Question

I have a strange issue with std::less.

indexedpriorityq.hpp(21): error C2661: 'std::less<_Ty>::less' : no overloaded function takes 2 arguments
1>          with
1>          [
1>              _Ty=float
1>          ]

But isn’t that what it’s supposed to do?

Here’s some of my code:

template<class KeyType, class binary_predicate = std::less<KeyType> >
class IndexedPriorityQ
{
 private:
    typedef typename std::vector<KeyType> KEYLIST;
    KEYLIST& m_Keys_V;

    [...]
};

template<class KeyType, class binary_predicate>
void IndexedPriorityQ<KeyType, binary_predicate>::
    ReorderUpwards(int size)
{
    while( (size>1) && 
        (binary_predicate(m_Keys_V[m_Heap_V[size]], m_Keys_V[m_Heap_V[size/2]])) //breaks here
         )
    {
        Swap(size/2, size);
        size /= 2;
    }
}

What exactly is causing the error, and how can I fix it?

Answer 1

std::less is a functor, and its constructor takes 0 arguments. That is, you create the objcet like this:

std::less<Key> a;

Then, you use it like this:

if(a(x,y)) ...

or even

if(std::less<Key>()(x,y)) ...

There are functors whose constructor takes more than 0 arguments, like std::bind1st. The rule is that if the functor is binary, it is its operator() that takes 2 arguments.