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Although em(class) templates may be partially specialized, em(function)
templates may not. At times this is annoying. Assume a function template is
available implementing a certain unary operator that could be used with the
tt(transform) generic algorithm (cf. section ref(TRANSFORM)):
verb( template <typename Return, typename Argument>
Return chop(Argument const &arg)
{
return Return{ arg };
})
Furthermore assume that if tt(Return) is tt(std::string) then the above
implementation should not be used. Instead, with tt(std::string) a second
argument tt(1) should always be provided. If tt(Argument) is a bf(C++) string,
this would allow us to, e.g., return a copy of tt(arg) from which its first
character has been chopped off.
Since tt(chop) is a function, it is not possible to define a partial
specialization like this:
verb( template <typename Argument> // This won't compile!
std::string chop<std::string, Argument>(Argument const &arg)
{
return std::string{ arg, 1 };
})
Although a function template cannot be partially specialized it em(is)
possible to use overloading, defining a second, dummy, tt(string) parameter:
verb( template <typename Argument>
std::string chop(Argument const &arg, std::string)
{
return std::string{ arg, 1 };
})
COMMENT(
Now it em(is) possible to distinguish the two cases, but at the expense of
a more complex function call. Moreover, in code this function may
require the use of the tt(bind2nd) binder (cf. section ref(FUNADAPT)) to
provide the dummy second argument, or it may require a (possibly expensive to
construct) dummy argument to allow the compiler to choose among the two
overloaded function templates.
END)
Instead of providing a tt(string) dummy argument the functions em(could)
use the tt(IntType) template (cf. section ref(INTTYPE)) to select the proper
overloaded version. E.g., tt(IntType<0>) could be defined as the type of the
second argument of the first overloaded tt(chop) function, and tt(IntType<1>)
could be used for the second overloaded function. From the point of view of
program efficiency this is an attractive option, as the provided tt(IntType)
objects are extremely lightweight. tt(IntType) objects contain no data at
all. But there's also an obvious disadvantage as there is no intuitively clear
association between the tt(int) value used and the intended type.
Instead of defining arbitrary tt(IntType) types it is more attractive to
use another lightweight solution, using an automatic type-to-type
association. The tt(struct TypeType) is a lightweight type wrapper, much like
tt(IntType). Here is its definition:
verb( template <typename T>
struct TypeType
{
using Type = T;
};)
tt(TypeType) is also a lightweight type as it doesn't have any data fields
either. tt(TypeType) allows us to use a natural type association for
tt(chop)'s second argument. E.g, the overloaded functions can now be defined
as follows:
verb( template <typename Return, typename Argument>
Return chop(Argument const &arg, TypeType<Argument> )
{
return Return{ arg };
}
template <typename Argument>
std::string chop(Argument const &arg, TypeType<std::string> )
{
return std::string{ arg, 1 };
})
Using the above implementations any type can be specified for
tt(Result). If it happens to be a tt(std::string) the appropriate overloaded
version is automatically selected. The following additional overload of the
function tt(chop) capitalizes on this:
verb( template <typename Result>
Result chop(char const *txt) // char const * could also be a 2nd
{ // template type parameter
return chop(std::string{ txt }, TypeType<Result>{});
})
Using the third tt(chop) function, the following statement produces the
text `tt(ello world)':
verb( cout << chop<string>{ "hello world" } << '\n';)
Template functions do not support partial specializations. But they can be
overloaded. By providing overloads with dummy type-arguments that depend
on other parameters and calling these overloads from a overloaded function
that does not require the dummy type argument a situation similar to partial
specializations with class templates can often be realized.
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