C++ returning an unknown type

Question

I'm making a Configuration class that will hold my user application's configuration, and it reads from a file as strings.

class ConfigKey
{
    public:
        string KeyLabel; //Will be used to identify this key
        string KeyValue; //The value
        bool IsEditable; //For developing uses only, I'm saving a few default and non editable keys for specific apps here
};
class Configuration
{
    public:
        void AddKey(char* keyLabel, char* keyValue, bool isEditable);
    private:
        vector<ConfigKey> configKeys;
};

So when I start the app, I read the config file line by line and add to my Config class:

//Constructor
Configuration::Configuration()
{
    //read from file, examples
    AddKey("windowWidth", "1024", false);
    AddKey("windowHeight", "768", false);
}

Now I want to retrieve these values somewhere else for use in the app, is there a way I can leave the cast for the Configuration class? Something like this:

//In the Configuration class
void* GetKey(char* keyLabel);

//And when I call it, I'd like to do something like this:
int windowAspectRatio = myApp.config.GetKey("windowWidth") / myApp.config.GetKey("windowHeight");

The reason is so I don't have a bunch of stringstreams elsewhere in the code converting the config values before I can use them. I would save the configKey's type as well in the ConfigKey so it can auto-convert itself.

Any advice or suggestions?

Edit for clarification:

I want to retrieve a configKey using this method:

//In the Configuration Class
public:
    int GetKey(char* keyLabel)
    { 
        //the value I saved in ConfigKey is a "string" type, but I'm converting it to Int before I return it
        //loop through the vector, find the keyLabel
        stringstream mySS(foundKey.KeyValue);
        int returnValue = 0;
        mySS >> returnValue; //converted the string to int

        return returnValue; //returned an int
    }

So elsewhere in the code I can call:

int myWidth = myConfig.GetKey("windowWidth"); //It's already converted

But I can have multiple configKeys that are either int , float , bool or even something else. I'm looking for a way to have the GetKey(char* keyLabel) to check for the keyType, then convert it, then return it.

Or any advice on a better solution!

Answer 1

Use boost::variant for this.

Boost variant allows you to represent multiple types with the condition that each type is copyable.

boost::get will allow you to get a type out of the variant.

Usage example:

using string_or_int = boost::variant<std::string, int>;

std::vector<string_or_int> data;

std::string& s = boost::get<std::string>(data[0]);

You can make a template using this concept to get whatever configuration information you want:

template<typename T> T get_config_data(const std::string& key) {
    return boost::get<T>( some_variant );
}

Usage:

std::string window_name = config.get_config_data<std::string>("WindowName");
int window_width = config.get_config_data<int>("WindowWidth");
int window_height = config.get_config_data<int>("WindowHeight");

Answer 2

Can you use templates? The following code may not be totally correct, but should at least help you evaluate whether this is a solution that will work for you:

//In the Configuration Class
public:
    template< typename T >
    T GetKey(char* keyLabel)
    { 
        stringstream mySS( foundKey.KeyValue );
        T returnValue;
        mySS >> returnValue;
        return returnValue;
    }

// elsewhere
int myWidth = myConfig.GetKey< int >("windowWidth");

Answer 3

You can override the conversion operator for ConfigKey class and return an instance of it.

class ConfigKey
{
    public:
        string KeyLabel; //Will be used to identify this key
        string KeyValue; //The value
        bool IsEditable; //For developing uses only, I'm saving a few default and non editable keys for specific apps here
        operator int();  //Returns an integer representation of KeyValue
};

You might have to perform the conversion explicitly in case the compiler complains:

//In the Configuration class
void* GetKey(char* keyLabel);

//And when I call it, I'd like to do something like this:
int windowAspectRatio = (int)myApp.config.GetKey("windowWidth") / (int)myApp.config.GetKey("windowHeight");

In case of an error in the conversion (eg the string does not represent a number) you can throw an exception. Also, would be nice to check if "windowHeight" does not have the value of 0.