如何按一个值属性对海图进行排序？

Question

I have a map<std::string, myStruct> I wonder how to sort items in map by int property that is in myStruct.order and if 2 or more of myStruct orders are same throw list of keys (strings) that make map unsortable by that field of myStruct? Is there any fancy way of doing in in C++03 (may be with boost)?

Answer 1

boost has a complete (and complex) functionality in MultiIndex , but if I understand your requirements, it's overkill in this case. A fairly easy way could be to build a list of pointers to myStruct and sort. Then you can easily check for duplicate keys (these became adjacents).

sort should use a functor of type less<const myStruct*> , ie

bool compare_orders(const myStruct* a, const myStruct* b) { return a->order < b->order; }

Answer 2

You can't do this, using std::map - the first in the pair is used for comparing (it's called key ). In your case, the std::string is the key .

You can use std::set< std::pair< std::string, MyStruct > > and then implement operator< for two std::pair< std::string, MyStruct > -s.

Or, you can change the std::map 's definition, if it's possible/allowed/suitable/etc. It really depends on what you're trying to do and what you're allowed to do.

Or some other container (that keeps the order of elements, as inserted - like std::list , std::vector , etc.), and then using std::sort or container's sort method, if exists.

Answer 3

What meaning is there in "sorting" a map? Sure, the map may be internally organized as a BST or something to make access faster, but conceptionally the elements of the map are in no particular order.

Other than that, you CAN supply an ordering method for the keys (because even if you do organize the map in a particular way, it is by the order of the keys, not by that of the values), but not for the values with the third template argument for map.

Answer 4

I agree with the other answers that it's not clear why you need to sort the map. But you can use a bidirectional map to let you view both myStruct and string as keys.

There's one in boost:

http://www.boost.org/doc/libs/1_48_0/libs/bimap/doc/html/index.html

There's an example here

http://www.boost.org/doc/libs/1_48_0/libs/bimap/doc/html/boost_bimap/examples/simple_bimap.html