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Consider tt(string)'s member tt(insert). tt(String::insert) has several
overloaded implementations. It can be used to insert text (completely or
partially) provided by a tt(string) or by a tt(char const *) argument; to
insert single characters a specified number of times; iterators can be used to
specify the range of characters to be inserted; etc., etc.. All in all,
tt(string) offers as many as five overloaded tt(insert) members.
Assume the existence of a class tt(Inserter) that is used to insert
information into all kinds of objects. Such a class could have a tt(string)
data member into which information can be inserted. tt(Inserter)'s interface
only partially has to copy tt(string)'s interface to realize this: only
tt(string::insert)'s interfaces must be duplicated. The members duplicating
interfaces often contain one statement (calling the appropriate member
function of the object's data member) and are for this reason often
implemented in-line. Such em(wrapper functions) merely emi(forward) their
parameters to the matching member functions of the object's data member.
Another example is found in em(factory functions) that also frequently forward
their parameters to the constructors of objects that they return.
bf(C++) simplifies and generalizes forwarding of parameters by
offering emi(perfect forwarding), implemented through rvalue references and
variadic templates. With perfect forwarding the arguments passed to functions
are `perfectly forwarded' to nested functions. Forwarding is called
em(perfect) as the arguments are forwarded in a type-safe way. To use perfect
forwarding nested functions must define parameter lists matching the
forwarding parameters both in types and number.
Perfect forwarding is easily implemented:
itemization(
it() The forwarding function is defined as a template+nl()
(usually a em(variadic) template, but single argument forwarding is
also possible. To define and forward a single argument omit the
ellipsis from the following code);
it() The forwarding function's parameter list is an
hi(parameter pack: rvalue reference)
em(rvalue reference parameter pack) (e.g., tt(Params &&...params));
itht(forward)(std::forward) is used to forward the forwarding function's
arguments to the nested function, keeping track of their types and
number. Before tt(forward) can be used the tthi(utility) header file
must be included.
it() The nested function is called using this stanza to specify its
arguments: nl()
tt(std::forward<Params>(params)...).
)
In the next example perfect forwarding is used to implement em(one) member
tt(Inserter::insert) that can be used to call any of the five overloaded
tt(string::insert) members. The tt(insert) function that's actually called now
simply depends on the types and number of arguments that are passed to
tt(Inserter::insert):
verb( class Inserter
{
std::string d_str; // somehow initialized
public:
// constructors not implemented,
// but see below
Inserter();
Inserter(std::string const &str);
Inserter(Inserter const &other);
Inserter(Inserter &&other);
template<typename ...Params>
void insert(Params &&...params)
{
d_str.insert(std::forward<Params>(params)...);
}
};)
A factory function returning an tt(Inserter) can also easily be implemented
using perfect forwarding. Rather than defining four overloaded factory
functions a single one now suffices. By providing the factory function with an
additional template type parameter specifying the class of the object to
construct the factory function is turned into a completely general factory
function:
verb( template <typename Class, typename ...Params>
Class factory(Params &&...params)
{
return Class(std::forward<Params>(params)...);
})
Here are some examples showing its use:
verb( Inserter inserter(factory<Inserter>("hello"));
string delimiter(factory<string>(10, '='));
Inserter copy(factory<Inserter>(inserter));)
The function tt(std::forward) is provided by the standard library. It
performs no magic, but merely returns tt(params) as a nameless object. That
way it acts like tt(std::move) that also removes the name from an object,
returning it as a nameless object. The unpack operator has nothing to do with
the use of tt(forward) but merely tells the compiler to apply tt(forward) to
each of the arguments in turn. Thus it behaves similarly to bf(C)'s ellipsis
operator used by variadic functions.
Perfect forwarding was introduced in section ref(CONTRACTIONS): a template
function defining a tt(Type &¶m), with tt(Type) being a template type
parameter converts tt(Type &&) to tt(Tp &) if the function is called with
an argument of type tt(Tp &). Otherwise it binds tt(Type) to tt(Tp),
with tt(param) being defined as tt(Tp &¶m). As a result an em(lvalue)
argument binds to an lvalue-type (tt(Tp &)), while an em(rvalue) argument
binds to an rvalue-type (tt(Tp &&)).
The function tt(std::forward) merely passes the argument (and its type) on to
the called function or object. Here is its simplified implementation:
verb( typedef <type T>
T &&forward(T &&a)
{
return a;
})
Since tt(T &&) turns into an lvalue reference when tt(forward) is called
with an lvalue (or lvalue reference) and remains an rvalue reference if
tt(forward) is called with an rvalue reference, and since tt(forward) (like
tt(std::move)) anonymizes the variable passed as argument to tt(forward), the
argument value is forwarded while passing its type from the function's
parameter to the called function's argument.
This is called em(perfect forwarding) as the nested function can only be
called if the types of the arguments that were used when calling the `outer'
function (e.g., tt(factory)) exactly match the types of the parameters of the
nested function (e.g., tt(Class)'s constructor). Perfect forwarding therefore
is a tool to realize type safety.
A cosmetic improvement to tt(forward) requires users of tt(forward) to
specify the type to use rather than to have the compiler deduce the type as a
result of the function template parameter type deduction's process. This is
realized by a small support struct template:
verb( template <typename T>
struct identity
{
using type = T;
};)
This struct merely defines tt(identity::type) as tt(T), but as it is a
struct it must be specified explicitly. It cannot be determined from the
function's argument itself. The subtle modification to the above
implementation of tt(forward) thus becomes (cf. section ref(DISTINGUISH) for
an explanation of the use of tt(typename)):
verb( typedef <type T>
T &&forward(typename identity<T>::type &&arg)
{
return arg;
})
Now tt(forward) must explicitly state tt(arg)'s type, as in:
verb( std::forward<Params>(params))
Using the tt(std::forward) function and the rvalue reference specification
is not restricted to the context of parameter packs. Because of the special
way rvalue references to template type parameters are treated (cf. section
ref(CONTRACTIONS)) they can profitably be used to forward individual function
hi(forward: parameters)
parameters as well. Here is an example showing how an argument to a function
can be forwarded from a template to a function that is itself passed to the
template as a pointer to an (unspecified) function:
verb( template<typename Fun, typename ArgType>
void caller(Fun fun, ArgType &&arg)
{
fun(std::forward<ArgType>(arg));
})
A function tt(display(ostream &out)) and tt(increment(int &x)) may now
both be called through tt(caller). Example:
verb( caller(display, cout);
int x = 0;
caller(increment, x);)
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