Compiler generated assignment operator= question

Damir Valiulin

Suppose I have a class with a few member variables, and one of them is a CArray object, e.g.:

CMyClass {
...
private:
  int var1, var2, var3;
  bool flag1, flag2, flag3;
  CArray  arSomeArray;
};

If I didn't have that CArray member, I could simply write:

CMyClass a, b;
a = b;

The assignment operator would be simply generated by compiler. But when CArray is there, compiler cannot generate default assigment operator, so I have to add to the class:

CMyClass& operator= (CMyClass& src){
  var1 = src.var1;
  var2 = src.var2;
  var3 = src.var3;
  flag1 = src.flag1;
  flag2 = src.flag2;
  flag3 = src.flag3;
  arSomArray.Copy (src.arSomeArray);
  return *this;
}

Not only that it is very tedious writing a few dozen of "x = src.x", it is also prone to bugs. Like later I will add a variable to the class and might forget to add it to the operator= function. Is there a way to write something like below?

CMyClass& operator= (CMyClass& src){
  dear_compiler_do_what_you_can_with_all_the_variables_i_ll_take_care_of_the_rest();
  arSomArray.Copy (src.arSomeArray);
  return *this;
}

Thanks

Ryan_Roberts

Not using CArray would be a good start :-D MFC's container classes suck. Try using a stl container instead, it might just change your life. Ryan

John M Drescher

I agree 100%. I've used the MFC containers for years but STL is much more powerful and more portable. I wish I switched a long time ago. Try std::vector John

Dave Bryant

You could declare all the members you want automatically copied in a structure defined within the class - C++ will then generate an assignment operator for that structure. e.g. class Blah { private: struct Hidden { int a; int b; }; Hidden h; CArray<int,int> array; Blah& operator=(const Blah& copy) { h = copy.h; // Copy the array... } }; However, as others have pointed out, if you use STL you don't run into these problems. Dave http://www.cloudsofheaven.org

Damir Valiulin

Thanks, but I am not going to rewrite a few hundred of classes, possibly breaking several projects along the way, just for this cause. Having CArray with all its upsides and downsides is a must.

John M Drescher

I know what you mean. I have a lot of code in several DLLs that sill has MFC containers and in most cases it is better not to rewrite but I wish I would have known stl in the first place. Anything new I will use stl containers instead of MFC. John

Damir Valiulin

The idea is good, however separating some variables into a structure just for coding convinience would not make sense to someoneelse reading the code.

Dean Goodman

What about writing a quick subclass of CArray that defines operator= and simply calls CArray::Copy()? That would still be compatible with your other class that just use CArray. --Dean

Dave Bryant

I agree it would look weird and non-intuative - which is while i recommend STL. Dave http://www.cloudsofheaven.org

John M Drescher

There is a problem in your assignment operator. If someone assigns an object to itself your code will fail on dynamic data such as CArray. For example: CMyClass A; A=A; //What happens to arSomArray? To solve this I create a private destroy() and copy() functions and implement the copy constructor and assignment operator as follows: PROPERTY::~PROPERTY() { destroy(); } PROPERTY::PROPERTY(PROPERTY & other ) { copy(other); } PROPERTY& PROPERTY::operator =(PROPERTY & other ) { if ( &other != this ) { destroy(); copy( other ); } return *this; } copy() does the copy of data from one class to the other similar to what you did in your assignment operator destroy() deletes any dynamically allocated data. I have an idea to solve your problem that I will post a little bit later. John

Damir Valiulin

Thank you for the reminder. I remember I've read some article on CodeProject about dangers of CArray such assigning to itself or from reference to itself. I will keep this in mind. As for the answer to my problem, Dean Goodman already answered it.

Damir Valiulin

Thank you. This is exactly the answer I was looking for! I derived a class from CArray and added operator= to it. Now compiler has no trouble generating default assignment. Everything works perfectly! :-D

John M Drescher

I was just writing some code for my idea... Anyways make sure you provide a copy constructor otherwise the complier would provide one and circumvent your code for the following code CMyClass a; CMyClass b = a; This code will not call the = operator. It uses the copy constructor... John