use?

Ulrich Eckhardt <>
Mon, 03 Mar 2008 15:02:06 +0100
rockdale wrote:

Sorry my code example has a lot of typos. I extract my current code
and changed those classes names.

So from what you wrote:

No. It works for any datatype that defines its own copy constructor
that you don't have to invoke explicitly, like any standard container,
strings, any POD, any classes containing those (which is actually
kind of recursive).

ItemSet set1;

ItemSet set2;

I simply assign set1 to set2 like this
set2 = set1;

will this assignment will copy whatever in set1 to set2? (deep copy,
even the vector items?),

well my coworker told me that I need to have a define my own copy
constructor for ItemSet, ItemListA and ItemListB for it works

in the copy contructor of ItemSet,

ItemSet(ItemSet& toCopy){
//copy constructor
  a = toCopy.a;
  b = toCopy.b;

Two things here:
1. A copy constructor usually takes a reference to a const object, not a
plain reference. If the book or the coworkers you are learning from have
this wrong I would strongly suggest replacing/upgrading them. Getting this
wrong means getting the very basics of C++ wrong and that is a really bad
2. If ItemSet only contains the fields 'a' and 'b', this copy constructor
almost does the same as the compiler-generated one would do (only that it's
using assignment instead of initialisation), so there is no need to write
it manually.

The compiler-generated one would look like this:

  ItemSet(ItemSet const& toCopy):

I need to define copy contructor in my ItemListA and ItelListB classes,
basically, to manually the vector from the toCopy, is his statement true?

Hard to say if this is true, I seriously don't understand what you mean

struct ItemA{
        int aInt;
        std::string aString;
} ;

struct ItemB{
        int bInt;
        std::string bString;
        time_t bTime;
} ;

Both of these types are freely copyable and assignable and you don't have to
define any copy constructors or assignment operators.

void load(){

One thing please: do not retype code but use cut'n'paste. The above is
syntactically wrong and this makes it impossible to guess what code you are
referring to.

std::vector <ItemA> m_vecA;

std::vector is also freely copyable and assignable, provided that is true
for its elements. This last restriction e.g. applies to vectors of
pointers, which are copyable but which only copy pointers and _not_ the
objects they point to.

[full quote]

Please refrain from top posting. The quoted text is for you to reference it,
if you don't use it please delete it. Google for "Usenet netiquette".


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