c++ - std::maps with user-defined types as key -


i'm wondering why can't use stl maps user-defined classes. when compile code below, cryptic error message. mean? also, why happening user-defined types? (primitive types okay when used key)

c:\mingw\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_function.h||in member function `bool std::less<_tp>::operator()(const _tp&, const _tp&) const [with _tp = class1]':|

c:\mingw\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_map.h|338|instantiated `_tp& std::map<_key, _tp, _compare, _alloc>::operator[](const _key&) [with _key = class1, _tp = int, _compare = std::less, _alloc = std::allocator >]'|

c:\users\admin\documents\dev\sandbox\sandbox\sandbox.cpp|24|instantiated here|

c:\mingw\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_function.h|227|error: no match 'operator<' in '__x < __y'| ||=== build finished: 1 errors, 0 warnings ===|

#include <iostream> #include <map>  using namespace std;  class class1 { public:     class1(int id);  private:     int id; };  class1::class1(int id): id(id) {}  int main() {     class1 c1(1);      map< class1 , int> c2int;     c2int[c1] = 12;      return 0; }

you don't have define operator< class, actually. can make comparator function object class it, , use specialize std::map. extend example:

struct class1compare {    bool operator() (const class1& lhs, const class1& rhs) const    {        return lhs.id < rhs.id;    } };  std::map<class1, int, class1compare> c2int;

it happens default third template parameter of std::map std::less, delegate operator< defined class (and fail if there none). want objects usable map keys, not have meaningful comparison semantics, , don't want confuse people providing operator< on class that. if that's case, can use above trick.

yet way achieve same specialize std::less:

namespace std {     template<> struct less<class1>     {        bool operator() (const class1& lhs, const class1& rhs) const        {            return lhs.id < rhs.id;        }     }; }

the advantage of picked std::map "by default", , yet not expose operator< client code otherwise.


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