Предикат как член класса

 std::count_if (v.begin(), v.end(), func);  // Reference to non-static member function must be called

#include <iostream>       // std::cout
#include <algorithm>        
#include <vector>
#include <functional>
 
class Foo
{
public:
    bool func(int c)
    {
        return c == 1;
    }
    size_t GetNum(const std::vector<int>& v) const
    {
        return  std::count_if (v.begin(), v.end(), func);  // Reference to non-static member function must be called
    }
 
};
 
int main ()
{
    std::vector<int> v = {1, 2, 3};
    std::cout << Foo().GetNum(v);
 
 
    return 0;
}

 bool func(int c)
    {
        return .....
    }

class Foo
{
public:
 
        bool func(int c)
        {
            return c == 1;
        }
 
    size_t GetNum(const std::vector<int>& v) const
    {
        return  std::count_if (v.begin(), v.end(), 
                std::bind(&Foo::func, *this, std::placeholders::_1));
    }
 
};

return  std::count_if (v.begin(), v.end(), [this](int c) -> bool {
    return func(c);
});

class Foo
{
public:
  bool func(int c) const
  {
    return c == 1;
  }
 
  size_t GetNum(const std::vector<int>& v) const
  {
    return std::count_if(v.begin(), v.end(), std::bind(&Foo::func, this, std::placeholders::_1));
  }
};