Simple way of moving private non-virtual methods out of .h

gistfile1.cpp

// -----------------------------------------------------------------------------
// in foo.h

#ifndef FOO_H_2011_12_23_INCLUDED
#define FOO_H_2011_12_23_INCLUDED

class Foo
{
    friend class FooImpl;

    int m_secret;
public:
    Foo();
};

#endif

// -----------------------------------------------------------------------------
// foo.cpp

#include "private_impl.h"

class FooImpl
{
public:
    static void helper(Foo* self)
    {
        ++self->m_secret;
    }
};

Foo::Foo()
{
    m_secret = 0;

    FooImpl::helper(this);
}