Re: [boost] Improving the assignment operators of various Boosttypes

Subject: Re: [boost] Improving the assignment operators of various Boosttypes
From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-09-09 20:03:03

Michael Marcin:
...
>>>  T& operator=(T arg)
>>>  {
>>>    arg.swap(*this);
>>>    return *this;
>>>  }

...

> It also allows you to support move assignment emulation rather elegantly 
> using the move library in the sandbox.

There is no need to use a move library. The point of the above assignment is 
to take advantage of copy elision, which allows the compiler to construct 
the rvalue argument directly into 'arg', without a copy. In other words, if 
you have

T t;
T f();

int main()
{
    t = f();
}

the return value of f() can be constructed into 'arg', producing the 
equivalent of

    f().swap(t);

If, in addition, the compiler implements RVO and f() is written in an 
RVO-friendly way, there will be no copies at all.

(I'm sure that this is explained well in the clc++m thread, which I 
apologize for not having read.)