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From: Ian Collins <ian-news@hotmail.com>
Newsgroups: comp.lang.c++
Subject: Re: STL map insert Options
Date: Thu, 14 Jul 2011 16:50:47 +1200
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On 07/14/11 04:07 PM, Pavel wrote:
> Ian Collins wrote:
>> On 07/14/11 02:31 AM, Stephen Howe wrote:
>>>> I'd be very surprised if all three didn't compile down to the same
>>>> assembly code. It certainly does with the two compilers I tried.
>>>>
>>>> Ian Collins
>>>
>>> I have just rechecked with Visual Studio 2008, 8 years later and I get
>>> the same results.
>>> And that is with full optimisations switched on.
>>> It is easy to see why as make_pair()/pair() are creating an actual
>>> object to insert, value_type does not. So that is an extra round of
>>> copy constructors/destructors.
>>> You might be using ints and the compilers you use might optimise out
>>> extra assignments.
>>>
>>> With the code referenced in the link, I get (and it is not too hard to
>>> work out what work map actually does).
>>
>> If you change the code to use the same type as the map::value_type, all things
>> are equal. For example:
>>
>> m.insert(pair<const Key, Value>(t2, tv));

> const is the key here; otherwise there will be the second copy; same with make_pair.
>
> e.g. reasonably looking
>
>
> std::map<int, int>  m;
> m.insert(std::make_pair(1,2));
>
> will lead to 2 copies: first std::pair<int, int>(), then std::pair<const int,
> int>(std::pair<int, int>);
>
> That's why for performance reasons, value_type or a typedef is a better option.
> For same reason make_pair is one of the "evil" functions that I never use: it's
> too easy to introduce an accidental extra copy.

Indeed.  A little helper like

template <typename Key, typename Value>
std::pair<const Key,Value> map_pair( const Key& key, Value& value )
{
   return std::pair<const Key,Value>(key, value);
}

can come in handy.

-- 
Ian Collins