C++ for Scientists - Technische Universität Dresden

2.6. FUNCTIONS 39
case 3: v[i] = w[i] + x[i]; i++;
case 2: v[i] = w[i] + x[i]; i++;
case 1: v[i] = w[i] + x[i];
case 0: ;
}
This technique is called Duff’s device. Although this is an interesting technique to realize an
iterative computation without a loop, the performance impact is probably limited in practice.
Such technique should be only considered in program parts with a significant fraction on the
overall run time; otherwise readability of sources is more important.
2.5.7 Goto
DO NOT USE IT. NEVER! EVER!
2.6 Functions
Functions are important building blocks of C ++ programs. The first example we have seen is
the main function in the hello-world program. main must be present in every executable and is
called when the program starts. Other than that there is noting special about main.
The general form of a C ++ function is:
[inline] return type function name (argument list)
{
body of the function
}
For instance, one can be implement a very simple function to square a value:
double square(double x)
{
return x ∗ x;
}
In C and C ++ each function has a return type. A function that does not return a value has the
pseudo-return-type “void”:
void print(double x)
{
std::cout ≪ ”x is ” ≪ x ≪ ’\n’;
}
void is not a real type but moreover a placeholder that enables us to omit returning a value.
We cannot define objects of it:
void nothing; // error