[boost] Re : BOOST_FOREACH like macro for map

Subject: [boost] Re : BOOST_FOREACH like macro for map
From: Pierre Morcello (pmorcell-cppfrance_at_[hidden])
Date: 2009-10-20 16:50:39

Hi mobi phil,

> One of the annoyances with std::map is that if you work
> with const maps, then
> there is no "one line" code to get an element knowing the
> key.

If that is really what you want and you don't care about performances, I can propose you for example the following :

[code]

#include <boost/foreach.hpp>
#include <stdexcept>

struct ConstMapHelper
{
	template <class T,class U> 
	static const U& getConst(const T& pKey,const std::map<T,U>& pMap)
	{
		typedef std::pair<const T,U> TPair;
		BOOST_FOREACH(const TPair& lPair,pMap)
		{
			if(lPair.first==pKey)
			{
				return lPair.second;
			}
		}
		throw std::domain_error("[ConstMapHelper::getConst] key not found"); // do what you want
	}
};
[/code]
And then when you want to use it :

// construction of the const map
std::map<std::string, std::string> lMap;
lMap.insert(std::make_pair("1","1"));
lMap.insert(std::make_pair("2","2"));
const std::map<std::string, std::string> lMapConst(lMap);

// access
const std::string key = "5"; // the key
const std::string& s = ConstMapHelper::getConst(key,lMapConst); // will throw exception because "5" is not in the map.

You can even make it a true one liner if you get rid of the reference in the 'key' parameter.

Hope this helps,

Best regards,

Pierre Morcello

--- En date de : Mar 20.10.09, mobi phil <mobi_at_[hidden]> a écrit :

> De: mobi phil <mobi_at_[hidden]>
> Objet: [boost] BOOST_FOREACH like macro for map
> À: boost_at_[hidden]
> Date: Mardi 20 Octobre 2009, 9h06
> Hello,
> 
> new to this list... did not find anything similar on net.
> 
> One of the annoyances with std::map is that if you work
> with const maps, then
> there is no "one line" code to get an element knowing the
> key.
> 
> 
> given the map
> 
> const map<string, string> aStringMap;
> 
> I cannot write
> 
> const string &s = aStringMap["key"];
> process(s);
> 
> I should write:
> 
> map<string, string>::const_iterator
> it=aStringMap..find("key")
> if( it != aStringMap.end() ) {
>    process(it->second)
> }
> 
> so would be nice to have a macro like
> 
> BOOST_CONSTMAPFIND(aStringMap, "key", data)
> {
>       processResult( data );
> }
> 
> the version
> 
> if(BOOST_CONSTMAPFIND(l_map, l_key, l_data) )
> {
> 
> }
> 
> Seems to be impossible to implement
> 
> 
> -- 
> rgrds,
> mobi phil
> 
> being mobile, but including technology
> http://mobiphil.com
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