boost for each problem

Question

std::map< std::string , std::string > matrix_int;
typedef std::pair< std::string , std::string > lp_type;
BOOST_FOREACH( lp_type &row, matrix_int ){

}

this can not be complied: error C2440: 'initializing' : cannot convert from 'std::pair<_Ty1,_Ty2>' to 'lp_type &'

when I have ',' in element type, boost doc says I can use typedef or predefine a var; but what should I do when I want to get a reference?

Are you going to modify `row`? – kennytm Jun 07 '10 at 14:29 — kennytm, Jun 07 '10 at 14:29

score 10 · Accepted Answer · answered Jun 07 '10 at 14:32

10

Your typedef is incorrect; it needs to be:

typedef std::pair< const std::string , std::string > lp_type;
                   ^ note the added const

The key element in the map pair is const-qualified.

It would be a bit cleaner to use the value_type typedef; this way you don't repeat the type information:

typedef std::map<std::string, std::string> map_t;
map_t matrix_int;
BOOST_FOREACH(map_t::value_type& row, matrix_int){

}

answered Jun 07 '10 at 14:32

James McNellis

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Although technically correct, I would argue that using the inner typedef `value_type` of the map would be better, as it does not expose this intricacy. – Matthieu M. Jun 07 '10 at 14:36
@Matthieu: Yeah; three people said that while I was editing to add that as a suggestion. :-P I agree wholeheartedly. – James McNellis Jun 07 '10 at 14:39

score 2 · Answer 2 · edited May 23 '17 at 10:32

2

See Is it possible to use boost::foreach with std::map?.

Looks like you need to do:

typedef std::map< std::string, std::string > MyMap;
BOOST_FOREACH( MyMap::value_type& row, matrix_int ) {
}

edited May 23 '17 at 10:32

Community

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answered Jun 07 '10 at 14:32

bshields

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score 1 · Answer 3 · answered Jun 07 '10 at 14:36

I think James McNellis is right. I'll add the suggestion that you take advantage of the value_type typedef that std::map provides. Then your code could look like this:

typedef std::map< std::string , std::string > MyMap;
MyMap matrix_int;

BOOST_FOREACH( MyMap::value_type &row, matrix_int ){

}

boost for each problem

3 Answers3