Q104650: INFO: Default Assignment Used, Not User-Defined operator=()

Article: Q104650
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kb
Last Modified: 29-NOV-2001

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The information in this article applies to:

- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0 
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SUMMARY
=======

The Microsoft "C++ Language Reference" for C/C++ version 7.0 states the
following:

  ...if the class declares a user-defined operator=() that takes an argument of
  type "reference to class-name", no default assignment operator is generated.

(See the Memberwise Assignment and Initialization section of the Special Member
Functions chapter.) This statement may be confusing. However, the term
"argument" means formal parameter, not actual parameter.

MORE INFORMATION
================

The following program outputs "Default assignment used". The default assignment
operator is used for the assignment and not Derived::operator=() as might be
expected. The Derived::operator(const Base &) function specifies a const
Base & for a formal parameter. Because there is no operator=() that
specifies a formal parameter of type "reference to class- name", the default
assignment operator is generated and used.

Some compiler vendors have interpreted the C++ language differently by using the
term "argument" in the specification to mean actual parameter. In that case, the
default assignment operator is not generated and the program above will output
"Derived::operator=() called" because an object of type Derived is an object of
type Base.

Sample Code
-----------

  /* Compile Options needed: None
  */ 

  #include <iostream.h>

  char* message1 = "Derived::operator=() called";
  char* message2 = "Default assignment used";

  class Base
  {
  public:
     char* OperatorCalled;
     Base() { OperatorCalled = message2; }
  };

  class Derived : public Base
  {
  public:
     void operator=(const Base&) { OperatorCalled = message1; }
     Derived() {}
  };

  void main()
  {
     Derived first, second;
     first = second;
     cout << first.OperatorCalled;
  }

Additional query words: 8.00 8.00c 9.00 9.10

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Keywords          : kbCompiler kbCPPonly kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC16bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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