c++ stl using compare predicate/functor with containers

Question

I haven't touched C++ in a few years so please bear with me. I need to do binary search in a sorted vector of other containers, let's say pairs, but whatever the reason I cannot get a few lines to compile.

So, for starters, this compiles fine:

bool cmp_pair( int l, int r )
{
    return l < r;
}

main ()
{
    vector<int> v;
    binary_search( v.begin(), v.end(), 0, cmp_pair );

  return 0;
}

But this doesn't:

bool cmp_pair( const pair<int,int>& l, const pair<int,int>& r )
{
    return l.first < r.first;
}

main ()
{
    vector< pair< int, int > > v;
    binary_search( v.begin(), v.end(), 0, cmp_pair );

  return 0;
}

Functor doesn't help either:

struct cmp_pair {
    bool operator()( const pair<int,int>& l, const pair<int,int>& r )
    {
        return l.first < r.first;
    }
};

main ()
{
    vector< pair< int, int > > v;
    binary_search( v.begin(), v.end(), 0, cmp_pair() );

  return 0;
}

Answer 1

Since you are doing binary search on vector of pairs so third argument should be pair so your code should be like this :

using namespace std;
bool cmp_pair( const pair<int,int>& l, const pair<int,int>& r )
{
    return l.first < r.first;
}

int main ()
{
    vector< pair< int, int > > v;
   if(binary_search( v.begin(), v.end(), std::make_pair(0,0), cmp_pair ))
   {
     std::cout<<"found...."<<std::endl;
   }
   else
   {
    std::cout<<"not found...."<<std::endl;
   }
  return 0;
}

Same problem exists in your comparator