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From: Ian Collins <ian-news@hotmail.com>
Newsgroups: comp.lang.c++
Subject: Re: STL map insert Options
Date: Wed, 06 Jul 2011 10:28:05 +1200
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On 07/ 6/11 09:13 AM, Mike Copeland wrote:
>     I am curious about the benefits of several STL map usages and
> interfaces.  Consider the following  declarations:
>
> class Entrant
> {
> public:
>      int          baseBib;
>      char         baseEvent;
>      unsigned int specTime;
> } ;
>
> int main()
> {
>      map<int, Entrant>  myEntrant;
>      map<int, Entrant>::iterator it;
>      Entrant a1;
>
>      a1.baseBib = 2066, a1.baseEvent = 'a', a1.specTime = 0;
>      myEntrant.insert(pair<int, Entrant>(a1.baseBib, a1));
> [etc.]
>
> ....and:
>
> typedef struct ChipRec
> {
>      int          bibNum;
>      char         cEvent;
>      unsigned int finTime;
> } TData;
>      TData tWork;
>
> typedef map<int, ChipRec>  BCI;
> 	BCI bci;
> 	map<int, ChipRec>::iterator bciIter;
> int main()
> {
>      tWork.bibNum = 2066, tWork.cEvent = 'a', tWork.finTime = 0;
>      bci.insert(BCI::value_type(tWork.bibNum, tWork));
> [etc.]
>
>     In both declarations I'm doing the same work, but I can't see any
> advantage to either (class or typedef struct declarations)...other than
> minor coding differences.  Am I missing something that _is_ a reason to
> use one technique versus another?
>     Also, I see some references to "make_pair", but I don't understand
> what that is nor why/how I might use that construct in my applications.
> Could someone explain the 3 constructs "pair", "make_pair" and
> "value_type"?  TIA

The value_type for a std::map is a std::pair<key,value>.

std::make_pair is a function template short cut for creating a 
std::pair.  The function template allows automatic template argument 
deduction.  So

myEntrant.insert(std::pair<int, Entrant>(a1.baseBib, a1));

is equivalent to

myEntrant.insert(std::make_pair(a1.baseBib, a1));

-- 
Ian Collins