Functions

In most non-trivial programs, functions need to be declared before their implementation is given (``prototypes''). Normally, this is done in the ``header''-file (ending on .h) belonging to a C$^{++}$-file. Variable names can be omitted in the prototype (and types in the implementation) if that does not lead to confusion. maximum.h

// Prototype:
int max(int , int );

maximum.cxx

#include "maximum.h"

// Implementation:
int max (int a, int b) {
   int max;
   if (a>b) max = a;
   else     max = b;
   return max;
}

• Parameter types can be references (marked by &) ³.
• Passing (and returning results) by value (int max(int a, int b)) makes a copy of the data (i.e., in the example, a and b would be copied at the beginning of the function, then the local variable max is copied again to provide a return value from the function).
• Passing by reference (int max(int &a, int &b)) does not copy data but may change the original (passed) data in the calling function.
• Marking reference parameters constant (Item max(const Item &a, const Item &b)) makes data in the calling function non-changeable.