动态类加载 [英] Dynamic Class Loading
问题描述
我之前的例子使用了Shape类层次结构的概念，所以我将继续使用它。
假设我有五十种不同形状的东西，我正在尝试将b $ b实例化其中一个。诀窍是，instatiation将基于一个
字符串，其名称与我试图实例化的类型完全相同。
显然，写了50个元素长switch语句是一个劣等的
解决方案。那么，我如何基于字符串实例化？标准中是否提供了一种方法
，或者我是没有划桨的小溪？
Aguilar， James写道：我之前的例子使用了Shape类层次结构的概念，所以我将继续这样做。
假设我有五十种不同的形状，我正在尝试实例化其中一个。诀窍是，instatiation将基于一个
字符串，其名称与我尝试实例化的类型完全相同。
类型的名称仅存在于源代码中。一旦代码已经编译并链接了
，就没有类型名称，并且通常还有
也没有可区分的类型。只是一堆函数和const
数据控制程序的行为，直到运行时数据
进来。
显然，写一个五十元素的长开关语句是一个劣等的解决方案。
这是AFA C ++的唯一解决方案。关于主题的变化
包括表格，地图等，但基本上它们导致相同的任务：
字符串与某些函数的关联导致实例化
相应的一个或多个类。
那么，我如何基于字符串进行实例化？是否有标准中提供的方法，或者我是没有划桨的小溪吗？
语言中没有提供关联字符串的方法任何
类型，除非你自己做。不要打电话给简单的编程任务
在没有桨的情况下上一条小溪。免费奶酪只存在于捕鼠器中。
V
" Victor Bazarov" <五******** @ comAcast.net>在消息中写道
news：jI ***************** @ ord-read.news.verio.net ...Aguilar，James写道：
[snip]
好的，所以没有语言结构可以让我这样做。我想，第二个最好的想法是HashMap，它包含字符串和类。但是，正如你所说的那样，没有办法在C ++中简单地表示一个类型。那么我应该把什么作为数据放在hashmap中？
？指向构造函数的指针？我不认为
有效，是吗？那么，指向静态成员方法的指针呢？东西
喜欢这个：
//在Something.h
class东西：public Foo
{
public：
void method1（）;
Something（int n）;
static Something * initThis（int n）;
}
//在Something.cpp中
static Something * Something :: initThis（int n）
{
返回新的东西（n）;
}
// In程序运行时填充hashmap的方法
void fillHashMap（）
{
...
map.put（" Something"，（Something :: initThis））;
...
//这假定map是< string的类型的模板，东西*（*）（int n）>
}
//然后，获取实例化Something的方法：
void someothermethod（string classToGet）
{
Something *（initFun *）（int n）= map.get（classToGet）;
}
那段代码可行，对吗？ （Something :: initThis）（int n）将解析为指向该方法的
，除非我的语法错误。请更正
我，如果我没有正确的话。
Aguilar，James写道：
我之前的例子使用了Shape类层次结构的概念，所以我将继续这样做。
假设我有五十种不同的形状，我正在努力
实例化其中一个。诀窍是，instatiation将基于
一个字符串，其名称与我尝试
实例化的类型完全相同。显然，编写一个50元长的switch语句是一个劣等的解决方案。那么，我如何基于字符串实例化？是否有标准中提供的方法，或者我是没有划桨的小溪吗？
读取/设计模式/，并使用原型，类工厂或工厂
方法。
另外，为什么需要实例？每个班级真的有州吗？可以
你使用Flyweight吗？
-
Phlip
http://industrialxp.org/community/bi...UserInterfaces
My previous example used the concept of a Shape class heirarchy, so I will
continue with that.
Suppose I have something like fifty different shapes, and I am trying to
instantiate one of them. The trick is, the instatiation will be based on a
string with the exact same name as the type that I am trying to instantiate.
Obviously, writing a fifty element long switch statement is an inferior
solution. So, how can I instantiate based on strings? Is there a method
provided in the standard, or am I up a creek without a paddle?
Aguilar, James wrote:My previous example used the concept of a Shape class heirarchy, so I will
continue with that.
Suppose I have something like fifty different shapes, and I am trying to
instantiate one of them. The trick is, the instatiation will be based on a
string with the exact same name as the type that I am trying to instantiate.
The name of a type only exists in the source code. Once the code has
been compiled and linked, there are no type names, and often there are
no distinguishable types either. Just a bunch of functions and const
data that control the behaviour of the program until the run-time data
come in.
Obviously, writing a fifty element long switch statement is an inferior
solution.
It''s the only solution, AFA C++ is concerned. Variations on the theme
include tables, maps, etc., but essentially they lead to the same task:
association of a string with some function that causes the instantiation
of the corresponding class or classes.
So, how can I instantiate based on strings? Is there a method
provided in the standard, or am I up a creek without a paddle?
No method is provided in the language to associate strings with any
types unless you do it yourself. Do not call simple programming task
"up a creek without a paddle". Free cheese exists only in a mousetrap.
V
"Victor Bazarov" <v.********@comAcast.net> wrote in message
news:jI*****************@ord-read.news.verio.net...Aguilar, James wrote:
[snip]
OK, so there''s no language construct that lets me do that. The second best
idea is a HashMap, I guess, with the string and the class. However, as you
said, there is no way to represent simply a type in C++. So what should I
put as the data in the hashmap? A pointer to a constructor? I don''t think
that works, does it? So, a pointer to a static member method? Something
like this:
//In Something.h
class Something : public Foo
{
public:
void method1();
Something(int n);
static Something* initThis(int n);
}
//In Something.cpp
static Something* Something::initThis(int n)
{
return new Something(n);
}
//In the method that fills the hashmap when the program runs
void fillHashMap()
{
...
map.put("Something", (Something::initThis));
...
//This assumes map is a template of type <string, Something* (*)(int n)>
}
//Then, to get the method that would instantiate Something:
void someothermethod(string classToGet)
{
Something* (initFun*)(int n) = map.get(classToGet);
}
That code would work, right? (Something::initThis)(int n) would resolve to
a pointer to that method, unless I''ve got my syntax wrong. Please correct
me if I don''t have it right.
Aguilar, James wrote:
My previous example used the concept of a Shape class heirarchy, so I will
continue with that.
Suppose I have something like fifty different shapes, and I am trying to
instantiate one of them. The trick is, the instatiation will be based on a string with the exact same name as the type that I am trying to instantiate. Obviously, writing a fifty element long switch statement is an inferior
solution. So, how can I instantiate based on strings? Is there a method
provided in the standard, or am I up a creek without a paddle?
Read /Design Patterns/, and use either Prototype, Class Factory, or Factory
Method.
Also, why do you need an instance? Does each class really have state? Could
you use Flyweight?
--
Phlip
http://industrialxp.org/community/bi...UserInterfaces
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