C++ for Scientists - Technische Universität Dresden

98 CHAPTER 4. GENERIC PROGRAMMING
class (on page 70) cannot be used for accumulate. What should be accumulated from a set of
solvers? All the requirements for the template T of the function accumulate can be summarized
as follows:
• T is CopyConstructable;
– Copy constructor T::T(const T&) exists so that ‘T a(b);’ is compilable if b is of type
T.
• T is PlusAssignable:
– PlusAssign operator T::operator+=(const T&) exists so that ‘a+= b;’ is compilable if
b is of type T.
• T is Constructible from int
– Constructor T::T(int) exists so that ‘T a(0);’ is compilable.
Such a set of type requirements is called a ‘Concept’. A concept CR that contains all requirements
of concept C and additional requirements is called a ‘Refinement’ of C. A type t that
holds all requirements of concept C is called a ‘Model’ of C.
A complete definition of a template function or type shall contain the list of required concepts
like it is done for functions from the Standard Template Library, see http://www.sgi.com/
tech/stl/.
Today such requirements are mere documentation. A prototype of a C ++ Concept Compiler [?]
that checks that
• Whether a function can be called with a certain type (combination 5 );
• Whether a class can be instantiated with a certain types (combination); and
• Whether a function’s requirement list covers all used expressions, including those in subfunctions.
The compiler generates short and comprehensive message when template functions or classes
are used erroneously. People interested in generic programming shall try the compiler, it helps
for better understanding. However, the compiler really is a prototype and must not be used in
production code. This functionality was even planned for the next language standard but the
committee could achieve a consensus on its details, to make a (very) long story short.
Discussion 4.1 The most vulnerable aspect of generic programming is the semantic conformance,
that is which Semantic Concepts are modelled. For instance, an algorithm might require
that a binary operation is associative to calculate correctly. One can express this requirement
in the functions documentation but if someone calls this function with an operation that is not
associative the compiler has no idea about this. If one violates a syntactical requirement than
the compiler will complain about the missing function or operator — often in a hardly readable
form — but it will be caught no matter what. If one violates a semantic requirement the
compiler generates erroneous executables and the compilation does not give not any warning
because it is entirely unaware of the user types’ semantic. The only way to find such semantic
errors in templates with today’s compilers is careful documentation (and its reading of course).
Latest research gives hope that future C ++ standards and compilers will provide more reliable
and elegant possibilities to ensure semantic correctness of template programs.
5 If you have multiple template arguments