How to convert a string to a class name

Question

Possible Duplicate:
C++: instantiate class from name?

For example, if I have

string s = "myObject";

So how can I use that String s to make this one?

return new myObject();

I tried this one but it's quite silly and it's wrong

return new s();

Thank you

What you're trying to do is hard in C++, at least for the general case. You might think about using a language that better supports this sort of operation. — Carl Norum, Mar 27 '11 at 06:46
because i'm trying to use factory pattern, i know this problem can be solved by Java, but i'm learning c++ so i want to give it a try — Xitrum, Mar 27 '11 at 06:50
@user552279 - That's your answer; Use the factory or registry pattern. You can find tons of examples with google. — Brian Roach, Mar 27 '11 at 06:53
The factory pattern is just **one class responsible for creating/instatiating** your objects. So when you need to change it you will change only one class. — gideon, Mar 27 '11 at 06:54
same question http://stackoverflow.com/questions/1096700/c-instantiate-class-from-name — Mark.Ablov, Mar 27 '11 at 06:47

score 10 · Accepted Answer · answered Mar 27 '11 at 07:14

You can create a simple factory and register the classes you want to be able to construct. This is a very light-weight sort-of reflection.

#include <map>
#include <string>

template <class T> void* constructor() { return (void*)new T(); }

struct factory
{
   typedef void*(*constructor_t)();
   typedef std::map<std::string, constructor_t> map_type;
   map_type m_classes;

   template <class T>
   void register_class(std::string const& n)
   { m_classes.insert(std::make_pair(n, &constructor<T>)); }

   void* construct(std::string const& n)
   {
      map_type::iterator i = m_classes.find(n);
      if (i == m_classes.end()) return 0; // or throw or whatever you want
      return i->second();
   }
};

factory g_factory;

#define REGISTER_CLASS(n) g_factory.register_class<n>(#n)

int main()
{
   using namespace std;

   REGISTER_CLASS(string);

   std::string* s = (std::string*)g_factory.construct("string");

   printf("s = '%s'\n", s->c_str());
   *s = "foobar";
   printf("s = '%s'\n", s->c_str());

   return 0;
}

score 1 · Answer 2 · answered Mar 27 '11 at 09:50

What you want is called virtual constructor (pattern). The availability of this feature in a language is not necessarily coupled to the language being interpreted or managed by a VM - it depends on how (and if at all) the information about types existing in a program (or library) is available at run time. This is not the case in "naked" C++ - but it can be implemented, as shown by Arvid, for example. The problem is that there is no standardized implementation of this feature, so everybody keeps re-inventing this again and again. To certain extent COM or it's platform independent counterpart XPCOM "standardize" this at component level.

score 0 · Answer 3 · answered Mar 27 '11 at 06:49

You cannot do that in c++, by design. In c++, there is a clear distinction between a type and an object, which are two very different entities of the language. You can thus only instantiate a type and get back an object, and decide to do so at compile time (c++ does not provide any kind of reflection system unlike c# or java).

However, you can use a factory / registry for the objects you want to instantiate that way.

score 0 · Answer 4 · answered Mar 27 '11 at 06:50

0

You'll need Reflection to do that. But C++ AFAIK doesn't support reflection.

Interestingly QT C++ Supports some kinda reflection. Look at the C# section for how its done there, and Javascript has one function to do this. Its called eval()

answered Mar 27 '11 at 06:50

gideon

19,329
11
72
113

Shamim Hafiz - MSFT · Answer 5 · 2011-03-27T06:57:53.653

-2

It's not possible to do this with C++ since the code run on it's own. Only interpreted languages, such as PHP, or programs that are run through interpreters allows this kind of feature.

edited Mar 27 '11 at 06:57

answered Mar 27 '11 at 06:47

Shamim Hafiz - MSFT

21,454
43
116
176

Objective-C, which is a compiled language, supports this behaviour quite easily. – Carl Norum Mar 27 '11 at 06:49
Java is compiled, and does this quite readily. – Brian Roach Mar 27 '11 at 06:49
There are work around to achieve the same in C++. The way the PO wants it, Objective-C and Java supports this because there is a VM running those. – Shamim Hafiz - MSFT Mar 27 '11 at 06:57
there is no VM running Objective-C. And I don't see how that matters for the Java case, either. – Carl Norum Mar 27 '11 at 07:05
1

Objective-C does *not* run in a VM. And the fact that Java is running in a VM has nothing to do with the fact that it's a reflective language. Last time I looked, Lisp is compiled as well. – Brian Roach Mar 27 '11 at 07:10

How to convert a string to a class name

5 Answers5