C++ for Scientists - Technische Universität Dresden

90 CHAPTER 4. GENERIC PROGRAMMING
}
else
return b;
Note that the function body is exactly the same for both int and double.
With the template mechanism we can write just one generic implementation:
template
T inline max (T a, T b)
{
if (a > b)
return a;
else
return b;
}
The function can be used in the same way as the overloaded functions:
std::cout ≪ ”The maximum of 3 and 5 is ” ≪ max(3, 5) ≪ ’\n’;
std::cout ≪ ”The maximum of 3l and 5l is ” ≪ max(3l, 5l) ≪ ’\n’;
std::cout ≪ ”The maximum of 3.0 and 5.0 is ” ≪ max(3.0, 5.0) ≪ ’\n’;
In the first case, ‘3’ and ‘5’ are literals of type int and the max function is instantiated to
int inline max (int, int);
Likewise the second and third call of max instantiate
long inline max (long, long);
double inline max (double, double);
as the literals are interpreted as long and double.
In the same way the template function can be called with variables and expressions:
unsigned u1= 2, u2= 8;
std::cout ≪ ”The maximum of u1 and u2 is ” ≪ max(u1, u2) ≪ ’\n’;
std::cout ≪ ”The maximum of u1∗u2 and u1+u2 is ” ≪ max(u1∗u2, u1+u2) ≪ ’\n’;
Here the function is instantiated for short.
Instead of typename one can also write class in this context but we do not recommend this
because typename expresses better the intention of a generic function.
What does instantiation mean? When you write a non-generic function, the compiler reads
its definition, checks for errors, and generates executable code. When the compiler processes
a generic function’s definition it only checks certain errors (parsing errors) and generates no
executable code. For instance:
template
T inline max (T a, T b)
{
if a > b // Error !
return a;
else
return b;
}