Understanding Function Overloading in C++

1. Computer Science 1620 Function Overloading

2. Review Question: • suppose I have the following function: • can I call this function with an integer? • yes – compiler will promote integer to double • same thing applies to demotion • double square(double x) { • return x * x; • } • int main() { • cout << square(3) << endl;

3. Function Overloading • Suppose you are in charge of writing a maximum function • The program must work with three different types: • int • double • float

4. Function Overloading • First Way: double maximum(double n1, double n2) { if (n1 > n2) return n1; else return n2; }

5. Function Overloading • The previous example works fine, but inefficient • consider following call int x, y; cin >> x >> y; int z = maximum(x,y); • consider what happens when compiling the program • the values x and y must be converted to type double in order to use the function • the return value is of type double … which must be converted back to an integer (which generates a compiler warning)

6. Function Overloading • Second Try: int maximum(int n1, int n2) { if (n1 > n2) return n1; else return n2; }

7. Function Overloading • The previous example works fine for integers, but what about doubles? • consider following call double x, y; cin >> x >> y; double z = maximum(x,y); cout << z << endl; • suppose I run this program, and type in the values 4.6 and 7.4 • what will the output be?

9. Function Overloading • Second try: double maximumDouble(double n1, double n2) { if (n1 > n2) return n1; else return n2; } float maximumFloat(float n1, float n2) { if (n1 > n2) return n1; else return n2; } int maximumInt(int n1, int n2) { if (n1 > n2) return n1; else return n2; }

10. Function Overloading • We can now change our original call int x, y; cin >> x >> y; int z = maximumInt(x,y); • no conversions take place • some problems though • what if a programmer writes a program using integers … and the supervisor decides to use doubles instead • not only are you changing your variable types but now you have to change all of your calls to maximum

11. Function Overloading • Solution: Function overloading • C++ allows you to use the same function name for different functions • the functions must have different parameter lists • the function being called will depend on the arguments being sent

12. Function Overloading • Last try: double maximum(double n1, double n2) { if (n1 > n2) return n1; else return n2; } float maximum(float n1, float n2) { if (n1 > n2) return n1; else return n2; } int maximum(int n1, int n2) { if (n1 > n2) return n1; else return n2; }

13. Function Overloading • Now when we make the following call: int x, y; cin >> x >> y; int z = maximum(x,y); • no conversions take place • if we wanted to change to doubles, we need only change the variable type double x, y; cin >> x >> y; double z = maximum(x,y);

14. Function Overloading • Resolving overloaded definitions • the compiler must be able to choose between two function declarations • always chooses the best match • overload resolution is a complex task • beyond scope of class • suffices to know that it tries for closest match