C++ for Scientists - Technische Universität Dresden

66 CHAPTER 3. CLASSES
apply symm blk2x2 rowmajor dnsvec multhr athlon(L.data addr, L.nrows, L.ncols,
L.ldim, L.blksch, v.data addr, v.size);
Developing software in this fashion is far from being fun. It wastes so much energy of the
programmer. Getting such calls right is of course much more work than the former notations.
If one of the arguments is stored in a different format, the function call must be meticulously
adapted. Remember the person who implements the linear projection wanted to do science,
actually.
The cardinal error of scientific software providing such interfaces — there is even worse than
our example — is to commit to too many technical details in the user interface. The reason lies
partly in the usage of simplistic programming languages as C and Fortran 77 or in the effort to
interoperate with software in these languages.
Advise
If you ever get forced to write software that interoperates with C or Fortran,
write your software first with a concise and intuitive interface in C ++ for
yourself and other C ++ programmer and add the C and Fortran interface on
top of it.
The elegant way of writing scientific software is to use and to provide the best abstraction. A
good implementation reduces the user interface to the essential behavior and omits all surplus
commitments to technical details. Applications with a concise and intuitive interface can be as
efficient as their ugly and detail-obsessed counterparts.
In our example, this is achieved by providing a class for every specific linear operator and implement
the projection type-dependently. 1 This way, we can apply the projection without given
all details and the user application is short and nice. This chapter will show the foundations of
how providing new abstraction in scientific software and the following chapters will elaborate
this.
3.2 Class members
Object types are called classes in C ++, defined by the class keyword. A class defines a new data
type, which can be used to create objects. A class is a collection of:
• data;
• functions which are also referred to as member functions or methods;
• types
Furthermore class members can be public or private and classes can inherit from each other.
Let us now give an example to illustrate the class concept. To have something tangible for
scientists, we refrain from foo and bar examples but implement gradually a class complex (al-
1 Specializations for specific platforms can also be handled with the type system.