How to approach programming

Lesson 3 

Value translation

  

Task description

Distance in a program may be represented in two units of measurement: miles and in kilometers. One set of algorithms/functions accepts value in miles while another in kilometers. In order to use the same distance value in both, value needs to be “translated” between miles and kilometers. Provide a way to ensure that correct parameter value is provided in both set of interface functions.

Solution

 

 2.1  Type definitions

Separate types/classes need to be defined for each different units of measurement to be used as parameters in different API functions. In our case there are two of those units – kilometers and miles but in other cases may be more. So, these types are DistanceInKilometers and DistanceInMiles. These types are to be used to pass values as parameters into APIs

Then, a main class needs to be defined which will hold the value of distance in the program – Distance. Value will be only one but there are accessor methods (empty operators) that will return values of this distance held by this object internally in different units.

Class UnsignedInteger

{

public:

    UnsignedInteger(unsigned int val) { Value = val; }

protected:

    unsigned int Value;

};

Class DistanceInMiles : public class UnsignedInteger

{

    DistanceInMiles(unsigned int value) // constructor – takes initial value in miles

    { Value = value; } // assigns parameter value to a member Value

};

Class DistanceInKilometers : public class UnsignedInteger

{

    DistanceInKilometers(unsigned int value) // constructor – takes initial value in kilometers

    { Value = value; } // assigns parameter value to a member Value

};

Class Distance : public class UnsignedInteger

{

    Distance(DistanceInKilometers Val) // constructor – takes initial value in kilometers

    { Value = Val.Value; } // assigns parameter value to a member Value

    operator DistanceInMiles () // empty operator method to get in miles

    { return DistanceInMiles(Value * 10 /16); }

    operator DistanceInKilometers () // empty operator method to get in kilometers

    { return DistanceInKilometers(Value); }

};

2.2  Usage in APIs

Now we will explain the usage of those classes in the program. Let's imagine we have two APIs – taking distance in different units. These APIs need to use these defined types sa parameters in their APIs.

Class APIUsingKilometers

{

    static Void API1Method( DistanceInKilometers dist);

}

Class APIUsingMiles

{

    static Void API2Method( DistanceInMiles dist);

}

2.3  Usage in program

Here will be the usage of the whole thing together. Let's say we calculated the distance in the program and then use it to pass to different APIs.

Class123::Method123

{

Distance distanceToSomething(DistanceInKilometers(2345)); // initialize with kilometers

    APIUsingKilometers::API1Method(distanceToSomething); // call API which accepts kilometers, the value will be automatically given in kilometers

    APIUsingMiles::API2Method(distanceToSomething); // call API which accepts miles, the value will be automatically given in miles

}

As we see in the example above, all these class/type definitions were made just in order to be able to call these two APIs with “transparent” Distance value and so that each API will get  automatically translated correct value. It is assumed, of course, that in real life the number of times that those APIs are called are much much bigger and there will be a lot of savings in programmer's effort on calling those APIs, so that all this “preparation” work on defining special classes/types is not wasted.