A default value can be set for a parameter(argument) of a function. When the user does not give a value the function will take the default value. An important thing remember is an argument cannot have a default value unless all arguments on its right side must have default value.
Functions with valid default arguments are given below
float area(int x, int y, int z=30);
float area(int x, int y=20, int z=30);
float area(int x=10, int y=20, int z=30);
Functions with invalid default arguments are given below
float area(int x=10, int y, int z);
float area(intx, inty=20, int z);
float area(int x=10, int y=20, int z);