...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
An interesting peculiarity of functions like at
when applied to a Forward
Sequence like list
is that what could have been linear runtime complexity effectively becomes
constant O(1) due to compiler optimization of C++ inlined functions, however
deeply recursive (up to a certain compiler limit of course). Compile time complexity
remains linear.
Associative sequences use function overloading to implement membership testing
and type associated key lookup. This amounts to constant runtime and amortized
constant compile time complexities. There is an overloaded function, f(k)
, for each key type k
. The compiler chooses the appropriate function
given a key, k
.
Tag dispatching is a generic programming technique for selecting template specializations. There are typically 3 components involved in the tag dispatching mechanism:
For example, the fusion result_of::begin
metafunction
is implemented as follows:
template <typename Sequence> struct begin { typedef typename result_of::begin_impl<typename traits::tag_of<Sequence>::type>:: template apply<Sequence>::type type; };
In the case:
Sequence
is the type for
which a suitable implementation of result_of::begin_impl
is required
traits::tag_of
is the metafunction that associates
Sequence
with an appropriate
tag
result_of::begin_impl
is the template which is specialized
to provide an implementation for each tag type
Unlike MPL, there is no
extensible sequence concept in fusion. This does not mean that Fusion sequences
are not extensible. In fact, all Fusion sequences are inherently extensible.
It is just that the manner of sequence extension in Fusion is different from
both STL
and MPL on account of the
lazy nature of fusion Algorithms.
STL
containers extend themselves in place though member functions such as push_back
and insert
. MPL
sequences, on the other hand, are extended through "intrinsic" functions
that actually return whole sequences. MPL
is purely functional and can not have side effects. For example, MPL's
push_back
does not actually
mutate an mpl::vector
. It can't do that. Instead, it returns
an extended mpl::vector
.
Like MPL, Fusion too is
purely functional and can not have side effects. With runtime efficiency in
mind, Fusion sequences are extended through generic functions that return
Views. Views
are sequences that do not actually contain data, but instead impart an alternative
presentation over the data from one or more underlying sequences. Views
are proxies. They provide an efficient yet purely functional way to work on
potentially expensive sequence operations. For example, given a vector
, Fusion's push_back
returns a joint_view
, instead of an actual extended
vector
.
A joint_view
holds a reference to the original sequence plus the appended data --making
it very cheap to pass around.
Functions that take in elemental values to form sequences (e.g. make_list
) convert their arguments
to something suitable to be stored as a sequence element. In general, the element
types are stored as plain values. Example:
make_list
(1, 'x')
returns a list
<int,
char>
.
There are a few exceptions, however.
Arrays:
Array arguments are deduced to reference to const types. For example [14]:
make_list
("Donald", "Daisy")
creates a list
of type
list
<const char (&)[7], const char (&)[6]>
Function pointers:
Function pointers are deduced to the plain non-reference type (i.e. to plain function pointer). Example:
void f(int i);
...
make_list
(&f);
creates a list
of type
list
<void (*)(int)>
Fusion's generation functions (e.g. make_list
) by default stores the element
types as plain non-reference types. Example:
void foo(const A& a, B& b) {
...
make_list
(a, b)
creates a list
of type
list
<A, B>
Sometimes the plain non-reference type is not desired. You can use boost::ref
and boost::cref
to store references or const references
(respectively) instead. The mechanism does not compromise const correctness
since a const object wrapped with ref results in a tuple element with const
reference type (see the fifth code line below). Examples:
For example:
A a; B b; const A ca = a;make_list
(cref(a), b); // creates list<const A&, B>make_list
(ref(a), b); // creates list<A&, B>make_list
(ref(a), cref(b)); // creates list<A&, const B&>make_list
(cref(ca)); // creates list<const A&>make_list
(ref(ca)); // creates list<const A&>
See Ref utility for details.
Since C++11, the standard reference wrappers (std::ref
and
std::cref
) work as well.
To adapt arbitrary data types that do not allow direct access to their members,
but allow indirect access via expressions (such as invocations of get- and
set-methods), fusion's BOOST_FUSION_ADAPT_xxxADTxxx
-family
(e.g. BOOST_FUSION_ADAPT_ADT
)
may be used. To bypass the restriction of not having actual lvalues that represent
the elements of the fusion sequence, but rather a sequence of paired expressions
that access the elements, the actual return type of fusion's intrinsic sequence
access functions (at
, at_c
, at_key
, deref
, and deref_data
) is a proxy type, an instance
of adt_attribute_proxy
, that
encapsulates these expressions.
adt_attribute_proxy
is defined
in the namespace boost::fusion::extension
and has three template arguments:
namespace boost { namespace fusion { namespace extension { template< typename Type , int Index , bool Const > struct adt_attribute_proxy; }}}
When adapting a class type, adt_attribute_proxy
is specialized for every element of the adapted sequence, with Type
being the class type that is adapted,
Index
the 0-based indices of
the elements, and Const
both
true
and false
.
The return type of fusion's intrinsic sequence access functions for the Nth
element of an adapted class type type_name
is adt_attribute_proxy<type_name, N, Const>
,
with Const
being true
for constant instances of type_name
and false
for non-constant ones.
Notation
type_name
The type to be adapted, with M attributes
inst
Object of type type_name
const_inst
Object of type type_name const
(attribute_typeN, attribute_const_typeN,
get_exprN, set_exprN)
Attribute descriptor of the Nth attribute of type_name
as passed to the adaption
macro, 0≤N<M
proxy_typeN
adt_attribute_proxy<type_name, N,
with N
being an integral constant, 0≤N<M
false
>
const_proxy_typeN
adt_attribute_proxy<type_name, N,
with N
being an integral constant, 0≤N<M
true
>
proxyN
Object of type proxy_typeN
const_proxyN
Object of type const_proxy_typeN
Expression Semantics
Expression |
Semantics |
---|---|
|
Creates an instance of |
|
Creates an instance of |
|
Another name for |
|
Another name for |
|
Invokes |
|
Invokes |
|
Invokes |
Additionally, proxy_typeN
and const_proxy_typeN
are copy constructible, copy assignable and implicitly convertible to proxy_typeN::type
or const_proxy_typeN::type
.
Tip | |
---|---|
To avoid the pitfalls of the proxy type, an arbitrary class type may also be adapted directly using fusion's intrinsic extension mechanism. |
[14]
Note that the type of a string literal is an array of const characters, not
const char*
. To get make_list
to create a list
with an element of a non-const
array type one must use the ref
wrapper (see Reference Wrappers
).