Effective STL 学习笔记 39 ~ 41

Effective STL 学习笔记 39 ~ 41

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Table of Contents

• Make Predicate pure Function
• Make 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。

1. Predicate Functions should be pure function.
2. 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 。）