std::function 是如何實現的？

Question

根據我發現的來源， lambda 表達式本質上是由編譯器實現的，它創建了一個類，該類具有重載的函數調用運算符和引用的變量作為成員。 這表明 lambda 表達式的大小各不相同，並且給定足夠多的引用變量，大小可以是任意大的。

std::function應該有一個固定的 size ，但它必須能夠包裝任何類型的可調用對象，包括任何同類的 lambdas。 它是如何實施的？ 如果std::function內部使用指向其目標的指針，那么當std::function實例被復制或移動時會發生什么？ 是否涉及任何堆分配？

Answer 1

std::function的實現可以從一個實現到另一個不同，但核心思想是它使用類型擦除。 雖然有多種方法可以做到這一點，但您可以想象一個微不足道的（不是最佳的）解決方案可能是這樣的（為了簡單起見，針對std::function<int (double)>的特定情況進行了簡化）：

struct callable_base {
   virtual int operator()(double d) = 0;
   virtual ~callable_base() {}
};
template <typename F>
struct callable : callable_base {
   F functor;
   callable(F functor) : functor(functor) {}
   virtual int operator()(double d) { return functor(d); }
};
class function_int_double {
   std::unique_ptr<callable_base> c;
public:
   template <typename F>
   function(F f) {
      c.reset(new callable<F>(f));
   }
   int operator()(double d) { return c(d); }
// ...
};

在這個簡單的方法中， function對象將只存儲一個unique_ptr到一個基本類型。 對於所使用的每個不同的函子function ，創建從基衍生的新的類型和該類型的對象動態實例化。 std::function對象始終具有相同的大小，並將根據需要為堆中的不同函子分配空間。

在現實生活中，有不同的優化可以提供性能優勢，但會使答案復雜化。 該類型可以使用小對象優化，動態分派可以由一個以函子為參數的自由函數指針代替，以避免一級間接......但想法基本相同。

---

關於std::function副本的行為問題，快速測試表明內部可調用對象的副本已完成，而不是共享狀態。

// g++4.8
int main() {
   int value = 5;
   typedef std::function<void()> fun;
   fun f1 = [=]() mutable { std::cout << value++ << '\n' };
   fun f2 = f1;
   f1();                    // prints 5
   fun f3 = f1;
   f2();                    // prints 5
   f3();                    // prints 6 (copy after first increment)
}

測試表明f2獲取可調用實體的副本，而不是引用。 如果可調用實體由不同的std::function<>對象共享，則程序的輸出將是 5、6、7。

Answer 2

來自@David Rodríguez 的答案 - dribeas 很好地演示了類型擦除，但還不夠好，因為類型擦除還包括如何復制類型（在該答案中，函數對象將不可復制構造）。 除了函子數據之外，這些行為也存儲在function對象中。

在 Ubuntu 14.04 gcc 4.8 的 STL 實現中使用的技巧是編寫一個通用函數，用每種可能的函子類型專門化它，並將它們轉換為通用函數指針類型。 因此類型信息被擦除。

我拼湊了一個簡化版本。 希望它會有所幫助

#include <iostream>
#include <memory>

template <typename T>
class function;

template <typename R, typename... Args>
class function<R(Args...)>
{
    // function pointer types for the type-erasure behaviors
    // all these char* parameters are actually casted from some functor type
    typedef R (*invoke_fn_t)(char*, Args&&...);
    typedef void (*construct_fn_t)(char*, char*);
    typedef void (*destroy_fn_t)(char*);

    // type-aware generic functions for invoking
    // the specialization of these functions won't be capable with
    //   the above function pointer types, so we need some cast
    template <typename Functor>
    static R invoke_fn(Functor* fn, Args&&... args)
    {
        return (*fn)(std::forward<Args>(args)...);
    }

    template <typename Functor>
    static void construct_fn(Functor* construct_dst, Functor* construct_src)
    {
        // the functor type must be copy-constructible
        new (construct_dst) Functor(*construct_src);
    }

    template <typename Functor>
    static void destroy_fn(Functor* f)
    {
        f->~Functor();
    }

    // these pointers are storing behaviors
    invoke_fn_t invoke_f;
    construct_fn_t construct_f;
    destroy_fn_t destroy_f;

    // erase the type of any functor and store it into a char*
    // so the storage size should be obtained as well
    std::unique_ptr<char[]> data_ptr;
    size_t data_size;
public:
    function()
        : invoke_f(nullptr)
        , construct_f(nullptr)
        , destroy_f(nullptr)
        , data_ptr(nullptr)
        , data_size(0)
    {}

    // construct from any functor type
    template <typename Functor>
    function(Functor f)
        // specialize functions and erase their type info by casting
        : invoke_f(reinterpret_cast<invoke_fn_t>(invoke_fn<Functor>))
        , construct_f(reinterpret_cast<construct_fn_t>(construct_fn<Functor>))
        , destroy_f(reinterpret_cast<destroy_fn_t>(destroy_fn<Functor>))
        , data_ptr(new char[sizeof(Functor)])
        , data_size(sizeof(Functor))
    {
        // copy the functor to internal storage
        this->construct_f(this->data_ptr.get(), reinterpret_cast<char*>(&f));
    }

    // copy constructor
    function(function const& rhs)
        : invoke_f(rhs.invoke_f)
        , construct_f(rhs.construct_f)
        , destroy_f(rhs.destroy_f)
        , data_size(rhs.data_size)
    {
        if (this->invoke_f) {
            // when the source is not a null function, copy its internal functor
            this->data_ptr.reset(new char[this->data_size]);
            this->construct_f(this->data_ptr.get(), rhs.data_ptr.get());
        }
    }

    ~function()
    {
        if (data_ptr != nullptr) {
            this->destroy_f(this->data_ptr.get());
        }
    }

    // other constructors, from nullptr, from function pointers

    R operator()(Args&&... args)
    {
        return this->invoke_f(this->data_ptr.get(), std::forward<Args>(args)...);
    }
};

// examples
int main()
{
    int i = 0;
    auto fn = [i](std::string const& s) mutable
    {
        std::cout << ++i << ". " << s << std::endl;
    };
    fn("first");                                   // 1. first
    fn("second");                                  // 2. second

    // construct from lambda
    ::function<void(std::string const&)> f(fn);
    f("third");                                    // 3. third

    // copy from another function
    ::function<void(std::string const&)> g(f);
    f("forth - f");                                // 4. forth - f
    g("forth - g");                                // 4. forth - g

    // capture and copy non-trivial types like std::string
    std::string x("xxxx");
    ::function<void()> h([x]() { std::cout << x << std::endl; });
    h();

    ::function<void()> k(h);
    k();
    return 0;
}

STL版本也有一些優化

• 為了節省一些字節， construct_f和destroy_f被混合成一個函數指針（帶有一個附加參數，告訴做什么）
• 原始指針是用來存儲函子對象，具有在一個函數指針沿着union ，這樣，當一個function對象從一個函數指針構造，它將被直接存儲在union而不是堆空間

也許 STL 實現不是最好的解決方案，因為我聽說過一些更快的實現。 但是我相信底層機制是相同的。

Answer 3

對於某些類型的參數（“如果 f 的目標是通過reference_wrapper或函數指針傳遞的可調用對象”）， std::function的構造函數不允許任何異常，因此使用動態內存是不可能的。 對於這種情況，所有數據必須直接存儲在std::function對象中。

在一般情況下（包括 lambda 情況），在實現認為合適時，允許使用動態內存（通過標准分配器或傳遞給std::function構造std::function的分配器）。 如果可以避免，標准建議實現不要使用動態內存，但正如您所說，如果函數對象（不是std::function對象，而是包裝在其中的對象）足夠大，則沒有辦法為了防止它，因為std::function具有固定大小。

這種拋出異常的權限被授予普通構造函數和復制構造函數，它們相當明確地允許在復制期間進行動態內存分配。 對於移動，沒有理由需要動態記憶。 該標准似乎沒有明確禁止它，如果移動可能調用包裝對象類型的移動構造函數，則可能不能，但您應該能夠假設，如果實現和您的對象都是合理的，移動不會導致任何分配。

Answer 4

@neuront 給出了准確的答案，技巧需要新的技術等等，來自 GNU std::function 的代碼

// Implementation of std::function -*- C++ -*-

// Copyright (C) 2004-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/bits/std_function.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{functional}
 */

#ifndef _GLIBCXX_STD_FUNCTION_H
#define _GLIBCXX_STD_FUNCTION_H 1

#pragma GCC system_header

#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else

#if __cpp_rtti
# include <typeinfo>
#endif
#include <bits/stl_function.h>
#include <bits/invoke.h>
#include <bits/refwrap.h>
#include <bits/functexcept.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   *  @brief Exception class thrown when class template function's
   *  operator() is called with an empty target.
   *  @ingroup exceptions
   */
  class bad_function_call : public std::exception
  {
  public:
    virtual ~bad_function_call() noexcept;

    const char* what() const noexcept;
  };

  /**
   *  Trait identifying "location-invariant" types, meaning that the
   *  address of the object (or any of its members) will not escape.
   *  Trivially copyable types are location-invariant and users can
   *  specialize this trait for other types.
   */
  template<typename _Tp>
    struct __is_location_invariant
    : is_trivially_copyable<_Tp>::type
    { };

  class _Undefined_class;

  union _Nocopy_types
  {
    void*       _M_object;
    const void* _M_const_object;
    void (*_M_function_pointer)();
    void (_Undefined_class::*_M_member_pointer)();
  };

  union [[gnu::may_alias]] _Any_data
  {
    void*       _M_access()       { return &_M_pod_data[0]; }
    const void* _M_access() const { return &_M_pod_data[0]; }

    template<typename _Tp>
      _Tp&
      _M_access()
      { return *static_cast<_Tp*>(_M_access()); }

    template<typename _Tp>
      const _Tp&
      _M_access() const
      { return *static_cast<const _Tp*>(_M_access()); }

    _Nocopy_types _M_unused;
    char _M_pod_data[sizeof(_Nocopy_types)];
  };

  enum _Manager_operation
  {
    __get_type_info,
    __get_functor_ptr,
    __clone_functor,
    __destroy_functor
  };

  // Simple type wrapper that helps avoid annoying const problems
  // when casting between void pointers and pointers-to-pointers.
  template<typename _Tp>
    struct _Simple_type_wrapper
    {
      _Simple_type_wrapper(_Tp __value) : __value(__value) { }

      _Tp __value;
    };

  template<typename _Tp>
    struct __is_location_invariant<_Simple_type_wrapper<_Tp> >
    : __is_location_invariant<_Tp>
    { };

  template<typename _Signature>
    class function;

  /// Base class of all polymorphic function object wrappers.
  class _Function_base
  {
  public:
    static const std::size_t _M_max_size = sizeof(_Nocopy_types);
    static const std::size_t _M_max_align = __alignof__(_Nocopy_types);

    template<typename _Functor>
      class _Base_manager
      {
      protected:
    static const bool __stored_locally =
    (__is_location_invariant<_Functor>::value
     && sizeof(_Functor) <= _M_max_size
     && __alignof__(_Functor) <= _M_max_align
     && (_M_max_align % __alignof__(_Functor) == 0));

    typedef integral_constant<bool, __stored_locally> _Local_storage;

    // Retrieve a pointer to the function object
    static _Functor*
    _M_get_pointer(const _Any_data& __source)
    {
      const _Functor* __ptr =
        __stored_locally? std::__addressof(__source._M_access<_Functor>())
        /* have stored a pointer */ : __source._M_access<_Functor*>();
      return const_cast<_Functor*>(__ptr);
    }

    // Clone a location-invariant function object that fits within
    // an _Any_data structure.
    static void
    _M_clone(_Any_data& __dest, const _Any_data& __source, true_type)
    {
      ::new (__dest._M_access()) _Functor(__source._M_access<_Functor>());
    }

    // Clone a function object that is not location-invariant or
    // that cannot fit into an _Any_data structure.
    static void
    _M_clone(_Any_data& __dest, const _Any_data& __source, false_type)
    {
      __dest._M_access<_Functor*>() =
        new _Functor(*__source._M_access<_Functor*>());
    }

    // Destroying a location-invariant object may still require
    // destruction.
    static void
    _M_destroy(_Any_data& __victim, true_type)
    {
      __victim._M_access<_Functor>().~_Functor();
    }

    // Destroying an object located on the heap.
    static void
    _M_destroy(_Any_data& __victim, false_type)
    {
      delete __victim._M_access<_Functor*>();
    }

      public:
    static bool
    _M_manager(_Any_data& __dest, const _Any_data& __source,
           _Manager_operation __op)
    {
      switch (__op)
        {
#if __cpp_rtti
        case __get_type_info:
          __dest._M_access<const type_info*>() = &typeid(_Functor);
          break;
#endif
        case __get_functor_ptr:
          __dest._M_access<_Functor*>() = _M_get_pointer(__source);
          break;

        case __clone_functor:
          _M_clone(__dest, __source, _Local_storage());
          break;

        case __destroy_functor:
          _M_destroy(__dest, _Local_storage());
          break;
        }
      return false;
    }

    static void
    _M_init_functor(_Any_data& __functor, _Functor&& __f)
    { _M_init_functor(__functor, std::move(__f), _Local_storage()); }

    template<typename _Signature>
      static bool
      _M_not_empty_function(const function<_Signature>& __f)
      { return static_cast<bool>(__f); }

    template<typename _Tp>
      static bool
      _M_not_empty_function(_Tp* __fp)
      { return __fp != nullptr; }

    template<typename _Class, typename _Tp>
      static bool
      _M_not_empty_function(_Tp _Class::* __mp)
      { return __mp != nullptr; }

    template<typename _Tp>
      static bool
      _M_not_empty_function(const _Tp&)
      { return true; }

      private:
    static void
    _M_init_functor(_Any_data& __functor, _Functor&& __f, true_type)
    { ::new (__functor._M_access()) _Functor(std::move(__f)); }

    static void
    _M_init_functor(_Any_data& __functor, _Functor&& __f, false_type)
    { __functor._M_access<_Functor*>() = new _Functor(std::move(__f)); }
      };

    _Function_base() : _M_manager(nullptr) { }

    ~_Function_base()
    {
      if (_M_manager)
    _M_manager(_M_functor, _M_functor, __destroy_functor);
    }

    bool _M_empty() const { return !_M_manager; }

    typedef bool (*_Manager_type)(_Any_data&, const _Any_data&,
                  _Manager_operation);

    _Any_data     _M_functor;
    _Manager_type _M_manager;
  };

  template<typename _Signature, typename _Functor>
    class _Function_handler;

  template<typename _Res, typename _Functor, typename... _ArgTypes>
    class _Function_handler<_Res(_ArgTypes...), _Functor>
    : public _Function_base::_Base_manager<_Functor>
    {
      typedef _Function_base::_Base_manager<_Functor> _Base;

    public:
      static _Res
      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
      {
    return (*_Base::_M_get_pointer(__functor))(
        std::forward<_ArgTypes>(__args)...);
      }
    };

  template<typename _Functor, typename... _ArgTypes>
    class _Function_handler<void(_ArgTypes...), _Functor>
    : public _Function_base::_Base_manager<_Functor>
    {
      typedef _Function_base::_Base_manager<_Functor> _Base;

     public:
      static void
      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
      {
    (*_Base::_M_get_pointer(__functor))(
        std::forward<_ArgTypes>(__args)...);
      }
    };

  template<typename _Class, typename _Member, typename _Res,
       typename... _ArgTypes>
    class _Function_handler<_Res(_ArgTypes...), _Member _Class::*>
    : public _Function_handler<void(_ArgTypes...), _Member _Class::*>
    {
      typedef _Function_handler<void(_ArgTypes...), _Member _Class::*>
    _Base;

     public:
      static _Res
      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
      {
    return std::__invoke(_Base::_M_get_pointer(__functor)->__value,
                 std::forward<_ArgTypes>(__args)...);
      }
    };

  template<typename _Class, typename _Member, typename... _ArgTypes>
    class _Function_handler<void(_ArgTypes...), _Member _Class::*>
    : public _Function_base::_Base_manager<
         _Simple_type_wrapper< _Member _Class::* > >
    {
      typedef _Member _Class::* _Functor;
      typedef _Simple_type_wrapper<_Functor> _Wrapper;
      typedef _Function_base::_Base_manager<_Wrapper> _Base;

    public:
      static bool
      _M_manager(_Any_data& __dest, const _Any_data& __source,
         _Manager_operation __op)
      {
    switch (__op)
      {
#if __cpp_rtti
      case __get_type_info:
        __dest._M_access<const type_info*>() = &typeid(_Functor);
        break;
#endif
      case __get_functor_ptr:
        __dest._M_access<_Functor*>() =
          &_Base::_M_get_pointer(__source)->__value;
        break;

      default:
        _Base::_M_manager(__dest, __source, __op);
      }
    return false;
      }

      static void
      _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
      {
    std::__invoke(_Base::_M_get_pointer(__functor)->__value,
              std::forward<_ArgTypes>(__args)...);
      }
    };

  template<typename _From, typename _To>
    using __check_func_return_type
      = __or_<is_void<_To>, is_same<_From, _To>, is_convertible<_From, _To>>;

  /**
   *  @brief Primary class template for std::function.
   *  @ingroup functors
   *
   *  Polymorphic function wrapper.
   */
  template<typename _Res, typename... _ArgTypes>
    class function<_Res(_ArgTypes...)>
    : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
      private _Function_base
    {
      template<typename _Func,
           typename _Res2 = typename result_of<_Func&(_ArgTypes...)>::type>
    struct _Callable : __check_func_return_type<_Res2, _Res> { };

      // Used so the return type convertibility checks aren't done when
      // performing overload resolution for copy construction/assignment.
      template<typename _Tp>
    struct _Callable<function, _Tp> : false_type { };

      template<typename _Cond, typename _Tp>
    using _Requires = typename enable_if<_Cond::value, _Tp>::type;

    public:
      typedef _Res result_type;

      // [3.7.2.1] construct/copy/destroy

      /**
       *  @brief Default construct creates an empty function call wrapper.
       *  @post @c !(bool)*this
       */
      function() noexcept
      : _Function_base() { }

      /**
       *  @brief Creates an empty function call wrapper.
       *  @post @c !(bool)*this
       */
      function(nullptr_t) noexcept
      : _Function_base() { }

      /**
       *  @brief %Function copy constructor.
       *  @param __x A %function object with identical call signature.
       *  @post @c bool(*this) == bool(__x)
       *
       *  The newly-created %function contains a copy of the target of @a
       *  __x (if it has one).
       */
      function(const function& __x);

      /**
       *  @brief %Function move constructor.
       *  @param __x A %function object rvalue with identical call signature.
       *
       *  The newly-created %function contains the target of @a __x
       *  (if it has one).
       */
      function(function&& __x) noexcept : _Function_base()
      {
    __x.swap(*this);
      }

      /**
       *  @brief Builds a %function that targets a copy of the incoming
       *  function object.
       *  @param __f A %function object that is callable with parameters of
       *  type @c T1, @c T2, ..., @c TN and returns a value convertible
       *  to @c Res.
       *
       *  The newly-created %function object will target a copy of
       *  @a __f. If @a __f is @c reference_wrapper<F>, then this function
       *  object will contain a reference to the function object @c
       *  __f.get(). If @a __f is a NULL function pointer or NULL
       *  pointer-to-member, the newly-created object will be empty.
       *
       *  If @a __f is a non-NULL function pointer or an object of type @c
       *  reference_wrapper<F>, this function will not throw.
       */
      template<typename _Functor,
           typename = _Requires<__not_<is_same<_Functor, function>>, void>,
           typename = _Requires<_Callable<_Functor>, void>>
    function(_Functor);

      /**
       *  @brief %Function assignment operator.
       *  @param __x A %function with identical call signature.
       *  @post @c (bool)*this == (bool)x
       *  @returns @c *this
       *
       *  The target of @a __x is copied to @c *this. If @a __x has no
       *  target, then @c *this will be empty.
       *
       *  If @a __x targets a function pointer or a reference to a function
       *  object, then this operation will not throw an %exception.
       */
      function&
      operator=(const function& __x)
      {
    function(__x).swap(*this);
    return *this;
      }

      /**
       *  @brief %Function move-assignment operator.
       *  @param __x A %function rvalue with identical call signature.
       *  @returns @c *this
       *
       *  The target of @a __x is moved to @c *this. If @a __x has no
       *  target, then @c *this will be empty.
       *
       *  If @a __x targets a function pointer or a reference to a function
       *  object, then this operation will not throw an %exception.
       */
      function&
      operator=(function&& __x) noexcept
      {
    function(std::move(__x)).swap(*this);
    return *this;
      }

      /**
       *  @brief %Function assignment to zero.
       *  @post @c !(bool)*this
       *  @returns @c *this
       *
       *  The target of @c *this is deallocated, leaving it empty.
       */
      function&
      operator=(nullptr_t) noexcept
      {
    if (_M_manager)
      {
        _M_manager(_M_functor, _M_functor, __destroy_functor);
        _M_manager = nullptr;
        _M_invoker = nullptr;
      }
    return *this;
      }

      /**
       *  @brief %Function assignment to a new target.
       *  @param __f A %function object that is callable with parameters of
       *  type @c T1, @c T2, ..., @c TN and returns a value convertible
       *  to @c Res.
       *  @return @c *this
       *
       *  This  %function object wrapper will target a copy of @a
       *  __f. If @a __f is @c reference_wrapper<F>, then this function
       *  object will contain a reference to the function object @c
       *  __f.get(). If @a __f is a NULL function pointer or NULL
       *  pointer-to-member, @c this object will be empty.
       *
       *  If @a __f is a non-NULL function pointer or an object of type @c
       *  reference_wrapper<F>, this function will not throw.
       */
      template<typename _Functor>
    _Requires<_Callable<typename decay<_Functor>::type>, function&>
    operator=(_Functor&& __f)
    {
      function(std::forward<_Functor>(__f)).swap(*this);
      return *this;
    }

      /// @overload
      template<typename _Functor>
    function&
    operator=(reference_wrapper<_Functor> __f) noexcept
    {
      function(__f).swap(*this);
      return *this;
    }

      // [3.7.2.2] function modifiers

      /**
       *  @brief Swap the targets of two %function objects.
       *  @param __x A %function with identical call signature.
       *
       *  Swap the targets of @c this function object and @a __f. This
       *  function will not throw an %exception.
       */
      void swap(function& __x) noexcept
      {
    std::swap(_M_functor, __x._M_functor);
    std::swap(_M_manager, __x._M_manager);
    std::swap(_M_invoker, __x._M_invoker);
      }

      // [3.7.2.3] function capacity

      /**
       *  @brief Determine if the %function wrapper has a target.
       *
       *  @return @c true when this %function object contains a target,
       *  or @c false when it is empty.
       *
       *  This function will not throw an %exception.
       */
      explicit operator bool() const noexcept
      { return !_M_empty(); }

      // [3.7.2.4] function invocation

      /**
       *  @brief Invokes the function targeted by @c *this.
       *  @returns the result of the target.
       *  @throws bad_function_call when @c !(bool)*this
       *
       *  The function call operator invokes the target function object
       *  stored by @c this.
       */
      _Res operator()(_ArgTypes... __args) const;

#if __cpp_rtti
      // [3.7.2.5] function target access
      /**
       *  @brief Determine the type of the target of this function object
       *  wrapper.
       *
       *  @returns the type identifier of the target function object, or
       *  @c typeid(void) if @c !(bool)*this.
       *
       *  This function will not throw an %exception.
       */
      const type_info& target_type() const noexcept;

      /**
       *  @brief Access the stored target function object.
       *
       *  @return Returns a pointer to the stored target function object,
       *  if @c typeid(_Functor).equals(target_type()); otherwise, a NULL
       *  pointer.
       *
       * This function does not throw exceptions.
       *
       * @{
       */
      template<typename _Functor>       _Functor* target() noexcept;

      template<typename _Functor> const _Functor* target() const noexcept;
      // @}
#endif

    private:
      using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);
      _Invoker_type _M_invoker;
  };

#if __cpp_deduction_guides >= 201606
  template<typename>
    struct __function_guide_helper
    { };

  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
    struct __function_guide_helper<
      _Res (_Tp::*) (_Args...) noexcept(_Nx)
    >
    { using type = _Res(_Args...); };

  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
    struct __function_guide_helper<
      _Res (_Tp::*) (_Args...) & noexcept(_Nx)
    >
    { using type = _Res(_Args...); };

  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
    struct __function_guide_helper<
      _Res (_Tp::*) (_Args...) const noexcept(_Nx)
    >
    { using type = _Res(_Args...); };

  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
    struct __function_guide_helper<
      _Res (_Tp::*) (_Args...) const & noexcept(_Nx)
    >
    { using type = _Res(_Args...); };

  template<typename _Res, typename... _ArgTypes>
    function(_Res(*)(_ArgTypes...)) -> function<_Res(_ArgTypes...)>;

  template<typename _Functor, typename _Signature = typename
       __function_guide_helper<decltype(&_Functor::operator())>::type>
    function(_Functor) -> function<_Signature>;
#endif

  // Out-of-line member definitions.
  template<typename _Res, typename... _ArgTypes>
    function<_Res(_ArgTypes...)>::
    function(const function& __x)
    : _Function_base()
    {
      if (static_cast<bool>(__x))
    {
      __x._M_manager(_M_functor, __x._M_functor, __clone_functor);
      _M_invoker = __x._M_invoker;
      _M_manager = __x._M_manager;
    }
    }

  template<typename _Res, typename... _ArgTypes>
    template<typename _Functor, typename, typename>
      function<_Res(_ArgTypes...)>::
      function(_Functor __f)
      : _Function_base()
      {
    typedef _Function_handler<_Res(_ArgTypes...), _Functor> _My_handler;

    if (_My_handler::_M_not_empty_function(__f))
      {
        _My_handler::_M_init_functor(_M_functor, std::move(__f));
        _M_invoker = &_My_handler::_M_invoke;
        _M_manager = &_My_handler::_M_manager;
      }
      }

  template<typename _Res, typename... _ArgTypes>
    _Res
    function<_Res(_ArgTypes...)>::
    operator()(_ArgTypes... __args) const
    {
      if (_M_empty())
    __throw_bad_function_call();
      return _M_invoker(_M_functor, std::forward<_ArgTypes>(__args)...);
    }

#if __cpp_rtti
  template<typename _Res, typename... _ArgTypes>
    const type_info&
    function<_Res(_ArgTypes...)>::
    target_type() const noexcept
    {
      if (_M_manager)
    {
      _Any_data __typeinfo_result;
      _M_manager(__typeinfo_result, _M_functor, __get_type_info);
      return *__typeinfo_result._M_access<const type_info*>();
    }
      else
    return typeid(void);
    }

  template<typename _Res, typename... _ArgTypes>
    template<typename _Functor>
      _Functor*
      function<_Res(_ArgTypes...)>::
      target() noexcept
      {
    const function* __const_this = this;
    const _Functor* __func = __const_this->template target<_Functor>();
    return const_cast<_Functor*>(__func);
      }

  template<typename _Res, typename... _ArgTypes>
    template<typename _Functor>
      const _Functor*
      function<_Res(_ArgTypes...)>::
      target() const noexcept
      {
    if (typeid(_Functor) == target_type() && _M_manager)
      {
        _Any_data __ptr;
        _M_manager(__ptr, _M_functor, __get_functor_ptr);
        return __ptr._M_access<const _Functor*>();
      }
    else
      return nullptr;
      }
#endif

  // [20.7.15.2.6] null pointer comparisons

  /**
   *  @brief Compares a polymorphic function object wrapper against 0
   *  (the NULL pointer).
   *  @returns @c true if the wrapper has no target, @c false otherwise
   *
   *  This function will not throw an %exception.
   */
  template<typename _Res, typename... _Args>
    inline bool
    operator==(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
    { return !static_cast<bool>(__f); }

  /// @overload
  template<typename _Res, typename... _Args>
    inline bool
    operator==(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
    { return !static_cast<bool>(__f); }

  /**
   *  @brief Compares a polymorphic function object wrapper against 0
   *  (the NULL pointer).
   *  @returns @c false if the wrapper has no target, @c true otherwise
   *
   *  This function will not throw an %exception.
   */
  template<typename _Res, typename... _Args>
    inline bool
    operator!=(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
    { return static_cast<bool>(__f); }

  /// @overload
  template<typename _Res, typename... _Args>
    inline bool
    operator!=(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
    { return static_cast<bool>(__f); }


  // [20.7.15.2.7] specialized algorithms

  /**
   *  @brief Swap the targets of two polymorphic function object wrappers.
   *
   *  This function will not throw an %exception.
   */
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2062. Effect contradictions w/o no-throw guarantee of std::function swaps
  template<typename _Res, typename... _Args>
    inline void
    swap(function<_Res(_Args...)>& __x, function<_Res(_Args...)>& __y) noexcept
    { __x.swap(__y); }

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++11

#endif // _GLIBCXX_STD_FUNCTION_H

Answer 5

std::function重載operator()使其成為函子對象，lambda 的工作方式相同。 它基本上創建了一個帶有成員變量的結構，可以在operator()函數內部訪問這些變量。 所以要記住的基本概念是 lambda 是一個對象（稱為函子或函數對象）而不是函數。 該標准表示，如果可以避免，則不要使用動態內存。