Effective STL 学习笔记 39 ~ 41
2013-11-21 12:45
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Effective STL 学习笔记 39 ~ 41
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Table of Contents
Make Predicate pure FunctionMake Functor classes adaptable
ptr_fun, mem_fun and mem_fun_ref
1 Make Predicate pure Function
纯函数 (Pure Function) 是指输出仅在输入变化时才发生变化的的函数,换句话说,该类型函数的输出不依赖于输入之外的任何东西,例如自身状态或者全局变量。这也是 Functional Programming 中的一个重要概念。 C++ 中用于 STL 算法的Functor 是一些 Predicate Class ,这些 Class 的 operator() 是预测函数,这些 Predicate Class
的 operator() 也应该是纯函数,且不能修改 Class 内部成员变量 —— 换句话说,典型的 Const Member
Function。
Predicate Functions should be pure function.
Predicate Class should make operator() const member function.
2 Make Functor classes adaptable
~~(╯﹏╰)b, 各种从 unary/binary_function 继承过来的东东。。。。见下面的代码。。。3 ptr_fun, mem_fun and mem_fun_ref
这几个函数用于生成前面提到的 unary/binary_function:// 20.3.7 adaptors pointers functions /** @defgroup pointer_adaptors Adaptors for pointers to functions * @ingroup functors * * The advantage of function objects over pointers to functions is that * the objects in the standard library declare nested typedefs describing * their argument and result types with uniform names (e.g., @c result_type * from the base classes @c unary_function and @c binary_function). * Sometimes those typedefs are required, not just optional. * * Adaptors are provided to turn pointers to unary (single-argument) and * binary (double-argument) functions into function objects. The * long-winded functor @c pointer_to_unary_function is constructed with a * function pointer @c f, and its @c operator() called with argument @c x * returns @c f(x). The functor @c pointer_to_binary_function does the same * thing, but with a double-argument @c f and @c operator(). * * The function @c ptr_fun takes a pointer-to-function @c f and constructs * an instance of the appropriate functor. * * @{ */ /// One of the @link pointer_adaptors adaptors for function pointers@endlink. template<typename _Arg, typename _Result> class pointer_to_unary_function : public unary_function<_Arg, _Result> { protected: _Result (*_M_ptr)(_Arg); public: pointer_to_unary_function() { } explicit pointer_to_unary_function(_Result (*__x)(_Arg)) : _M_ptr(__x) { } _Result operator()(_Arg __x) const { return _M_ptr(__x); } }; /// One of the @link pointer_adaptors adaptors for function pointers@endlink. template<typename _Arg1, typename _Arg2, typename _Result> class pointer_to_binary_function : public binary_function<_Arg1, _Arg2, _Result> { protected: _Result (*_M_ptr)(_Arg1, _Arg2); public: pointer_to_binary_function() { } explicit pointer_to_binary_function(_Result (*__x)(_Arg1, _Arg2)) : _M_ptr(__x) { } _Result operator()(_Arg1 __x, _Arg2 __y) const { return _M_ptr(__x, __y); } }; /// One of the @link pointer_adaptors adaptors for function pointers@endlink. template<typename _Arg, typename _Result> inline pointer_to_unary_function<_Arg, _Result> ptr_fun(_Result (*__x)(_Arg)) { return pointer_to_unary_function<_Arg, _Result>(__x); } /// One of the @link pointer_adaptors adaptors for function pointers@endlink. template<typename _Arg1, typename _Arg2, typename _Result> inline pointer_to_binary_function<_Arg1, _Arg2, _Result> ptr_fun(_Result (*__x)(_Arg1, _Arg2)) { return pointer_to_binary_function<_Arg1, _Arg2, _Result>(__x); } /** * This is one of the @link functors functor base classes@endlink. */ template<typename _Arg, typename _Result> struct unary_function { /// @c argument_type is the type of the argument typedef _Arg argument_type; /// @c result_type is the return type typedef _Result result_type; }; /** * This is one of the @link functors functor base classes@endlink. */ template<typename _Arg1, typename _Arg2, typename _Result> struct binary_function { /// @c first_argument_type is the type of the first argument typedef _Arg1 first_argument_type; /// @c second_argument_type is the type of the second argument typedef _Arg2 second_argument_type; /// @c result_type is the return type typedef _Result result_type; }; // 20.3.8 adaptors pointers members /** @defgroup memory_adaptors Adaptors for pointers to members * @ingroup functors * * There are a total of 8 = 2^3 function objects in this family. * (1) Member functions taking no arguments vs member functions taking * one argument. * (2) Call through pointer vs call through reference. * (3) Const vs non-const member function. * * All of this complexity is in the function objects themselves. You can * ignore it by using the helper function mem_fun and mem_fun_ref, * which create whichever type of adaptor is appropriate. * * @{ */ /// One of the @link memory_adaptors adaptors for member /// pointers@endlink. template<typename _Ret, typename _Tp> class mem_fun_t : public unary_function<_Tp*, _Ret> { public: explicit mem_fun_t(_Ret (_Tp::*__pf)()) : _M_f(__pf) { } _Ret operator()(_Tp* __p) const { return (__p->*_M_f)(); } private: _Ret (_Tp::*_M_f)(); }; /// One of the @link memory_adaptors adaptors for member /// pointers@endlink. template<typename _Ret, typename _Tp> class mem_fun_ref_t : public unary_function<_Tp, _Ret> { public: explicit mem_fun_ref_t(_Ret (_Tp::*__pf)()) : _M_f(__pf) { } _Ret operator()(_Tp& __r) const { return (__r.*_M_f)(); } private: _Ret (_Tp::*_M_f)(); }; // Mem_fun adaptor helper functions. There are only two: // mem_fun and mem_fun_ref. template<typename _Ret, typename _Tp> inline mem_fun_t<_Ret, _Tp> mem_fun(_Ret (_Tp::*__f)()) { return mem_fun_t<_Ret, _Tp>(__f); } template<typename _Ret, typename _Tp> inline mem_fun_ref_t<_Ret, _Tp> mem_fun_ref(_Ret (_Tp::*__f)()) { return mem_fun_ref_t<_Ret, _Tp>(__f); }
简单来说:
ptr_fun 用于将函数指针转换成 unay_function 或者 binary_function
mem_fun 用于将成员函数指针转换成 unay_function 或者 binary_function
mem_fun_ref
同 mem_fun ,不同之处在于 mem_fun 返回的 Functor 接受的是对象指针,而 mem_fun_ref 返回的
Functor 接受的参数为对象引用。
在使用 STL 时候,尽量使用上述的三个函数来生成 functor。
(转载请注明出处,使用许可:署名-非商业性使用-相同方式共享 3.0 中国大陆许可协议 。)
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