Explicit instantion of boost multi_index container

Question

First I would like to show the working code and then explain, how i want to change things. This is simple boost multi_index example:

//main.cpp    
    #include <boost/multi_index_container.hpp>
    #include <boost/multi_index/ordered_index.hpp>
    #include <boost/multi_index/identity.hpp>
    #include <boost/multi_index/member.hpp>
    #include <string>

    struct employee
    {
        int         id;
        std::string name;

        employee(int id, const std::string& name) :id(id), name(name){}

        bool operator<(const employee& e)const{ return id<e.id; }
    };

    typedef boost::multi_index::multi_index_container<
        employee,
        boost::multi_index:: indexed_by<
        // sort by employee::operator<
        boost::multi_index:: ordered_unique< boost::multi_index:: identity<employee> >,

        // sort by less<string> on name
        boost::multi_index::ordered_non_unique<boost::multi_index::member<employee, std::string, &employee::name> >
        >
    > employee_set;

    int main()
    {
        employee_set es;
        es.insert(employee(0, "Bob"));
    }

Imagine if main.cpp is another module, without boost dependency. I want to udnerstand how to:

include some header file with boost multiindex container class being forward declared into main.cpp define multiindex container of employees in additional .cpp file I have tried tons of variants, but none if this works. Is it possible to create something like this?

//notmain.cpp
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/member.hpp>
#include "notmain.h"

typedef boost::multi_index::multi_index_container<
    employee,
    boost::multi_index::indexed_by<
    // sort by employee::operator<
    boost::multi_index::ordered_unique< boost::multi_index::identity<employee> >,

    // sort by less<string> on name
    boost::multi_index::ordered_non_unique<boost::multi_index::member<employee, std::string, &employee::name> >
    >
> employee_set;

Now comes h.file I need to fill with forward declaration (or explicit initiation) of container. I may be misunderstanding these terms, but I am new to c++ and boost.

//notmain.h

#include <string>

/*
    Some how here I need forward declaration or explicit initiation of boost container 
    class employee_set ???

*/

struct employee
{
    int         id;
    std::string name;

    employee(int id, const std::string& name) :id(id), name(name){}

    bool operator<(const employee& e)const{ return id<e.id; }
};

This is final goal. I want to remind that main.cpp is imagined to be .cpp of another module, without boost dependency.

//main.cpp
#include "notmain.h"

int main()
{
    employee_set es;
    es.insert(employee(0, "Bob"));
}

Answer 1

If the type is part of a class' visible interface then any headers that class is dependent on have to be included, no way around that. If you really don't want it to be part of the visible interface consider using the pImpl idiom:

Public header

#if !defined(MYCLASS_PUBLIC_H_)
#define MYCLASS_PUBLIC_H_

struct MyClassImpl;
class MyClass {
  MyClassImpl * pImpl;
public:
  void SomeOperation();
};
#endif

Implementation header:

#if !defined(MYCLASS_IMPL_H_)
#define MYCLASS_IMPL_H_
#include <private_type.h>
#include "MyClass.h"
struct MyClassImpl
{
  void Operation();

private:
  SomePrivateType member;
};
#endif

Implementation file:

#include "MyClassImpl.h"
void MyClass::SomeOperation()
{
  pImpl->Operation();
}

void MyClassImpl::Operation()
{
  // do something with 'member'
}

Code that only sees the public interface:

#include "MyClass.h"
void foo()
{
  MyClass inst;
  inst.SomeOperation();
}