Re: assignment of const class members

From:

"Tadeusz B. Kopec" <tkopec@NOSPAMPLEASElife.pl>

Newsgroups:

comp.lang.c++

Date:

21 Dec 2007 21:46:12 +0100

Message-ID:

<476c2614$1@news.home.net.pl>

On Fri, 21 Dec 2007 01:38:09 -0800, Salt_Peter wrote:

On Dec 21, 3:38 am, hweek...@yahoo.com wrote:

hi,

it seems i can't assign the const variable u in class A, one way to
solve the problem may be to build a copy constructor. however, why does
C++ or vector class not like this code? my g++ is: gcc version 4.0.1
(Apple Inc. build 5465). thanks for the help.

summary of compile error:
--------------------------------------- cpp.C:4: error: non-static
const member 'const unsigned int A::u', can't use default assignment
operator /usr/include/c++/4.0.0/bits/vector.tcc:260: warning:
synthesized method 'A& A::operator=(const A&)' first required here

code:
-------
  1
  2 #include <vector>
  3
  4 struct A {
  5 A(const unsigned int a) : u(a) { } 6 private: const unsigned
  int u;
  7 };
  8
  9 int main () {
10
11 std::vector<A> y;
12 y.push_back(A(2));
13 }

[ snip ]

One of the requirements of containers such as std::vector is that its
elements be copyable and assigneable. Your type has a const member, so
default assignment fails (if you think about it - thats safer and quite
brilliant). Solution: define your own op=(...) assuming that assigning
that const member is ok.

In your code (below) the member isn't const any more. Assigning const
member isn't OK. If it ceases to be const you needn't define assignment
operator - generated one will do.

struct A
{
  A(const unsigned a) : u(a) { }
  A& operator=(const A& rhs)
  {
    if(&rhs == this) // self check!
      return *this;
    u = rhs.u;
    return *this;
  }
private:
  unsigned u;
};

--
Tadeusz B. Kopec (tkopec@NOSPAMPLEASElife.pl)
The trouble with being punctual is that nobody's there to appreciate it.
-- Franklin P. Jones