C++ for Scientists - Technische Universität Dresden

5.2. PROVIDING TYPE INFORMATION 143
Constant types match both definitions but the second one is more specific and therefore picked
by the compiler. Non-constant types match only the first one. Note that the constancy of
template parameters is not considered, e.g., view is not regarded constant.
Compile-time Branching
The other tool we need for our view is a type selection depending on a logical condition. Introduced
was this technology by Krzysztof Czarnecki 9 and Ulrich W. Eisenecker [CE00]
This can be achieved by a rather simple implementation
1 template
2 struct if c
3 {
4 typedef ThenType type;
5 };
6
7 template
8 struct if c
9 {
10 typedef ElseType type;
11 };
Listing 5.2: Compile-time if
When this template is instantiated with a logical expressions and two type, only the general
definition in line 1 matches when the first argument evaluates to true and the ‘ThenType’ is used
in the type definition. If the first argument evaluates to false then the specialization in line 7
is more specific so that the ‘ElseType’ is used. Like many ingenious inventions it is very simple
once it is found.
This allows us to define funny things like using double for temporaries when our maximal
iteration number is larger than 100 other float:
typedef tst::if c 100, double, float>::type tmp type;
std::cout ≪ ”typeid = ” ≪ typeid(tmp type).name() ≪ ’\n’;
Needless to say that ‘max iter’ must be known at compile time. Admittedly, the example does
not look extremely useful and the meta-if is not so important in small isolated code snippets.
On the other hand, for the development of large generic software packages, it becomes extremely
important.
A convenience function as defined in the Meta-Programming Library [GA04] is ‘if ’
template
struct if
: if c
{};
It expects as first argument a type with a static const member named value and convertible to
bool. In other words, it selects the type based on the value of condition (and saves typing 8
characters).
9 Zu dem Zeitpunkt war er Doktorand an der TU Ilmenau.