std::function 的实现可能因不同的实现而异，但其核心思想是使用类型擦除。虽然有多种方法可以实现它，但你可以想象一个简单（不是最优）的解决方案可能像这样（为了简化起见，特定情况下的std::function<int(double)>）：
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); }
// ...
};
在这种简单的方法中，function对象仅存储对基类型的unique_ptr。对于每个使用function的不同函数对象，都会创建一个从基类派生的新类型，并动态实例化该类型的对象。 std::function对象始终具有相同的大小，并且将为堆中所需的不同函数对象分配空间。
在现实生活中，存在不同的优化措施可提供性能优势，但会使答案变得复杂。该类型可以使用小对象优化，动态调度可以被替换为以函数指针作为参数接受函数对象，以避免一级间接引用...但基本思想是相同的。
关于std::function副本如何行事的问题，快速测试表明内部可调用对象的副本是被执行的，而不是共享状态。
// 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)
}
测试表明，f2 获得的是可调用实体的副本，而不是引用。如果这个可调用实体被不同的 std::function<> 对象共享，那么程序的输出将会是5、6、7。
std::function的副本将触发分配。 - David Rodríguez - dribeas@David Rodríguez - dribeas的回答很好地展示了类型擦除，但并不够好，因为类型擦除还包括如何复制类型（在该回答中，函数对象将无法进行复制构造）。这些行为也存储在function对象中，除了函数对象数据之外。
Ubuntu 14.04 gcc 4.8的STL实现中使用的技巧是编写一个通用函数，对每个可能的函数对象类型进行特化，并将它们转换为通用函数指针类型。因此，类型信息被擦除。
我已经简化了这个过程，希望能有所帮助。
#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版本还进行了一些优化：
construct_f和destroy_f被混合成一个函数指针（带有一个额外的参数来告诉它该执行什么操作），以节省一些字节。union中，因此当从函数指针构建function对象时，它将直接存储在union而不是堆空间中。也许STL实现并不是最好的解决方案，因为我听说过一些更快的实现。但是我认为底层机制是相同的。
fn = reinterpret_cast<Functor*（fn）;代码也能正常工作？ - Roger Smithreference_wrapper或函数指针传递的可调用对象”），std::function的构造函数不允许任何异常，因此使用动态内存是不可能的。对于这种情况，所有数据必须直接存储在std::function对象内部。std::function构造函数的分配器）是允许的，由实现自行决定。标准建议实现避免使用动态内存，但正如您所说，如果函数对象（而不是std::function对象，而是包装在其中的对象）足够大，则无法防止它，因为std::function具有固定大小。
这个允许抛出异常的权限被授予给普通构造函数和复制构造函数，这明确允许在复制过程中进行动态内存分配。对于移动操作，没有理由需要动态内存。标准似乎并没有明确禁止它，如果移动可能调用包装对象类型的移动构造函数，则可能无法禁止，但您应该能够假设如果实现和您的对象都是合理的，移动不会导致任何分配。
#include <type_traits>
#include <utility>
#include <stdexcept>
template <typename... Args> class function;
template <typename R, typename... Args>
class function<R(Args...)> {
public:
/** construct empty function object */
function(): m_functor(nullptr), m_invoke(nullptr), m_delete(nullptr), m_copy(nullptr) {}
/** destruct function object */
~function() {
if (m_delete) m_delete(*this);
}
/** construct a valid function object from the copy of the given function pointer or functor */
template <typename F, typename std::enable_if<
!std::is_same<typename std::decay<F>::type, function>::value, int>::type = 1>
function(F&& functor): function(1, cast_mem_fn(std::forward<F>(functor))) {}
/** run function call */
R operator()(Args... args) const {
if (!m_functor || !m_invoke) throw std::runtime_error("call an empty function object");
return (*m_invoke)(*this, static_cast<Args>(args)...);
}
/** copy constructor */
function(const function &other): m_functor(other.m_copy ? (*other.m_copy)(other) : nullptr),
m_invoke(other.m_invoke), m_delete(other.m_delete), m_copy(other.m_copy) {}
/** move constructor */
function(function &&other): m_functor(other.m_functor), m_invoke(other.m_invoke),
m_delete(other.m_delete), m_copy(other.m_copy) {
other.m_functor = nullptr;
other.m_invoke = nullptr;
other.m_delete = nullptr;
other.m_copy = nullptr;
}
/** copy assignment */
function& operator=(const function &other) {
if (this == &other) return *this;
if (m_delete) m_delete(*this);
m_functor = other.m_copy ? (*other.m_copy)(other) : nullptr;
m_invoke = other.m_invoke;
m_delete = other.m_delete;
m_copy = other.m_copy;
return *this;
}
/** move assignment */
function& operator=(function &&other) {
if (m_delete) m_delete(*this);
m_functor = other.m_functor;
m_invoke = other.m_invoke;
m_delete = other.m_delete;
m_copy = other.m_copy;
other.m_functor = nullptr;
other.m_invoke = nullptr;
other.m_delete = nullptr;
other.m_copy = nullptr;
return *this;
}
private:
template <typename F>
function(int dummy, F&& functor): m_functor(reinterpret_cast<char*>(new
typename std::decay<F>::type(std::forward<F>(functor)))) {
typedef typename std::decay<F>::type functor_type;
static_assert(std::is_same<R, decltype(invoke_impl<functor_type>(
function(), std::declval<Args>()...))>::value, "invalid functor type");
static_assert(std::is_copy_constructible<functor_type>::value,
"uncopyable functor type");
m_invoke = invoke_impl<functor_type>;
m_delete = delete_impl<functor_type>;
m_copy = copy_impl<functor_type>;
}
template <typename F>
F&& cast_mem_fn(F&& f) {
return static_cast<F&&>(f);
}
/** a pointer to class member cannot be casted to char* */
/** conversion to a functor can solve this issue */
template <typename CLASS, typename METHOD>
class pmf_wrapper {
METHOD CLASS::*m_pmf;
public:
pmf_wrapper(METHOD CLASS::*pmf): m_pmf(pmf) {}
R operator()(Args... args) {
return invoke<METHOD>(static_cast<Args>(args)...);
}
private:
template <typename T, typename std::enable_if<std::is_same<CLASS,
typename std::decay<T>::type>::value, int>::type = 1>
T&& transfer(T&& inst) {
return static_cast<T&&>(inst);
}
template <typename T, typename std::enable_if<std::is_same<CLASS,
typename std::decay<decltype(*T())>::type>::value, int>::type = 1>
auto transfer(T&& ptr)->decltype(*ptr) {
return *ptr;
}
/** member function pointer */
template <typename MT,
typename std::enable_if<std::is_function<MT>::value, int>::type = 1,
typename T, typename... SUBARGS>
R invoke(T&& inst, SUBARGS&&... args) {
return (transfer(std::forward<T>(inst)).*m_pmf)(std::forward<SUBARGS>(args)...);
}
/** data member pointer */
template <typename MT,
typename std::enable_if<!std::is_function<MT>::value, int>::type = 1,
typename T>
R invoke(T&& inst) {
return transfer(std::forward<T>(inst)).*m_pmf;
}
};
template <typename CLASS, typename METHOD>
pmf_wrapper<CLASS, METHOD> cast_mem_fn(METHOD CLASS::*pmf) {
return pmf_wrapper<CLASS, METHOD>(pmf);
}
template <typename F>
static auto invoke_impl(const function &obj, Args... args)->
decltype(std::declval<F>()(static_cast<Args>(args)...)) {
return (*reinterpret_cast<F*>(obj.m_functor))(static_cast<Args>(args)...);
}
template <typename F>
static void delete_impl(const function &obj) {
delete reinterpret_cast<F*>(obj.m_functor);
}
template <typename F>
static char *copy_impl(const function &obj) {
return reinterpret_cast<char*>(new F(*reinterpret_cast<const F*>(obj.m_functor)));
}
/** pointer to the internal function pointer/functor object (on heap) */
char *m_functor;
/** call m_functor */
R (*m_invoke)(const function &obj, Args... args);
/** destroy m_functor */
void (*m_delete)(const function &obj);
/** copy m_functor */
char *(*m_copy)(const function &obj);
};
// 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
std::function 重载了 operator()，使其成为一个函数对象，lambda 表达式也是同样的方式。它基本上创建了一个带有成员变量的结构体，这些成员变量可以在 operator() 函数内部访问。因此，需要记住的基本概念是 lambda 是一个对象（称为函数对象或函数器），而不是函数。标准规定尽可能避免使用动态内存。std::function中，如何可能容纳任意大的lambda函数呢？这就是关键问题。 - user2545918std::function对象的大小都相同，并且不是包含的lambda表达式的大小。 - Mooing Duckstd::vector<T...>对象独立于实际分配器实例/元素数量具有（编译时）固定大小的方式相同。 - sehe
std::function的实现。它本质上是一个多态对象的句柄类。内部基类的派生类被创建来保存参数，分配在堆上 - 然后指向这个派生类的指针作为std::function的子对象被保留。我相信它使用像std::shared_ptr一样的引用计数来处理复制和移动。 - Andrew Tomazos