C ++-根据上下文实例化派生类

Question

let's say I have 200 classes named class1, class2, etc derived from class, and a integer between 1 and 200. Is there a way to instanciate specifically one of the derived class depending on the value of my integer? Obviously I could just manually check for every value but I am wondering is there is anything in C++ that is more flexible

Answer 1

You will have to do a switch on your integer, and instantiate the specific class. If you have this int -> class logic somewhere else in your program, you could consider representing it with a compile-time map. Look at Boost.MPL, specifically boost::mpl::map .

Answer 2

This may be just a longer way of doing the same things, but you could have a factory method for each derived class like so...

std::auto_ptr<BaseClass> createClass0()
{
    return std::auto_ptr<BaseClass>(new Class0());
}

Then define an array of these functions

typedef std::auto_ptr<BaseClass> (*pt2Creator)();
pt2Creator creators[] = {createClass0, ...};

Then, you can do

std::auto_ptr<BaseClass> createClass(int n)
{
   return creators[n]();
}

If you were trying to get out of having to write code for each class, this doesn't help you, but if the problem is figuring out at run-time which class to create based on an integer, this will do it.

You may also want to use a standard collection type rather than a C-style array.

Answer 3

If you are looking for a reflection type system where you can see all of classes and then instantiate them dynamically no. You can use typeid on runtime objects to see if they are the same but I believe its behavior is implementation defined.

Answer 4

If you can template your class, ie name them

template <int I> class Foo;

instead of

class FooI

you can use compile-time or run-time data structures to select the appropriate class.