根据C ++中的用户输入为特定对象分配不同的类

Question

I have considered an object 'genObj'. This object can be of the class phones, laptops, shoes, or books. I wish to take user input varying from 1-4, and accordingly assign genObj to a praticular class.

I wish to do this because I have to add these objects to data files. I am maintaining 4 separate data files for the different classes, and a value to fileName is also assigned accordingly.

 switch(mode)
    {
     case 1:
        phones genObj;
        strcpy(fileName,"phonesDatabase.dat");
        break;
     case 2:
        computers genObj;
        strcpy(fileName,"computersDatabase.dat");
        break;
     case 3:
        shoes genObj;
        strcpy(fileName,"shoesDatabase.dat)";
        break;
     case 4:
        books genObj;
        strcpy(fileName,"booksDatabase.dat");
        break;
    }

Doing this, hovewer, gives the following error:

"Multiple declaration for genObj in function add(int)"

The values are assigned to fileName without problem as I have declared

char fileName[20];

globally, and it gets initialized later. However, since the object genObj can be of different classes, it cannot be initialized like this.

If I am not able to assign genObj different classes according to user input, I will have to separate the code into 4 parts, each for one class. and that would be too cumbersome, and inelegant.

Thanks.

Answer 1

You cannot decide the type of an object dynamically at run-time.

Seems like you should use polymorphism in this situation:

First define an abstract class, say object , that defines the behavior that you use to access your genObj . That is, everything phones , computers , shoes , and books have in commons.

Then make those four sub-classes of object and then something like this:

std::unique_ptr<object> genObj;

switch(mode)
{
    case 1:
        genObj = std::make_unique<phones>();
        strcpy(fileName, "phonesDatabase.dat");
        break;
    case 2:
        genObj = std::make_unique<computers>();
        strcpy(fileName, "computersDatabase.dat");
        break;
    case 3:
        genObj = std::make_unique<shoes>();
        strcpy(fileName, "shoesDatabase.dat)";
        break;
    case 4:
        genObj = std::make_unique<books>();
        strcpy(fileName, "booksDatabase.dat");
        break;
}

(Note that make_unique is a feature only in the most recent C++ standard, so if you're writing your code for an old standard then you will have to use new instead, eg genObj.reset(new books) .)

Also as a side note, make mode of an enum type, so that you don't have to handle magic numbers and can replace 1 , 2 , 3 , and 4 with some descriptive names such as mode::phones , mode::computers , mode::shoes , and mode::books respectively.

Answer 2

switch construction won't handle dynamic allocation of objects for you. It'll be much better to employ a fabric here with common interface or abstract class (depending on your needs), say class IFace , and then have a local variable IFace *genObj and then make it equal to new some_derived_from_IFace .