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Subject: [boost] Improving the assignment operators of various Boost types
From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-09-09 18:39:10

David Abrahams wrote:
> In the case of copy-elision for by-value arguments and return values,
> the compiler is explicitly allowed to _assume_ there is no semantic
> difference between the original rvalue and its copy. That's low-hanging
> fruit for a compiler writer, that pays huge dividends.

So far I've found 7 copy assignment operators that could be improved by
having their argument "by value", instead of "by const reference", at
the following locations, in trunk/boost:

 - any.hpp(64): boost::any
 - function\function_template.hpp(916): boost::function
 - intrusive_ptr.hpp(114): intrusive_ptr
 - multi_index_container.hpp(269): multi_index_container
 - interprocess\allocators\adaptive_pool.hpp(136): adaptive_pool_base
 - spirit\home\classic\tree\common.hpp(101): spirit::tree_node
 - spirit\home\classic\tree\common.hpp(624): spirit::tree_match

Am I correct, or am I possibly missing some others? I wouldn't mind
creating some tickets (one per source file), requesting to have their
argument "by value", as we discussed at comp.lang.c++.moderated, "Re: Is
self assignment test valid?".

Basically it was concluded that /if/ an assignment operator is
implemented by means of copy-and-swap, it should preferably be done as

  T& operator=(T arg)
    return *this;

And preferably /not/ as follows:

  T& operator=(const T & arg)
    return *this;

Kind regards,

Niels Dekker
Scientific programmer at LKEB, Leiden University Medical Center

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