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An essential characteristic of programming languages is that they allow the
conditional execution of code. For this bf(C++) offers the tt(if) and
tt(switch) statements. If we want to be able to `program the compiler'
this feature must also be offered by templates.
Like templates storing values templates making choices do not require any code
to be executed at run-time. The selection is purely made by the compiler, at
compile-time. The essence of template meta programming is that we are em(not)
using or relying on any executable code. The result of a template meta
program often is executable code, but that code is a function of decisions
merely made by the compiler.
Template (member) functions are only instantiated when they are actually
used. Consequently we can define specializations of functions that are
mutually exclusive. Thus it is possible to define a specialization that can be
compiled in situation one, but not in situation two and to define another
specialization that can be compiled in situation two, but not in situation
one. Using specializations code can be generated that is tailored to the
demands of a particular situation.
A feature like this cannot be implemented in run-time executable code. For
example, when designing a generic storage class the software engineer may
intend to store em(value class) type objects as well as objects of
em(polymorphic class) types in the final storage class. Thus the software
engineer may conclude that the storage class should contain pointers to
objects, rather than the objects themselves. The initial implementation
attempt could look like this:
verb( template <typename Type>
void Storage::add(Type const &obj)
{
d_data.push_back(
d_ispolymorphic ?
obj.clone()
:
new Type{obj}
);
})
The intent is to use the tt(clone) member function of the class tt(Type)
if tt(Type) is a polymorphic class and the standard copy constructor if
tt(Type) is a value class.
Unfortunately, this scheme usually fails as value classes do not define
tt(clone) member functions and polymorphic base classes should em(delete)
their copy constructors (cf. section ref(DEFAULTED)). It doesn't matter to the
compiler that tt(clone) is never called for value classes and that the copy
constructor is only available in value classes and not in polymorphic
classes. It merely has some code to compile, and can't do that because of
missing members. It's as simple as that.
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