C++

 // This class converts temperature between Celsius and Fahrenheit.
 // It may be used by assigning a value to either Celsius or Fahrenheit 
 // and then retrieving the other value, or by calling the ToCelsius or
 // ToFahrenheit methods directly.
 //
 // References:
 //     https://www.mathsisfun.com/temperature-conversion.html
 //     https://en.wikibooks.org/wiki/C%2B%2B_Programming
 
 #include <iostream>
 
 using namespace std;
 
 class Temperature {
     public:
         double getCelsius(void);
         void setCelsius(double value);
         double getFahrenheit(void);
         void setFahrenheit(double value);
         double toCelsius(double fahrenheit);
         double toFahrenheit(double celsius);
     
     private:
         double celsius;
         double fahrenheit;
 };
 
 double Temperature::getCelsius(void) {
     return celsius;
 }
 
 void Temperature::setCelsius(double value) {
     celsius = value;
     fahrenheit = toFahrenheit(celsius);
 }
 
 double Temperature::getFahrenheit(void) {
     return fahrenheit;
 }
 
 void Temperature::setFahrenheit(double value) {
     fahrenheit = value;
     celsius = toCelsius(fahrenheit);
 }
     
 double Temperature::toCelsius(double fahrenheit) {
     return (fahrenheit - 32) * 5 / 9;
 }
 
 double Temperature::toFahrenheit(double celsius) {
     return celsius * 9 / 5 + 32;
 }
 
 // This program creates instances of the Temperature class to convert Celsius 
 // and Fahrenheit temperatures.
 //
 // References:
 //     https://www.mathsisfun.com/temperature-conversion.html
 //     https://en.wikibooks.org/wiki/C%2B%2B_Programming
 
 int main() {
     Temperature temp1;
     temp1.setCelsius(100.0);
     cout << "temp1.celsius = " << temp1.getCelsius() << endl;
     cout << "temp1.fahrenheit = " << temp1.getFahrenheit() << endl;
     cout << endl;
     
     Temperature temp2;
     temp2.setFahrenheit(100.0);
     cout << "temp2.fahrenheit = " << temp2.getFahrenheit() << endl;
     cout << "temp2.celsius = " << temp2.getCelsius() << endl;
 }