如何在STL容器中存储模板化的异构对象 [英] How to store templated heterogeneous objects in an STL container
问题描述
这个问题是关于在MS Visual C ++ 11中开发的代码,只能访问STL,没有提升。
有一个包装模板类, header:
template< typename Payload>
class Wrapper {
Payload p;
std :: string src;
包装器(std :: string,Payload);
Payload get(); //返回有效载荷
void set(Payload); //替换有效载荷
void operator()(); //使用负载
}
Payload
可能是任何东西 - 指针,int,甚至重对象。
稍后, Wrapper
需要进入一个容器,像 std :: vector
- 但是不管它们具体的参数类型。这给我带来麻烦,因为容器需要同质元素。
我已经尝试了基类建议像KennyTM这样的,然而它给我提供了一些方法 get()
和 set()
- 当从一个向量中使用时,需要使用cast(?),因为如果在该答案建议的模式中使用了元素,那么元素看起来像是一个基类。
template< typename Payload>
struct Wrapper;
struct WrapperBase {
std :: string src;
WrapperBase(std :: string s):src(s){}
模板< typename Payload>
Payload get()const;
模板< typename Payload>
void set(Payload);
virtual void operator()()= 0; //使用有效载荷
};
模板< typename Payload>
struct Wrapper {
Payload payload;
Wrapper(std :: string s,Payload p):WrapperBase(s),payload(p){}
Payload get()const {return payload; }; //返回有效载荷
void set(Payload p){payload = p; }; //替换有效载荷
virtual void operator()()override; // todo
}
模板< typename Payload>
Payload WrapperBase :: get()const {
Assert(dynamic_cast< Wrapper< Payload> const *>(this));
返回static_cast< Wrapper< Payload>常量*>(这个) - GT;获得();
}
模板< typename Payload>
void WrapperBase :: set(Payload p){
Assert(dynamic_cast< Wrapper< Payload> *>(this));
static_cast< Wrapper< Payload> *>(this) - > set(p);
WrapperBase
,如果他们想要设置/获取有效载荷,则需要知道有效载荷的类型。您可以使用 dynamic_cast< Wrapper< Payload> *>
来判断给定的 WrapperBase
是否为特定类型if你不知道。
这没有价值语义,所以你需要存储一个 vector
的智能指针指向 WrapperBase
,而不是实际的实例。 std :: shared_ptr
或 std :: unique_ptr
是非常不同行为的好候选人。
如果有一个有限的
如果你需要价值语义,一个存储实际有效负载的 pImpl
模式可以通过 operator =
来完成。克隆 pImpl
。
询问你是否持有类型 T
是可能的,但通常类似于 dynamic_cast
。
The question is about a code developed in MS Visual C++ 11, with access only to STL, no Boost.
There is a wrapper template class, roughly with this header:
template <typename Payload>
class Wrapper {
Payload p;
std::string src;
Wrapper( std::string, Payload );
Payload get(); // returns payload
void set(Payload); // replaces payload
void operator ()(); // uses payload
}
Payload
may be anything - pointer, int, even heavy object.
Later, Wrapper
s need to go in a container, like std::vector
- but regardless of their specific parameter type. And that gives me trouble because container needs homogeneous elements.
I have tried the base class suggestions like this from KennyTM, however it gives me some issues with methods get()
and set()
- those need cast (?) when used from a vector because elements look like a base class if used in the pattern suggested by that answer.
template<typename Payload>
struct Wrapper;
struct WrapperBase {
std::string src;
WrapperBase( std::string s ):src(s) {}
template<typename Payload>
Payload get() const;
template<typename Payload>
void set(Payload);
virtual void operator ()() = 0; // uses payload
};
template <typename Payload>
struct Wrapper {
Payload payload;
Wrapper( std::string s, Payload p ):WrapperBase(s),payload(p) {}
Payload get() const { return payload; }; // returns payload
void set(Payload p) { payload = p; }; // replaces payload
virtual void operator()() override; // todo
}
template<typename Payload>
Payload WrapperBase::get() const {
Assert(dynamic_cast<Wrapper<Payload> const*>(this));
return static_cast<Wrapper<Payload> const*>(this)->get();
}
template<typename Payload>
void WrapperBase::set(Payload p) {
Assert(dynamic_cast<Wrapper<Payload>*>(this));
static_cast<Wrapper<Payload>*>(this)->set(p);
}
Users of a WrapperBase
, if they want to set/get the payload, need to know what the type of the payload is. You can use dynamic_cast<Wrapper<Payload>*>
to figure out if a given WrapperBase
is a specific kind if you don't know.
This doesn't have value semantics, so you'll want to store a vector
of smart pointers to WrapperBase
rather than actual instances. std::shared_ptr
or std::unique_ptr
are good candidates with very different behavior.
If there is a finite bounded set of Payloads
, a visitor pattern can work.
If you need value semantics, a pImpl
pattern that stores the actual payload can do that with manual operator=
that clones the pImpl
.
Asking "do you hold type T
" is possible, but generally is similar to a dynamic_cast
.
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