阅读Loki中Funtion源码之后的个人理解，该库归纳起来可以说有三层（C++设计新思维列举到2个参数，此处列举到3个参数），要记住C++的模板其实就是C语言高级的宏定义，如果用户没有用到对应的模板编译器是不会生成对应代码的。
第一层：
template <typename R, template <class, class> class ThreadingModel>
class FunctorImpl<R, NullType, ThreadingModel>
: public Private::FunctorImplBase<R, ThreadingModel>
{
public:
typedef R ResultType;
virtual R operator()() = 0;
}; ////////////////////////////////////////////////////////////////////////////////
// class template FunctorImpl
// Specialization for 1 parameter
//////////////////////////////////////////////////////////////////////////////// template <typename R, typename P1, template <class, class> class ThreadingModel>
class FunctorImpl<R, Seq<P1>, ThreadingModel>
: public Private::FunctorImplBase<R, ThreadingModel>
{
public:
typedef R ResultType;
typedef typename TypeTraits<P1>::ParameterType Parm1;
virtual R operator()(Parm1) = 0;
}; ////////////////////////////////////////////////////////////////////////////////
// class template FunctorImpl
// Specialization for 2 parameters
//////////////////////////////////////////////////////////////////////////////// template <typename R, typename P1, typename P2,
template <class, class> class ThreadingModel>
class FunctorImpl<R, Seq<P1, P2>, ThreadingModel>
: public Private::FunctorImplBase<R, ThreadingModel>
{
public:
typedef R ResultType;
typedef typename TypeTraits<P1>::ParameterType Parm1;
typedef typename TypeTraits<P2>::ParameterType Parm2;
virtual R operator()(Parm1, Parm2) = 0;
};
对应的还有virtual R operator()(Parm1, Parm2, Pram3) = 0; virtual R operator()(Parm1, Parm2, Pram3,Pram4...) = 0总共有15个参数为止;每个operator（）操作符都是纯虚函数，这样上层类继承之实现多态。
需要注意的是每个模板特化的FunctorImpl只有一个R operator()，父类FunctorImplBase暂时不用关注
第二层：
template <class ParentFunctor, typename Fun>
class FunctorHandler
: public ParentFunctor::Impl
{
typedef typename ParentFunctor::Impl Base;
private:
Fun f_;
public:
typedef typename Base::ResultType ResultType;
typedef typename Base::Parm1 Parm1;
typedef typename Base::Parm2 Parm2;
typedef typename Base::Parm3 Parm3;
typedef typename Base::Parm4 Parm4;
typedef typename Base::Parm5 Parm5;
typedef typename Base::Parm6 Parm6;
typedef typename Base::Parm7 Parm7;
typedef typename Base::Parm8 Parm8;
typedef typename Base::Parm9 Parm9;
typedef typename Base::Parm10 Parm10;
typedef typename Base::Parm11 Parm11;
typedef typename Base::Parm12 Parm12;
typedef typename Base::Parm13 Parm13;
typedef typename Base::Parm14 Parm14;
typedef typename Base::Parm15 Parm15; FunctorHandler(const Fun& fun) : f_(fun) {} LOKI_DEFINE_CLONE_FUNCTORIMPL(FunctorHandler) #ifdef LOKI_FUNCTORS_ARE_COMPARABLE bool operator==(const typename Base::FunctorImplBaseType& rhs) const
{
// there is no static information if Functor holds a member function
// or a free function; this is the main difference to tr1::function
if(typeid(*this) != typeid(rhs))
return false; // cannot be equal const FunctorHandler& fh = static_cast<const FunctorHandler&>(rhs);
// if this line gives a compiler error, you are using a function object.
// you need to implement bool MyFnObj::operator == (const MyFnObj&) const;
return f_==fh.f_;
}
#endif
// operator() implementations for up to 15 arguments ResultType operator()()
{ return f_(); } ResultType operator()(Parm1 p1)
{ return f_(p1); } ResultType operator()(Parm1 p1, Parm2 p2)
{ return f_(p1, p2); } ResultType operator()(Parm1 p1, Parm2 p2, Parm3 p3)
{ return f_(p1, p2, p3); }
//后面还有operator()(Parm1 p1, Parm2 p2, Parm3 p3, Pram4 p4....)
FunctorHandler继承自模板参数ParentFunctor内部定义的类型Impl，待会看到第三层的时候就会发现其实是对应参数类型的FunctorImpl的某个特化版本，这样FunctorHandler实现了FunctorImpl中的重载operator()的虚函数，
而其中的成员Fun f_可能为函数指针或者函数对象，即用户传递进来的参数
第三层:
template <typename R = void, class TList = NullType,
template<class, class> class ThreadingModel = LOKI_DEFAULT_THREADING_NO_OBJ_LEVEL>
class Functor
{ private:
std::auto_ptr<Impl> spImpl_;
public:
// Handy type definitions for the body type
typedef FunctorImpl<R, TList, ThreadingModel> Impl;
typedef R ResultType;
typedef TList ParmList;
typedef typename Impl::Parm1 Parm1;
typedef typename Impl::Parm2 Parm2;
typedef typename Impl::Parm3 Parm3;
//...... // Member functions Functor() : spImpl_(0)
{} Functor(const Functor& rhs) : spImpl_(Impl::Clone(rhs.spImpl_.get()))
{} Functor(std::auto_ptr<Impl> spImpl) : spImpl_(spImpl)
{} template <typename Fun>
Functor(Fun fun)
: spImpl_(new FunctorHandler<Functor, Fun>(fun))
{} template <class PtrObj, typename MemFn>
Functor(const PtrObj& p, MemFn memFn)
: spImpl_(new MemFunHandler<Functor, PtrObj, MemFn>(p, memFn))
{} ResultType operator()() const
{
LOKI_FUNCTION_THROW_BAD_FUNCTION_CALL
return (*spImpl_)();
} ResultType operator()(Parm1 p1) const
{
LOKI_FUNCTION_THROW_BAD_FUNCTION_CALL
return (*spImpl_)(p1);
}
ResultType operator()(Parm1 p1, Parm2 p2) const
{
LOKI_FUNCTION_THROW_BAD_FUNCTION_CALL
return (*spImpl_)(p1, p2);
}
ResultType operator()(Parm1 p1, Parm2 p2, Parm3 p3) const
{
LOKI_FUNCTION_THROW_BAD_FUNCTION_CALL
return (*spImpl_)(p1, p2, p3);
}
//......
注意成员std::auto_ptr<Impl> spImpl_;其类型是FunctorImpl根据模板参数TList会选择对应的特化版本，它是FunctorHandler的基类，为什么这么说请看对应的构造函数就明白
template <typename Fun>
Functor(Fun fun)
: spImpl_(new FunctorHandler<Functor, Fun>(fun))
{}
该构造函数也是模板函数，模板参数fun是用户传递进来对应的函数指针或者仿函数，还记得FunctorHandler继承自Functor::Impl吗？这时候就是子类FunctorHandler赋值给基类指针spImpl实现了多态，当调用operator()时候转发的过程是
(*spImpl_)(...)
spImpl的operator()为虚函数转发给子类FunctorHandler的operator();FunctorHandler调用用户传递进来的函数指针或者仿函数
暂时写到这，后期完善