Question

I have the following program where two variables are to be passed by reference to a function where their values will be determined based on external factors before being returned to main() so that they can be used by other functions. The first variable I am trying to pass is an int, and that goes fine, but the other is an array of strings, which is causing me some problems.

I’ve done enough research into this to know that you can’t have an array or references (though I’ve yet to figure out why) and I was wondering if anyone could help me figure out how to do this? The various methods I’ve tried have all resulted in segmentation faults.

NB: The code below has the array being passed by value since I just don’t know what to write for it.

Update: I’m required to use an array for my coursework. Some other data structure, such as the vector that has been suggested, would be great, but I have to use specific structures.

void initialise_existing_devices(int& no_of_existing_devices, string existing_devices[100]);

int main()
{
    int no_of_existing_devices = 0;
    string existing_devices[100];

    initialise_existing_devices(no_of_existing_devices, existing_devices[100]);
}

void initialise_existing_devices(int& no_of_existing_devices, string existing_devices[100])
{
    string line;
    ifstream DeviceList;
    DeviceList.open("devices/device_list");
    while (true)
    {
        getline(DeviceList, line, '\n');
        if (DeviceList.eof())
        {
            break;
        }
        ++ no_of_existing_devices;
    }
    DeviceList.close();

    DeviceList.open("devices/device_list");
    for (int i = 0; i < no_of_existing_devices; i ++)
    {
        getline(DeviceList, line, '\n');
        existing_devices[i] = line;
    }
}

Answer 1

A reference to an array looks like:

void f(std::string (&a)[N]) { }

where a is the name of the parameter and N is the number of elements in the array.

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However, usually in C++ you don’t pass an array by reference (you can; it’s just not common). Other options include:

• Pass a pointer to the initial element of the array; in this case, consider passing the size of the array as a second argument to the function.

• Use a std::vector<std::string> or a std::array<std::string, N> instead and pass it by reference (you can also find the array psuedo-container in Boost; barring that, consider writing your own. If you take a look at the Boost source code, it’s quite simple and straightforward).

• Pass a pair of iterators (begin and end) to the function and use them to manipulate the range.

The last option is the most idiomatic C++ approach; it is also the most generic because you can use any type of container, including arrays, standard library containers, or containers that you’ve written yourself.

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Since you are actually trying to use the parameter as an “out” parameter, it’s probably better just to return a std::vector<string> or a std::array<string, 100> containing the results; this is much cleaner.