Question

How to use std::less in a vector that contains classes, all google results are with plain int examples.

When it comes to classes, for example:

class A{
   public:
      A( int value = 0 ):m_value(value){};
      int m_value;
};

How to do something like:

std::count_if( m_cells.begin(), m_cells.end(), std::less< int >() );

Less will receive an A, not an int. std::less< A >

Seems like less needs a functor operator()(), how to avoid this? I need to implement operator<( int a ) ? Make a bind? What else?

Code:

  1 #include <iostream>
  2 #include <vector>
  3 #include <algorithm>
  4
  5 using namespace std;
  6
  7 class A
  8 {
  9    public:
 10       A(int a = 0 ):m_value(a) {}
 11       bool operator!=( int a )
 12       {
 13          return m_value != a;
 14       }
 15
 16       bool operator<( A &a )
 17       {
 18          return m_value < a.m_value;
 19       }
 20
 21       int m_value;
 22 };
 23
 24
 25 int main(){
 26    std::vector< A > m_cells( 5 );
 27
 28    m_cells[2].m_value = 3;
 29    m_cells[3].m_value = 4;
 30    m_cells[4].m_value = 4;
 31    std::count_if( m_cells.begin(), m_cells.end(), std::less< A >() );
 32    return 0;
 33 }
 34

This results in:

/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_algo.h:4437: error: no match for call to ‘(std::less<A>) (A&)’
/usr/lib/gcc/x86_64-redhat-linux/4.4.6/../../../../include/c++/4.4.6/bits/stl_function.h:229: note: candidates are: bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = A]

Answer 1

Implement an operator< for A, and then pass in std::less<A>, other options (for example you could implement a conversion operator to int – YUCKYUCKYUCKYUCK)

EDIT: To continue…

Like James says, std::less<> requires two arguments, if you are stuck on c++03 (and don’t have boost), you can do something like the following:

std::count_if( m_cells.begin(), m_cells.end() ,std::bind1st(std::less<A>(), 3) )

Basically one of the arguments is bound to 3 – this the first argument (consider the left hand side), you can also bind to the second argument (right hand side – using std::bind2nd), depending on how you want the predicate to work.

This approach requires that you properly implement operator<, i.e.

bool operator<(A const& a) const
{
}

EDIT2: for c++11, you can try any of the following:

 std::cout << std::count_if( m_cells.begin(), m_cells.end() , [](A const& a) { return 3 < a.m_value; } ) << std::endl; // lambda
 std::cout << std::count_if( m_cells.begin(), m_cells.end() , std::bind(std::less<A>(), _1, 4) ) << std::endl; // bind second argument (rhs)
 std::cout << std::count_if( m_cells.begin(), m_cells.end() , std::bind(std::less<A>(), 3, _1) ) << std::endl; // bind first argument (lhs)