[boost] [move] auto-generated operator=

Subject: [boost] [move] auto-generated operator=
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2009-09-03 02:55:02

Hi (again) Ion,

I recently ran into the following issue.  I defined a 
copyable-and-movable class X, and class Y has a member variable of type 
X and no other member variables.  It thus (apparently) happens that Y's 
auto-generated operator= takes a non-const reference to Y, which I would 
guess is generally not desired, or at least an operator= overload taking 
a const reference to Y should be available.

For whatever reason, MSVC9 didn't complain at all about using the 
auto-generated operator=, but when a friend tried compiling code with 
GCC on Ubuntu, GCC complained about no matching function call to 
operator=(const Y) (or something like that).  The error occurred within 
a concept checking class, and I would've assumed that MSVC9 was looking 
at the same code and should've come to the same conclusion; I haven't 
investigated this further.  In any case, we added a trivial 
Y::operator=(const Y&) explicitly, but GCC didn't like this overload, 
and it finally quieted down once we added *both* Y::operator=(Y&) and 
Y::operator=(const Y&).  Unfortunately, MSVC9 gives warnings about 
several operator= overloads, so right now we only have those explicit 
overloads of operator= when compiling with GCC.

The only point I want to make is that it seems worth mentioning this 
subtlety (at least it was a subtlety to me) in the move documentation 
somewhere, and perhaps how to most easily and portably address this (I'm 
not sure what that would be), as I would expect it to be a not uncommon 
situation.

- Jeff