Re: Inserting objects into a std::map?

saneman <>
Thu, 27 Mar 2008 23:48:09 +0100
Ian Collins wrote:

saneman wrote:

Ian Collins wrote:

saneman wrote:

I am reading section 23 in the C++ standard and cannot seem to find
where it says that the key_type must be defined for '<' when inserting
into std::map.

That's because it doesn't have to.

std::map has four template arguments, the third is the comparison, which
defaults to std::less. By definition, std::less requires operator <, so
that's where the requirement for a map key originates. You are free to
provide your own comparison object which does not require an operator <.

Assuming that std::map is based on some kind of balanced tree structure
it wouldn't make much sense if no ordering based on key_type was
supplied (<, >, < 5 etc).

I realize that the dependency comes from the comparator but since the
comparator is based on the key_type some kind of ordering operator still
needs to be valid for the key_type.

Did you read what I said? You are free to provide your own comparison
object which does not require an operator <. That's the way maps work.
 Say you had a map of pointers and wanted to sort based on the objects
pointed to. In that case you would provide a comparison object.

So it would in my opinion make sense if the standard says that the
key_type needs to be valid for the comparator used. Or is this to
obvious to mention?

NO, because it does not!

I don't understand. Does std::map not require that the comparison object
  implement a boolean operator () ? In the below example I have made a
comparison object independent of the key_type:

// Dummy comparator.

class myCmp {

  myCmp() {}
  bool operator()() const {
       return 0;

// Testing with myCmp
std::map<int, std::string, myCmp> m;
std::pair<int, std::string> p;
p.first = 22; // key.
p.second = "33"; // key.

And it gives the error:

error: no match for call to ?(myCmp) (const int&, const int&)?
bob.cpp:42: note: candidates are: bool myCmp::operator()() const

I read the first error line as it tries to call a function operator on
the two key_types, which I intensionally have omitted in the operator in

Here is what the standard says:
"This comparison object may be a pointer to function or an object of a
type with an appropriate function call operator."

If I instead do:

template <typename T>
class myCmp {

    myCmp() {}

    bool operator()(const T& x, const T& y) const {
        return 0;

and declare the map with:

std::map<int, std::string, myCmp<int> > m;

it works.

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