C++ STL map , std::pair as the key

Question

This is how I have defined by map.

std::map<std::pair<std::string,std::string>, int>  edMap;

I am confuse on how to insert values , I am getting compilation error always. This is how I am trying to insert.

    std::pair<std::string,std::string> key;
    edMap.insert(key,d);

Compilation error is

1>------ Build started: Project: spellsuggest, Configuration: Debug Win32 ------
1>Compiling...
1>breathalyzer.cpp
1>d:\personal\spellsuggest\spellsuggest\breathalyzer.cpp(70) : error C2664: 'std::_Tree<_Traits>::iterator std::_Tree<_Traits>::insert(std::_Tree<_Traits>::iterator,const std::pair<_Ty1,_Ty2> &)' : cannot convert parameter 1 from 'std::pair<_Ty1,_Ty2>' to 'std::_Tree<_Traits>::iterator'
1>        with
1>        [
1>            _Traits=std::_Tmap_traits<std::pair<std::string,std::string>,int,std::less<std::pair<std::string,std::string>>,std::allocator<std::pair<const std::pair<std::string,std::string>,int>>,false>,
1>            _Ty1=const std::pair<std::string,std::string>,
1>            _Ty2=int
1>        ]
1>        and
1>        [
1>            _Ty1=std::string,
1>            _Ty2=std::string
1>        ]
1>        and
1>        [
1>            _Traits=std::_Tmap_traits<std::pair<std::string,std::string>,int,std::less<std::pair<std::string,std::string>>,std::allocator<std::pair<const std::pair<std::string,std::string>,int>>,false>
1>        ]
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>Build log was saved at "file://d:\personal\spellsuggest\spellsuggest\Debug\BuildLog.htm"
1>spellsuggest - 1 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

Let's try:

typedef std::pair<std::string, std::string> my_key_type;
typedef std::map<my_key_type, int>          my_map_type;

my_map_type m;

m.insert(my_map_type::value_type(my_key_type("A", "B"), 43));

Observe that the map's value_type is always std::pair<const key_type, mapped_type> , so in your case it's std::pair<my_key_type, int> -- a pair whose first member is itself a pair!

With that in mind you can alternatively use make_pair :

m.insert(std::make_pair(my_key_type("C", "D"), -5));

Finally, as Sven points out, there may or may not be a comparison operator for pairs (I think there is, though); so if there isn't, you have to write one yourself. Lexicographic comparison on the two elements should do. Sophie awaits :-)

(Here's the lexicographic pair comparison; you don't need to write this, it's already there :)

template<typename S, typename T>
bool operator<(const std::pair<S, T> & a, const std::pair<S, T> & b)
{
   return (a.first < b.first) || (a.first == b.first && a.second < b.second);
}

Answer 2

insert方法需要一对完整的map类型，所以你必须这样做：

edMap.insert(make_pair(key, d));

Answer 3

Note that this:

std::pair<std::string,std::string> key;
edMap.insert(make_pair(key,d));

will will fail to insert anything if there is already a key with the same value present.

This, on the other hand:

std::pair<std::string,std::string> key;
edMap[key] = d;

will either create a new item in the map, or overwrite the previous value, if one existed.