Question

I’m not good at OOP design, but I need to demonstrate my knowledge to potential employer.
The situation is following:

I’ve got a file with parameters of key-value type, called parameters.txt.
I’ve got a map <string, CommonParamValue> as a container. I need to fill it with elements, using the file with parameters. The key is always a std::string parameter, the CommonParamValue can be represented with int, double, string and a standard function call. To implement this, the CommonParamValue is a base class with virtual methods. It has childs – StringParamValue, DoubleParamValue, CurTimeParamValue. The base class and every child have a method virtual string GetValue() that returns a string representation of inner data; virtual void SetValue(string) to set the value.

The question is how to fill the container map <string, CommonParamValue> with appropriate data in runtime using polymorphism? Now I’ve got such situation:

parameters.txt

*user_name=Jane
*order_number=1325
current_date=

Routine for filling map

ifstream in(fileParamsPath);
    if (! in)
    {
        cout << "Cannot open file with parameters, program is terminating." << endl;
        cin.get();
        exit(-1);
    }
    string key = "", value = "";
    while(in)
    {
        getline(in, key, '=');
        getline(in, value, '\n');

        // put key and value into the container
        // right here we need to analyze the data type and choose appropriate container.
        // CommonParamValue *paramValue = new DoubleParamValue(); or
        // CommonParamValue *paramValue = new CurTimeParamValue(); or
        // CommonParamValue *paramValue = new StringParamValue();

        paramValue->SetValue(value);
        params.insert(make_pair(key, *paramValue)); // params is a map <string, CommonParamValue>
        delete paramValue;
    }
    in.close();

There’s an idea to keep the type of value-parameter in file parameters.txt and analyze it when filling map <string, CommonParamValue>

parameters.txt

*user_name=Jane
string

*order_number=1325
int

current_date=
function

And modify routine for filling map <string, CommonParamValue> this way:

string key = "", value = "", type = "";
    while(in)
    {
        getline(in, key, '=');
        getline(in, value, '\n');
        getline(in, type, '\n');
        in.get(); // got the dividing empty string
        // put key and value into the container
        // right here we need to analyze the data type and choose appropriate container.
        if(type == "int")
        {
            CommonParamValue *paramValue = new IntParamValue();
        }
        else if(type == "function")
        {
            CommonParamValue *paramValue = new CurTimeParamValue();
        }
        else if(type == "string")
        {
            CommonParamValue *paramValue = new StringParamValue();
        }
        else
        {
            // error
            exit(1);
        }

        paramValue->SetValue(value);
        params.insert(make_pair(key, *paramValue)); // params is a map <string, CommonParamValue>
        delete paramValue;
    }

Is that a good decision or bad? Maybe my potential employer wanted me to implement it in other way, but I have only this decision. Is there any better one for a junior C++ programmer?

Answer 1

The most classic design applicable is that of a factory method design.

CommonParamValue* createParamValue( const std::string &value );

The createParamValue method takes the responsibility of creating the appropriate derived ParamValue and return it as a CommonParamValue.

You can avoid pumping additional strings for “type” into parameters.txt by inferring the type, inside createParamValue, using the value string. A regular expression sounds like the most elegant for int, double and CurTime. Boost regex is an example library lets you do this in c++. But of course if you’re looking for a quick solution, then writing your own 30-line decision tree should not be a problem for these simple grammars.