stl predicate with different types

Question

I have a vector of ordered container classes where I need to know the index of the container that has a given element

so, I would like to do the following, but this obviously doesn't work. I could create a dummy Container to house the date to find, but I was wondering if there was a nicer way.

struct FooAccDateComp 
{
  bool operator()(const Container& d1, const MyDate&  f1) const 
  { return   d1->myDate < f1; } 
};

class Container
{
   MyDate myDate;
   ...
};

vector<Container> mystuff; 
MyDate temp(2008, 3, 15);

//add stuff to variable mystuff

int index = int(upper_bound(events.begin(), events.end(),temp, FooAccDateComp())-events.begin());

EDIT: The container class can contain other dates.

Answer 1

You don't necessarily need a special predicate, just enable comparison between Container and MyDate.

#include <vector>

struct MyDate {
   MyDate(int, int, int);
};

struct Container {
   MyDate myDate;
};

// enable comparison between Container and MyDate
bool operator<(Container const&, MyDate const&);
bool operator==(Container const&, MyDate const&);

std::vector<Container> v; 
//add stuff to variable mystuff
MyDate temp(2008, 3, 15);
std::vector<Container>::iterator i = std::lower_bound(v.begin(), v.end(), temp);
ptrdiff_t index = i != v.end() && *i == temp ? i - v.begin() : -1;

Answer 2

You can use find_if if you don't mind degrading performance (you said that you have a vector of sorted Container, so binary search would be faster) Or you can add

struct Container {
    MyDate myDate;
    operator MyDate () {return myDate}; 
}
bool operator <(MyDate  const&, MyDate const&)
{
  return // your logic here
};    

Now you can use binary search functions

std::vector<Container>::iterator i = std::upper_bound(v.begin(), v.end(), MyDateObject);

Surely, it will work only if your vector is sorted by Container.myDate

Answer 3

upper_bound needs to be able to evaluate expressions like Comp(date,container), but you've only provided Comp(container,date). You'll need to provide both:

struct FooAccDateComp 
{
    bool operator()(const Container& c, const MyDate& d) const 
        { return c.myDate < d; } 

    bool operator()(const MyDate& d, const Container& c) const 
        { return d < c.myDate; } 
};

Remember that the vector must be sorted according to this comparison for upper_bound and friends to work.