这个shared_ptr如何自动转换为原始指针? [英] How is this shared_ptr automatically converted to a raw pointer?
问题描述
我正在研究C ++ 11的 enable_shared_from_this
;一个例子让我感到困惑: shared_from_this()
返回的 shared_ptr
类型如何转换为该原始指针?
I'm studying enable_shared_from_this
of C++11 now; one example made me confused: how the shared_ptr
type returned by shared_from_this()
can convert to this raw pointer?
#include <iostream>
#include <memory>
#include <functional>
struct Bar {
Bar(int a) : a(a) {}
int a;
};
struct Foo : public std::enable_shared_from_this<Foo> {
Foo() { std::cout << "Foo::Foo\n"; }
~Foo() { std::cout << "Foo::~Foo\n"; }
std::shared_ptr<Bar> getBar(int a)
{
std::shared_ptr<Bar> pb(
new Bar{a}, std::bind(&Foo::showInfo, shared_from_this(), std::placeholders::_1)
);
return pb;
}
void showInfo(Bar *pb)
{
std::cout << "Foo::showInfo()\n";
delete pb;
}
};
int main()
{
std::shared_ptr<Foo> pf(new Foo);
std::shared_ptr<Bar> pb = pf->getBar(10);
std::cout << "pf use_count: " << pf.use_count() << std::endl;
}
推荐答案
这是 std :: bind
被
如Callable中所述,当调用指向非静态成员函数的指针或指向非静态数据的指针时成员,第一个参数必须是对其成员将被访问的对象的引用或指针(可能包括智能指针,如std :: shared_ptr和std :: unique_ptr)。
As described in Callable, when invoking a pointer to non-static member function or pointer to non-static data member, the first argument has to be a reference or pointer (including, possibly, smart pointer such as std::shared_ptr and std::unique_ptr) to an object whose member will be accessed.
bind
已实现,因此它可以接受智能指针代替原始指针。
bind
is implemented so it can accept a smart pointer in lieu of a raw pointer.
您可以在 glibc ++实现比 bind
内部调用 invoke
:
You can see in the glibc++ implementation than bind
internally calls invoke
:
// Call unqualified
template<typename _Result, typename... _Args, std::size_t... _Indexes>
_Result
__call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>)
{
return std::__invoke(_M_f,
_Mu<_Bound_args>()(std::get<_Indexes>(_M_bound_args), __args)...
);
}
和 std :: invoke
可用于智能事物(指针,引用包装器等),开箱即用:
INVOKE(f,t1,t2,...,tN)
的定义如下:
如果 f
是指向类 T
的成员函数的指针:
If f
is a pointer to member function of class T
:
- 如果
std :: is_base_of< T,则std :: decay_t< decltype(t1)> :: value
是如果为true,则INVOKE(f,t1,t2,...,tN)
等效于(t1。* f)(t2,.. 。,tN)
- 如果
std :: decay_t< decltype(t1)>
是专业化std :: reference_wrapper
的值,则INVOKE(f,t1,t2,...,tN)
等效于(t1.get()。* f)(t2,...,tN)
- 如果
t1
确实不满足前面的项目,则INVOKE(f,t1,t2,...,tN)
等效于((* t1)。* f)(t2,...,tN)
。
- If
std::is_base_of<T, std::decay_t<decltype(t1)>>::value
is true, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to(t1.*f)(t2, ..., tN)
- If
std::decay_t<decltype(t1)>
is a specialization ofstd::reference_wrapper
, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to(t1.get().*f)(t2, ..., tN)
- If
t1
does not satisfy the previous items, thenINVOKE(f, t1, t2, ..., tN)
is equivalent to((*t1).*f)(t2, ..., tN)
.
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