我正在重新使用我的游戏引擎来使用smart-pointers
。我有一个Object
类,一切都继承自。我有一个可渲染的GameObject,因此它继承自IRenderable
(一个定义纯虚拟渲染函数的类),不继承自Object 。我有一个RenderSystem
,它应该为场景中的所有IRenderable保留shared_ptr
。
我遇到的问题是如何将GameObject shared_ptr转换为RenderSystem的IRenderable?
我尝试过的想法:
这完全可以使用原始指针,因此,我觉得有一些方法可以通过智能指针实现相同的结果
示例:
// Object.h
class Object : public enable_shared_from_this<Object> { ... }
// GameObject.h
class GameObject : public Object { ... }
// MeshRenderer.h
class MeshRenderer : public GameObject, IRenderable {
public:
void initialize()
{
// Not able to cast Object to IRenderable
RenderSystem::instance().addRenderable(getShared());
// AND
// Not able to cast Object to IRenderable
RenderSystem::instance().addRenderable(std::static_pointer_cast<IRenderable>(getShared()));
}
}
// RenderSystem.h
class RenderSystem
{
std::list<std::shared_ptr<IRenderable>> m_renderables;
public:
void addRenderable(std::shared_ptr<IRenderable> ptr)
{
m_renderables.push_back(ptr);
}
}
// main.cpp
...
auto meshRenderer = std::shared_ptr<MeshRenderer>();
...
答案 0 :(得分:5)
您无法直接从$computerName = Get-Wmiobject Win32_ComputerSystem
$computerOS = Get-Wmiobject Win32_OperatingSystem
$computerHDD = Get-Wmiobject Win32_LogicalDisk -Filter drivetype=3
ForEach($HDD in $computerHDD){
$txtObject = New-Object PSObject -property @{
'ComputerName' = $computerName.Name
'ComputerModel' = $computerName.Model
'SerialNumber' = $computerName.SerialNumber
'HDDSize' = "{0:N2}" -f ($HDD.Size/1GB)
'HDDFree' = "{0:P2}" -f ($HDD.FreeSpace/$HDD.Size)
'OS' = $computerOS.caption
'OS_type' = $computerOS.OSArchitecture
'User' = $computerName.UserName
}
}
$txtObject | Select-Object ComputerName, ComputerModel, SerialNumber, HDDSize, HDDFree, OS, Os_type, User | Out-File "$PSSCriptRoot\computer_info.txt" -Append
到static_pointer_cast
执行shared_ptr<Object>
,因为这些类型不相关。
但是你可以转换为派生类型,然后允许隐式转换为shared_ptr<IRenderable>
:
shared_ptr<IRenderable>
执行从第一个基类到派生类型的显式转换,然后隐式转换为第二个基类型。这是静态允许的,因为已知派生类型与两个碱基相关,但不相关碱基之间的直接转换是不可能的。
答案 1 :(得分:3)
像这样:
#include <memory>
// everything is an Object - yuk, but ok, if you wish...
struct Object : std::enable_shared_from_this<Object>
{
};
struct GameObject : Object
{
};
struct IRenderable
{
virtual void render() {};
};
struct RederableGameObject : GameObject, IRenderable
{
auto as_shared_renderable() -> std::shared_ptr<IRenderable>
{
// builds a new shared pointer to IRenderable which
// uses the same lifetime control block as me
return std::shared_ptr<IRenderable>
{
this->shared_from_this(), // where to get the control block
this // what to point to
};
}
};
文档:
参见构造函数编号(8)
http://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
更新
以下是自由函数从任何对象获取正确的shared_pointer的起点,前提是它最终是从std::enable_shared_from_this
#include <memory>
#include <type_traits>
namespace notstd
{
// stuff that I think *should be* std
using namespace std;
// a trait to determine whether class T is derived from template
// Tmpl<...>
template <typename T, template <class...> class Tmpl>
struct is_derived_from_template_impl
{
static std::false_type test(...);
template <typename...Us>
static std::true_type test(Tmpl<Us...> const &);
using result = decltype(test(std::declval<T>()));
};
template <typename T, template <class...> class Tmpl>
using is_derived_from_template = typename is_derived_from_template_impl<T, Tmpl>::result;
template <typename T, template <class...> class Tmpl>
constexpr auto is_derived_from_template_v = is_derived_from_template<T, Tmpl>::value;
// free function version of shared_from_this
template<class T>
auto shared_from(enable_shared_from_this<T>* p)
-> std::shared_ptr<T>
{
return p->shared_from_this();
}
// specific shared_ptr construction from type T
template<class T>
auto shared_from(T*p)
-> enable_if_t
<
is_derived_from_template_v
<
T,
enable_shared_from_this
>,
std::shared_ptr<T>
>
{
return std::shared_ptr<T>(p->shared_from_this(), p);
}
}
// everything is an Object - yuk, but ok, if you wish...
struct Object : std::enable_shared_from_this<Object>
{
};
struct GameObject : Object
{
};
struct IRenderable
{
virtual void render() {};
};
extern int emit(const char* str);
struct RederableGameObject : GameObject, IRenderable
{
auto as_shared_renderable() -> std::shared_ptr<RederableGameObject>
{
return notstd::shared_from(this);
}
auto as_shared_renderable() const -> std::shared_ptr<const RederableGameObject>
{
return notstd::shared_from(this);
}
void e() const {
emit("const");
}
void e() {
emit("mutable");
}
};
int main()
{
auto rgo = std::make_shared<RederableGameObject>();
// prove it works
auto p1 = rgo->as_shared_renderable();
// prove it works with a const object also
auto p2 = static_cast<const RederableGameObject&>(*rgo).as_shared_renderable();
p1->e();
p2->e();
}