c ++继承和共享/非共享函数

Question

Thank you in advance for your help. Here's my problem :

I've got sub-class (x, y, z) included in a class (A). Some functions are shared (declared in A) and other not (declared in the subclass).

All the objects are stored in one map map<string,A>Groups .

Then, I want to do a loop for all the functions using an iterator, but there come the problem of the functions that belonged only to one class, it says that class A doesn't contain functions ...

I would like to say for(it=Groups.begin(); it!=Groups.end(); ++it) { it->second.functionShared1() if objects1 belongs to class x : it->second.functionsOfClassX and it understand it has to find the function in class x.

I suppose this will be impossible but if you have an idea of how I can resolve this problem I will be really grateful.

I though about creating virtual functions but it will be a mess, or to create a map for each class and a big map containing all the maps. But then I don't know how to iterate it and to declared outermap["x"]=innermap[x]. So that's it, sorry I just began C++, I hope I explained well.

Answer 1

Why not just make a virtual function:

class A
{
    //...
    virtual void executeMyJunk();
};

for( it=Groups.begin(); it!=Groups.end(); ++it ) it->second.executeMyJunk();

Provide an implementation for that in your subclasses. You can also provide an implementation in A to call functions that are common to all classes.

void A::executeMyJunk()
{
    EveryoneHasThisFunction();
}

void x::executeMyJunk()
{
    // Call common functions
    A::executeMyJunk();

    // Call functions specific to this class
    DoExxyStuff();
}

One thing I should point out is that if you plan to have virtual methods, you will need to store A* (or a smart pointer, eg std::unique_ptr<A> ) in your map, not just an instance of A .

I'm not sure if I interpreted your question correctly though. Perhaps this is not useful to you. If you actually meant that you want to execute a specific function only if a class supports that function, then you can use dynamic_cast . Here I assume that your map stores pointers:

for( it=Groups.begin(); it!=Groups.end(); ++it ) {
    x *xinst = dynamic_cast<x*>(it->second);
    if( x ) x->DoExxyStuff();
}

Answer 2

First off, when you put your objects into you std::map<std::string, A> the objects get sliced : Even if you objects of type x , y , or z into the map, only the base part of the object actually gets copied and stored. If you really mean to store objects of potentially derived objects in the map, you'll need to store some sort of pointer, probably some sort of managed pointer, into the map. For example, you could use

std::map<std::string, std::shared_ptr<A>> Group;

Assuming your class A has at least one virtual function, you could detect if any type is of a specific derived type, eg, using

if (x* ptr = dynamic_cast<x*>(it->second.get())) {
    ...
}