[英]C++ Shared pointers. How can I change the underlying object's pointer for all copies?
虚拟课我有一个奇怪的情况,我需要设计帮助。
我有一个解决方案,但我正在寻找一个更好的解决方案。 这是我编写的代码,它完成了我正在寻找的东西,但它依赖于指向指针的指针。
我觉得如果我可以直接操作共享指针的底层数据指针,我可以减少一层抽象。
我想用一个InterfaceWrapper,而不是两个。
#include <stdio.h>
#include <memory>
#include <vector>
class Interface
{
public:
virtual void WriteIt() = 0;
virtual ~Interface() { }
};
class Foo : public Interface
{
void WriteIt() { printf ("Foo\n"); }
};
class Bar : public Interface
{
void WriteIt() { printf ("Bar\n"); }
};
// This class wraps Interface so we can call WriteIt on desctruction
// I'd like to do this in the Interface class, but you can't call virtual methods during destruction.
class InterfaceWrapper : public std::unique_ptr<Interface>
{
public:
InterfaceWrapper(Interface * i) : std::unique_ptr<Interface>(i) { }
~InterfaceWrapper() { (*this)->WriteIt(); }
};
// This class provides counted destruction to InterfaceWrapper
class InterfaceWrapper2
{
public:
InterfaceWrapper2 () : _ptr(new InterfaceWrapper(new Foo)) { }
void MakeBar() { _ptr->reset(new Bar); }
private:
std::shared_ptr<InterfaceWrapper> _ptr;
};
int main (void)
{
std::vector<InterfaceWrapper2> thing_vector;
// The default implementation will, print "Foo" on destruction.
InterfaceWrapper2 thing;
// That destructor should only happen once for all copies of 'thing'
thing_vector.push_back(thing);
// MakeBar changes the virtual implementation so it prints "Bar" instead of "Foo"
thing.MakeBar();
// When destructors happen, the program should just print "Bar" once.
return 0;
}
欢迎任何事情,但我特别感兴趣的是使用boost在C ++ 03上工作的解决方案(我的例子是C ++ 11,但我的'真正的'代码是使用boost :: shared_ptr的C ++ 03)。
我基本上是在寻找在我的示例代码中实现InterfaceWrapper2的更好方法。 main()
是我想要完成的最好的解释。 只记得行为被困在那些虚拟类中。
这就是你要追求的吗?
更新3
如果你想保留接口机制,以下是通过编写标准库特性来编写包装器的一种非常简洁的方法:
class InterfaceWrapper {
using UI = std::unique_ptr<Interface>;
std::shared_ptr<UI> _sui {new UI{new Foo}, [](UI*p){ (*p)->WriteIt(); delete p; }};
public:
void MakeBar() { _sui->reset(new Bar); }
};
更新2
在意识到std::function<>
已经是一个可以绑定到任何有状态仿函数的动态,单函数,可变接口之后,我想到了以下简化版本:
#include <memory> #include <iostream> #include <vector> struct dynamic_release { template <typename F> dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~dynamic_release() { _f(); } private: std::function<void()> _f; }; void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } int main(void) { using InterfaceWrapper = std::shared_ptr<dynamic_release>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once }
要在想要使用无状态basic_dynamic_release
函数实现更高效率的情况下启用优化,请添加允许使用不同basic_dynamic_release
函数类型的basic_dynamic_release
模板(例如, void(*)()
):
#include <memory> #include <iostream> namespace detail { template <typename InterfaceCallable> struct basic_dynamic_release { basic_dynamic_release() = default; template <typename F> basic_dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> basic_dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~basic_dynamic_release() { _f(); } private: InterfaceCallable _f; }; } using dynamic_release = detail::basic_dynamic_release<std::function<void()>>; #include <vector> void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } int main(void) { using InterfaceWrapper = std::shared_ptr<detail::basic_dynamic_release<void(*)(void)>>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(do_foo); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once }
为了允许默认构造的实例具有良好定义的接口实现,添加工厂(这是为了使它非常通用):
#include <memory> #include <iostream> namespace detail { template <typename T> struct default_construction final { T operator()() const { return {}; } }; template <typename InterfaceCallable, typename Factory = default_construction<InterfaceCallable> > struct basic_dynamic_release { basic_dynamic_release() = default; template <typename F> basic_dynamic_release(F&& f) : _f(std::forward<F>(f)) { } template <typename F> basic_dynamic_release& operator=(F&& f) { _f = std::forward<F>(f); return *this; } ~basic_dynamic_release() { _f(); } private: InterfaceCallable _f = Factory()(); }; using dynamic_interface = std::function<void()>; template <typename Factory = default_construction<dynamic_interface> > using dynamic_release = basic_dynamic_release<dynamic_interface, Factory>; } #include <vector> void do_foo() { std::cout << "Foo\\n"; } void do_bar() { std::cout << "Bar\\n"; } struct foo_default { detail::dynamic_interface operator()() const { return do_foo; } }; int main(void) { using InterfaceWrapper = std::shared_ptr<detail::dynamic_release<foo_default> >; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); *thing = &do_bar; // Prints nothing thing_vector.push_back(thing); } // prints "Bar" once }
旧的答案倾向于使用boost::variant
静态多态性,代价是管理稍微复杂一些,但具有更大的灵活性:
我选择用静态多态替换动态多态,这会消除额外的分配,这也会带来生命周期管理(曾经是
unique_ptr
)。我认为这使得最终解决方案有点简化,同时更通用(自然提供一些扩展点)。
#include <boost/variant.hpp> #include <memory> #include <iostream> namespace nature { // detail namespace template <typename> struct Nature; template<> struct Nature<struct FooTag> { void do_it() { std::cout << "Foo" << "\\n"; } }; template<> struct Nature<struct BarTag> { void do_it() { std::cout << "Bar" << "\\n"; } }; using FooNature = Nature<FooTag>; using BarNature = Nature<BarTag>; using AnyNature = boost::variant<FooNature, BarNature>; struct Holder { AnyNature held; ~Holder() { DoIt()(held); } private: struct DoIt : boost::static_visitor<> { void operator()(AnyNature& any) const { return boost::apply_visitor(*this, any); } template <typename N> void operator()(N& nature) const { return nature.do_it(); } }; }; } #include <vector> int main(void) { using InterfaceWrapper = std::shared_ptr<nature::Holder>; using Thing = InterfaceWrapper::element_type; { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); // FooNature is default thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); } // prints "Foo" once { std::vector<InterfaceWrapper> thing_vector; auto thing = std::make_shared<Thing>(); thing_vector.push_back(thing); thing_vector.push_back(thing); thing_vector.push_back(thing); thing->held = nature::BarNature {}; // prints nothing thing_vector.push_back(thing); } // prints "Bar" once }
打印
Foo Bar
借用sehe的精彩洞察力 ,这个版本为您提供了一些与您在C ++中可以做的更接近的东西.2003:
class InterfaceWrapper {
typedef std::unique_ptr<Interface> UI;
std::shared_ptr<UI> p_;
public:
InterfaceWrapper () : p_(std::make_shared<UI>(new Foo)) {}
~InterfaceWrapper () { if (p_.unique()) (*p_)->WriteIt(); }
void MakeBar() { p_->reset(new Bar); }
const UI & operator -> () { return *p_.get(); }
};
我选择在InterfaceWrapper
的析构函数中添加条件,而不是自定义删除器。 这允许构造函数使用std::make_shared
。
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