Question

I have a stl::map which key type is a custom struct. I want to know if this map already has a key with a specific string as component (noted as “id” below), whatever the value of its other components. Inspired by this answer and this one also, I try to use stl::find_if with a custom functor:

map<myStruct, vector<size_t> > myMap;

struct myStruct
{
  string a, b, c, id;
};

struct checkId : unary_function<pair<myStruct, vector<size_t> >, bool>
{
private:
  string _exp;
public:
  checkId (myStruct x) : _exp(x.id) {}
  bool operator() (const pair<myStruct, vector<size_t> > & p) const
  {
    return p.first.id.compare(_exp) == 0;
  }
};

map<myStruct, vector<size_t> >::iterator it;
myStruct newS;  // to be initialized, but not shown here
it_mP2P = find_if(myMap.begin(), myMap.end(), checkId(newS));

When I compile this, gcc returns me:

/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = myStruct]’:
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_map.h:347:    instantiated from ‘_Tp& std::map<_Key,_Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = myStruct, _Tp = std::vector<long unsigned int, std::allocator<long unsigned int> >, _Compare = std::less<myStruct>, _Alloc = std::allocator<std::pair<const myStruct, std::vector<long unsigned int, std::allocator<long unsigned int> > > >]’
myprogram.cpp:386:    instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_function.h:227:  error: no match for ‘operator<’ in ‘__x < __y’

Does this mean that I have to overload the operator “<” to work with my custom struct if I want to use my functor “checkId”? How can I do this? I am not a C++ expert,so thanks in advance for any piece of code.

Answer 1

http://codepad.org/VYNqdZeF

struct myStruct
{
  std::string a, b, c, id;
  bool operator<(const myStruct& rhs) const //HERES THE MAGIC
  {return id < rhs.id;}                     //WHEEEEEEEEEEEE!
};

This is required to make std::map<myStruct, stuff> work, without passing a custom comparison functor.