Is there anyway to construct a object of class with its class name obtained dynamically? Prototype and Factory method along with Abstract factory design pattern can be used to do it. But it involves using of if-else statement at the factory level. To avoid using the if-else statements (there would be plenty in my case), is there any way to construct the object based on the value of a string variable ? So that when the class name is assigned to the string variable, the string value (class name) can be used in construction of the object.
On Aug 20, 2:41 pm, Sami <s.saminat...@gmail.com> wrote:
> Hi,
> Is there anyway to construct a object of class with its class name > obtained dynamically? Prototype and Factory method along with Abstract > factory design pattern can be used to do it. But it involves using of > if-else statement at the factory level. To avoid using the if-else > statements (there would be plenty in my case), is there any way to > construct the object based on the value of a string variable ? So that > when the class name is assigned to the string variable, the string > value (class name) can be used in construction of the object.
> Your response is appreciated.
> Thanks, > Sami
The implementation needed is in C++, hence posted in this group. Thanks.
> Is there anyway to construct a object of class with its class name > obtained dynamically? Prototype and Factory method along with Abstract > factory design pattern can be used to do it. But it involves using of > if-else statement at the factory level. To avoid using the if-else > statements (there would be plenty in my case), is there any way to > construct the object based on the value of a string variable ?
Use a map of strings (names) and factory objects to create the required class.
You can't create a class 'on the fly' from its name in C++, you have to know the classes at compile time.
On Aug 20, 10:41 am, Sami <s.saminat...@gmail.com> wrote:
> Hi,
> Is there anyway to construct a object of class with its class name > obtained dynamically? Prototype and Factory method along with Abstract > factory design pattern can be used to do it. But it involves using of > if-else statement at the factory level. To avoid using the if-else > statements (there would be plenty in my case), is there any way to > construct the object based on the value of a string variable ? So that > when the class name is assigned to the string variable, the string > value (class name) can be used in construction of the object.
> Your response is appreciated.
> Thanks, > Sami
A virtual copy constructor, or a clone function might help you?
On Aug 20, 5:41 am, Sami <s.saminat...@gmail.com> wrote:
> Hi,
> Is there anyway to construct a object of class with its class name > obtained dynamically? Prototype and Factory method along with Abstract > factory design pattern can be used to do it. But it involves using of > if-else statement at the factory level. To avoid using the if-else > statements (there would be plenty in my case), is there any way to > construct the object based on the value of a string variable ? So that > when the class name is assigned to the string variable, the string > value (class name) can be used in construction of the object.
> Your response is appreciated.
> Thanks, > Sami
Take a look at Modern C++ Design. It's factory implementation uses no if-else statements. Essentially, it uses a map of std::strings to function pointers.
> On Aug 20, 5:41 am, Sami <s.saminat...@gmail.com> wrote:
> > Hi,
> > Is there anyway to construct a object of class with its class name > > obtained dynamically? Prototype and Factory method along with Abstract > > factory design pattern can be used to do it. But it involves using of > > if-else statement at the factory level. To avoid using the if-else > > statements (there would be plenty in my case), is there any way to > > construct the object based on the value of a string variable ? So that > > when the class name is assigned to the string variable, the string > > value (class name) can be used in construction of the object.
> > Your response is appreciated.
> > Thanks, > > Sami
> Take a look at Modern C++ Design. It's factory implementation uses no > if-else statements. Essentially, it uses a map of std::strings to > function pointers.
> HTH
Thanks Ian, Harsh Puri for the responses.
Thanks HTH, it is a nice idea to have a map.
Also is there any way to implement reflection concept of java in C++. I know there is no support in native C++ language. Is there any pattern similar to that in C++?
> Also is there any way to implement reflection concept of java in C++. > I know there is no support in native C++ language. Is there any > pattern similar to that in C++?
have a look at Pattern-Oriented Software Architecture: A System of Patterns. There must be a description of implementing reflection. The mapping from strings to Functions Pointers is known as dispatch table.
On Aug 22, 11:31 am, r.gr...@science-computing.de wrote:
> Hallo,
> > Also is there any way to implement reflection concept of java in C++. > > I know there is no support in native C++ language. Is there any > > pattern similar to that in C++?
> have a look at Pattern-Oriented Software Architecture: A System of > Patterns. > There must be a description of implementing reflection. The mapping > from > strings to Functions Pointers is known as dispatch table.
On Aug 20, 11:47 am, Ian Collins <ian-n...@hotmail.com> wrote:
> Sami wrote: > You can't create a class 'on the fly' from its name in C++, > you have to know the classes at compile time.
That's true for pretty much every language, isn't it. It's not sufficient to just have a name; you need more information from somewhere. And you can make things pretty dynamic in C++; I have one case where I look for a dynamically linked object for the class if it isn't already loaded.
-- James Kanze (GABI Software) email:james.ka...@gmail.com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
James Kanze wrote: > On Aug 20, 11:47 am, Ian Collins <ian-n...@hotmail.com> wrote: >> Sami wrote: >> You can't create a class 'on the fly' from its name in C++, >> you have to know the classes at compile time.
> That's true for pretty much every language, isn't it. It's not > sufficient to just have a name; you need more information from > somewhere. And you can make things pretty dynamic in C++; I > have one case where I look for a dynamically linked object for > the class if it isn't already loaded.
True, but a compiled language will never match the runtime flexibility of an interpreted one. For example in PHP to process an RPC request I can write
Where $object is the name of a class, $method is the method to call and $parameters an array of parameters. The interpreter will find the class, construct it and call the method. I could write the same function in C++, but the tables of classes and methods would have to explicitly constructed.
On Oct 9, 3:59 am, Ian Collins <ian-n...@hotmail.com> wrote:
> James Kanze wrote: > > On Aug 20, 11:47 am, Ian Collins <ian-n...@hotmail.com> wrote: > >> Sami wrote: > >> You can't create a class 'on the fly' from its name in C++, > >> you have to know the classes at compile time. > > That's true for pretty much every language, isn't it. It's not > > sufficient to just have a name; you need more information from > > somewhere. And you can make things pretty dynamic in C++; I > > have one case where I look for a dynamically linked object for > > the class if it isn't already loaded. > True, but a compiled language will never match the runtime > flexibility of an interpreted one.
Yes. There are, in fact, several different levels of flexibility possible (with, generally, more flexibility implying less robustness). In totally dynamic languages, with fully dynamic typing, a "struct" or a "class" is really nothing more than an associative array; an array whose elements are indexed by the name of the field. In such cases, you can dynamically read a set of attribute value pairs, and use the results as a struct. In most compiled languages, and some interpreted ones as well, I think, the program can only deal with structs known to it. With dynamic linking, however, it is possible to make the set of such structs open, to add to it at runtime.
> For example in PHP to process an RPC request I can write > call_user_func_array( array($object,$method), $parameters ); > Where $object is the name of a class, $method is the method to > call and $parameters an array of parameters. The interpreter > will find the class, construct it and call the method. I > could write the same function in C++, but the tables of > classes and methods would have to explicitly constructed.
Or automatically generated by some other program:-). I'm not familiar with PHP, but what you are describing doesn't sound too different from Java's java.lang.Class.forName( "className" ) .newInstance(). As I said, I've actually implemented this once in C++, with the programming looking for a corresponding .so/.dll in a path if the desired class wasn't already present. (It's interesting to note that when I did more or less the same thing in Java, it was almost as many lines of code. Since, of course, not just any class would do; the class had to meet certain requirements, and I needed code to test those. In C++, the .so/.dll wouldn't link if the class didn't meet my requirements.)
Implementing total flexibility is also more or less possible; your actual instance is basically an std::map< std::string, boost::any >. Of course, total flexibility often results in total chaos, but like everything else, with a good enough process and some programmer discipline, it can be made to work. If you really, really need it.
-- James Kanze (GABI Software) email:james.ka...@gmail.com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
