Re: assignment operator implementation
dasjotre wrote:
I frequently see operator= implemented
through copy constructor + swap
struct c
{
c(c const & c) ...
void swap(c const & c) ...
surely you mean: void swap (c&) - a reference to non-const
c & operator=(c const & c)
{
c tmp(c);
swap(tmp);
return *this;
}
};
IMHO, it is better to skip naming the temporary "tmp":
c& operator=(c const & arg) {
c(arg).swap(*this);
}
However, I usually prefer this signature:
c& operator=(c tmp) {
swap(tmp);
}
Consider the code if you were to write string="hello";
with your signature, a temporary string would be constructed as the
function parameter. The temporary would then be copied into the object
named "tmp", so that it could be swapped. So, the characters "hello"
are copied twice.
With the improved signature, "hello" is copied once, into the function
argument named "tmp", which is then swapped.
This is one case where pass by value can really help you out.
A=B;
if B is the same type as A, A's copy constructor is invoked and you get
a temporary A that you can swap. If B is a different type than A, a
single argument constructor is called to convert B to an A and you get
a temporary that you can swap.
joshua lehrer
http://www.lehrerfamily.com/
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