How can I use std::maps with user-defined types as key?
c++, containers, dictionary, stdmap, stl
Solution
You don't have to define `operator<` for your class, actually. You can also make a comparator function object class for it, and use that to specialize `std::map`. To extend your example:
struct Class1Compare
{
bool operator() (const Class1& lhs, const Class1& rhs) const
{
return lhs.id < rhs.id;
}
};
std::map<Class1, int, Class1Compare> c2int;
It just so happens that the default for the third template parameter of `std::map` is `std::less`, which will delegate to `operator<` defined for your class (and fail if there is none). But sometimes you want objects to be usable as map keys, but you do not actually have any meaningful comparison semantics, and so you don't want to confuse people by providing `operator<` on your class just for that. If that's the case, you can use the above trick.
Yet another way to achieve the same is to 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 this is that it will be picked by `std::map` "by default", and yet you do not expose `operator<` to client code otherwise.
Problem
I'm wondering why I can't use STL maps with user-defined classes. When I compile the code below, I get the following cryptic error message. What does it mean? Also, why is it only happening with user-defined types? (Primitive types are okay when they are used as 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 from `_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 from here| C:\MinGW\bin..\lib\gcc\mingw32\3.4.5........\include\c++\3.4.5\bits\stl_function.h|227|error: no match for '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; } ```