C++ for Scientists - Technische Universität Dresden

4.4. CONCEPTS AND MODELING 97
type parameters. For instance the destructor uses the pointer data with a template type but
the delete function can deduce its type automatically and for the null pointer test it does not
matter either.
Template arguments can have default values. Assume, our vector class has in addition to the
value type also two parameters for the orientation and location:
template
class vector;
The arguments of a vector can be fully declared:
vector v;
The last argument is equal to the default value and can be omitted:
vector v;
As for functions, only the last arguments can be omitted. For instance, if the second argument
is the default and the last one is not we must write them all:
vector w;
If all template arguments are the default values, we can of course omit them all. However the
type is still a template class and the compiler gets confused if we skip the brackets in the type:
vector x; // wrong, it is considered a non−template class
vector y; // looks a bit strange but is correct
Other than the defaults of function arguments, the template defaults can refer to previous
template arguments:
template
class pair;
This is a class for two values that might have different types. If they do not we do not want to
declare it twice:
pair p1; // object with an int and float value
pair p2; // object with two int values
The dependency on previous arguments can be more complex than just equality when using
meta-functions that we will introduce in Chapter ??.
TODO: transition to next section
4.4 Concepts and Modeling
In the previous sections one could get the impression that template parameters can be replaced
by any type. This is in fact not entirely true. The programmer of templated classes and functions
makes assumptions about the operations that can be performed on the templated variables. So
it is very important to know which types may correctly be substituted for the formal template
parameters, in C ++ lingo which types the template function or class can be instantiated with.
Clearly, accumulate can be instantiated with int or double. Types without addition like a solver