Can I write functors using a private nested structure?

Question

Given this class:

class C
{
    private:
        struct Foo
        {
            int key1, key2, value;
        };
        std::vector<Foo> fooList;
};

The idea here is that fooListcan be indexed by either key1or key2Foo structure. I am trying to write functors to go to std::find_ifso that I can search for elements in fooListeach key. But I can not get them to compile because it Foois private in the class (it is not part of the C interface). Is there a way to do this without exposing Foothe rest of the world?

Here is an example of code that will not compile because it Foois private in my class:

struct MatchKey1 : public std::unary_function<Foo, bool>
{
    int key;
    MatchKey1(int k) : key(k) {}
    bool operator()(const Foo& elem) const
    {
        return key == elem.key1;
    }
};

Answer 1

- .

:

class C
{
private:
    struct Foo
    {
        int index;
        Bar bar;
    };

    // Predicates used to find Notification instances.
    struct EqualIndex;
    struct EqualBar;

    std::vector<Foo> fooList;
};

:

// Predicate for finding a Foo instance by index.
struct C::EqualIndex : std::unary_function<C::Foo, bool>
{
    EqualIndex(int index) : index(index) { }
    bool operator()(const C::Foo& foo) const { return foo.index == index; }
    const int index;
};

// Predicate for finding a Foo instance by Bar.
struct C::EqualBar : std::unary_function<C::Foo, bool>
{
    EqualBar(const Bar& bar) : bar(bar) { }
    bool operator()(const C::Foo& foo) const { return foo.bar == bar; }
    const Bar& bar;
};

:

// Find the element containing the Bar instance someBar.
std::vector<Foo>::iterator it = std::find_if(fooList.begin(),
                                             fooList.end(),
                                             EqualBar(someBar));

if (it != fooList.end())
{
    // Found it.
}

...

Answer 2

. functor C std::find_if C.

:

#include "stdafx.h"
#include <vector>
#include <cassert>
#include <algorithm>
#include <iostream>

class C
{
    private:
        struct Foo
        {
            int key1, key2, value;
        };

        std::vector<Foo> fooList;

    struct Finder
    {
    private:
      int key1, key2;

    public:
      Finder(int k1, int k2)
      {
        key1 = k1;
        key2 = k2;
      }

      bool operator ()(Foo const& foo) const
      {
        return foo.key1 == key1 || foo.key2 == key2;
      }
    };

public:
  C()
  {
    Foo foo1, foo2;
    foo1.key1 = 5;
    foo1.key2 = 6;
    foo1.value = 1;
    foo2.key1 = 7;
    foo2.key2 = 8;
    foo2.value = 10;

    fooList.insert(fooList.begin(), foo1);
    fooList.insert(fooList.begin(), foo2);
  }

  int Find(int key1, int key2)
  {
    return std::find_if(fooList.begin(), fooList.end(), Finder(key1, key2))->value;
  }
};

int _tmain(int argc, _TCHAR* argv[])
{
  C c;

  std::cout << c.Find(5, 3) << std::endl;
  std::cout << c.Find(3, 6) << std::endl;
  std::cout << c.Find(7, 3) << std::endl;
  std::cout << c.Find(3, 8) << std::endl;

  return 0;
}

Answer 3

functor C.

Answer 4

, fooList boost::multi_index_container, key1 key2.

Answer 5

, , ( , C-like static).

This gives you a cleaner header that is not obscured by implementation details.

Answer 6

I could use Pimpl Idiom to hide a private section Cinside another class. Since everything in CImplcan be safely published, I should be able to do whatever I want with help Foo.