Re: Newbie: Exception safe assignment
On Sep 2, 12:08 pm, Barry <magnus.morab...@gmail.com> wrote:
I'm learning C++ and am currently trying to get my head around memory
leakage.
I have a class, C, which contains a number of vectors, as follows -
vector<A> a;
vector<B> b;
I'm trying to implement an assignment operator for this class which is
exception safe and am wondering if the following meets this criteria
and if I'm doing it correctly -
C& C::operator=(const C& c)
{
C temp(c);
temp.swap(*this);
return *this;
}
void C::swap (C &c) throw ()
{
std::swap(this->a,c.a);
std::swap(this->b,c.b);
}
In this case, there is no magic under the hood. operator= can throw
iff the copy constructor C::C throws or C::swap throws. C::swap throws
iff std::swap throws for a or b.
Also, I'm not sure I would suggest using the no-throws specification
"throw()" if you're unsure if it can throw. First off, some compilers
(I'm looking at you windows) don't actually correctly implement it.
The correct implementation is to call terminate or something if an
exception would be thrown and escape swap. (Windows visual studios
IIRC lets it escape without calling terminate or whatever.) I would
suggest using throw() only on functions which are actually no-throws,
and not as a runtime check as guaranteed by the standard. It's both
documentation and an optimization allowance to some implementations.
Small aside: Last time I checked, there is a "defect report" related
to swap use. If a or b are types defined in another library, then you
will be using std::swap and not the swap from that library. std::swap
will probably make a copy instead of efficiently swapping the
internals like the swap from the library for "a" would do. I think the
current suggested course of action is use std::swap for built-in types
or types from the standard library, but otherwise do:
void C::swap (C &c) throw ()
{
using std::swap;
swap(this->a,c.a);
swap(this->b,c.b);
}
With Koenig lookup (aka ADL), it will use swap from the library of "a"
if such a swap exists. Otherwise it will use std::swap. This is
exactly what you want.