Issues converting istream to ifstream while passing std::cin into an argument

Question

I am currently implementing a text-file based testing UI that will take mocked up user inputs from a text file (line-by-line) to simulate real user input instead of using std::cin.

The issue arises when I attempt to pass std::cin into std::ifstream parameter; the issue persists whether it is by reference or by value.

Function:

void ZoinkersEngine::displayMainMenu(User& currentUser, std::ifstream& in) {
//need a function to check the role level of the user here

//DEMO (Mock-up) of Menu
std::string option = "";
do {
    std::cout << std::string(40, '\n');
    std::cout << "Successfully logged in...\n\n\n";
    std::cout << "Main Menu:\n";
    std::cout << "[0] Order Plan\n";
    std::cout << "[1] Generate Plan\n\n";

    std::cout << "Please input number of selected option: ";
    // std::cin >> option;
    in >> option;

    if (option == "0") {
        currentUser.calculateExhibitFav(zoinkersDirectory);
        currentUser.orderPlan(zoinkersDirectory);
    }
    else if (option == "1") {
        currentUser.calculateExhibitFav(zoinkersDirectory);
        currentUser.generatePlan(zoinkersDirectory);
    }
    else if (option == "cancel" || option == "Cancel") {
        break;
    }
} while (option != "cancel" || option != "Cancel");}

Call To Function:

engine.displayMainMenu(currentUser, std::cin);

Error:

cannot convert argument 2 from 'std::istream' to 'std::ifstream'

I can not figure this out; as far as I know ifstream is derived from the istream base class, thus the compiler should be able to cast it.

EDIT #1: Current IDE is Visual Studios 2017; answers must also compile on g++ and work on linux.

Such a conversion is not possible. Make the parameter in the function definition be an istream reference. — , Apr 16 '18 at 21:16
You cannot pass `std::cin` "into" a `std::ifstream` parameter because `std::cin` is not a `std::ifstream`. Furthermore, even if it was, you would not be able to pass it by value anyway, as you're doing. You won't be able to pass any real `std::ifstream` into your function, not just `std::cin`. — Sam Varshavchik, Apr 16 '18 at 21:16
"ifstream is derived from the istream base class". Another way to say that is, "ifstream 'is-a' istream". No cast implied. No cast required. — 2785528, Apr 16 '18 at 21:23
@SamVarshavchik because that is not possible is there a way to implement the same logic without creating separate function calls for the different input sources? — Jared F., Apr 16 '18 at 21:24

score 2 · Answer 1 · edited Apr 16 '18 at 21:40

2

There is no implicit down casting. If you want the function to take an input stream then it should have the signature

void ZoinkersEngine::displayMainMenu(User& currentUser, std::istream& in)
                                                             ^      ^
                                                             |      reference here
                                                             istream, not ifstream

And now you can pass to it any stream that is an istream or derived from one.

This is necessary as

you cannot pass a stream by value if it is an lvalue since streams are not copyable
casting from a base class to a derived class can only be done when the base class was originally a derived class. This is also only done with pointers otherwise the derived to base class conversion would slice and you lose the derived class part

edited Apr 16 '18 at 21:40

user4581301

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answered Apr 16 '18 at 21:17

NathanOliver

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Thank you for reply! This is all clear and makes sense to me. However, I am now at square one and cannot seem to think of a way to implement a selective input stream (from a file and std::cin). Any help at all would be greatly appreciated! – Jared F. Apr 16 '18 at 21:34
@JaredF. What do you mean? With the way I have it in my answer you can pass `cin` or your own file stream to the function. – NathanOliver Apr 16 '18 at 21:36
Ahhhh, please excuse my ignorance. I didn't realize you could use a file as an input stream with istream. Now that I say it outloud it seems like common sense! Thanks for all the help!!! – Jared F. Apr 16 '18 at 21:39
@JaredF. No problem. Glad to help. – NathanOliver Apr 16 '18 at 21:40

Issues converting istream to ifstream while passing std::cin into an argument

1 Answers1