C++ for Scientists - Technische Universität Dresden

5.2. PROVIDING TYPE INFORMATION 145
5.2.3 More Useful Meta-functions
The Boost Type Traits library [?] provides a large spectrum of meta-functions to test or manipulate
attributes of types. Some of them are rather easy to implement — like the previously
introduced is const — and others — like has trivial constructor or is base — require deep insight
into C ++ subtleties and often into compiler internals as well. Unless one only uses very simple
type traits and wants absolutely avoid the dependency of an external library, it is advisable to
favor the extensively tested implementations from the type traits library over rewriting it.
With the boost::is xyz we can implement special behavior for certain sets of types. One can easily
add tests for domain specific type sets:
template
struct is matrix
: boost::mpl::false
{};
template
struct is matrix
: boost::mpl::true
{};
// more matrix classes ...
template
struct is matrix
: is matrix
{};
// more views ...
Our program snippet is in line with the implementations in Boost. Instead of defining a static
constant as in Section 5.2.2 we derive the meta function from boost::mpl::false and boost::mpl::true
where static constants are defined with some additional typedefs. This not only shorter but
requires also a bit less compile time, see [?]. 10
The code is quite self-explanatory. Type we do not know are considered not being a matrix.
Then we specialize for known matrix classes. For views we can further refer to the matrix-ness
of the template argument.
Alternatively, we can say in the type trait that every transposed view is a matrix and instead
require for template arguments of transposed view that they are matrices.
#include
template
class transposed view
{
BOOST STATIC ASSERT((is matrix::value)); // Make sure that the argument is a matrix type
// ...
};
This additional assertion guarantees that the view class can only be instantiated with known
matrix types. For other argument types the compilation, will terminate in this line. Unfortunately,
the error message is not very informative for not saying confusing:
10 TODO: page