Re: Sorting a map<T*>

Pete Becker <>
Mon, 07 May 2007 21:00:59 -0400
Greg Herlihy wrote:

On 5/7/07 5:01 PM, in article, "Markus
Schoder" <> wrote:

On Mon, 07 May 2007 19:45:00 -0400, Pete Becker wrote:

Markus Schoder wrote:

On Mon, 07 May 2007 16:11:52 -0700, Michael wrote:

On May 7, 3:01 pm, "barcaroller" <> wrote:

I have a set<T*> that stores pointers to objects. How can I tell
set<T*> to use the objects' operator<() and not the value of the
pointers for sorting?

There's a way to create a set with two parameters: set<MyType,
key_comp>, where
key_comp is a key comparison function you define. For example,
set<int, less<int> >.

So then you'd just need to provide the appropriate comparison function
that uses
the objects' operator<(), suitably wrapped.

Actually you need a class with an appropriate operator(). A plain
function won't do here since you cannot pass it as a template

Sure you can. Try it.

No you cannot. I guess you are confusing this with algorithms like
std::lower_bound where you can specify a predicate as a _function_
parameter. As a template parameter for std::set or std::map this does not
work. Think about it -- it needs to be a type.

But std::set's Compare type may be a function type - it does not need to be
a class type that has an overloaded operator() defined. So Pete's objection
is correct - and here is some code to demonstrate:

    #include <set>

    bool MyComp(int *a, int *b)
        return *a < *b;

    int main()
       std::set<int*, bool (*)(int*, int*)> s(MyComp);

       s.insert( new int(1) );
       s.insert( new int(2) );

You're right. Spoke too quickly.


    -- Pete
Roundhouse Consulting, Ltd. (
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (

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