How to check the type of objects (which class they belong to) in C++

Question

The problem i have is this one. I have an array of type Event (Event is the base class of class Conference). In the array i put 5 objects (three of type Event and 2 from Conference in my case). In the exercise I am asked to create two binary files, one for objects of type Event and one for objects of type Conference. Next I have to check the type of the objects in the array, if it is from type Event it is written in Events.bin(binary file for objects of type event) if they are of type Conference they are written in Conference.bin. For that i use typeid() but when i read the files, instead of three objects in Events.bin i have all five there, the same goes for the other file. Here is the code i use to check the type and write them in the file:

for (int k = 0; k < N; k++) {
        if (typeid(arr[k]) == typeid(Conference)) {
            boutconferences.write((char*)&arr[k], sizeof(Conference));

        }else {
            boutevents.write((char*)&arr[k], sizeof(Event));
        }
    }

Second attempt:

 for (int k = 0; k < N; k++) {
            if (typeid(arr[k]) == typeid(Conference)) {
                boutconferences.write((char*)&arr[k], sizeof(Conference));
            }
            if(typeid(arr[k])==typeid(Event)) {
                boutevents.write((char*)&arr[k], sizeof(Event));
            }
        }

Both do the same

Thanks in advance.

not quite clear. If `Conference` inherits from `Event` then a `Converence` is an `Event`. Should conferences be included in both files? — 463035818_is_not_an_ai, May 29 '20 at 12:29
For polymorphic types and run-time type information, use `dynamic_cast<>()`. — Fureeish, May 29 '20 at 12:30
If you have an array of `Event` elements, and assign an object of type `Conference` to some of them, you will end up with [_object slicing_](https://stackoverflow.com/q/274626/580083). If you want polymorphism, you need to have an array of _pointers_ to `Event`. — Daniel Langr, May 29 '20 at 12:31
For `Event arr[]`, you cannot put `Conference` into arr. Actually if you try, you just _copy_ the `Event` part of the `Conference` object, see "object slicing". — mrksngl, May 29 '20 at 12:32
@idclev463035818 In the task given it says to extend class Event, in other words make a class Conference with its own fields and make it inherit from Event — ToniTonagata, May 29 '20 at 12:33
this I understand, but conferences are events, so should conferences go to the conference file and the events file or only to the conference file? — 463035818_is_not_an_ai, May 29 '20 at 12:34
@idclev463035818 Events go in the events file only and conferences go in the conference fileonly — ToniTonagata, May 29 '20 at 12:38
https://en.cppreference.com/w/cpp/language/typeid typeid(xyzObject).name() — Build Succeeded, May 29 '20 at 12:45

How to check the type of objects (which class they belong to) in C++

0 Answers0