Function Objects

Boost C++ Libraries

Function Objects

adapted_to
always
identity
apply
bll_N
construct
construct_braced1
construct_braced2
construct_variadic1
get
pack

It is difficult or even impossible to get a Function Object from a function template. Egg no longer considers function templates to be useful, so that it provides "objectified" functions which replace some famous function templates. Note that a Function Object type defined in this section is Default Constructible and Copy Assignable unless otherwise specified.

adapted_to

Description

adapted_to provides the way to access underlying "base" Function Object or Little Function from adapted Function Object.

Header

<boost/egg/adapted_to.hpp>

Notation

b is a "base" object such that __decltype(b) is B const &.

Valid expressions

Valid expression	Semantics
`X_adapted_to<B>`	A Major Function Object type
`X_adapted_to<B>()(f)`	Extracts `b` from `f`.
`egg::adapted_to<B>(f)`	`X_adapted_to<B>()(f)`

Preconditions

f is a Function Object which Function Adaptors or Function Builders build.
B is a "base" Function Object or Little Function type.

Example

struct base_plus
{
    int id;

    typedef int result_type;

    result_type operator()(int x, int y) const
    {
        return x + y;
    }
};

typedef result_of_curry2<base_plus>::type T_curried_plus;
T_curried_plus const curried_plus777 = BOOST_EGG_CURRY2({777});
T_curried_plus const curried_plus999 = BOOST_EGG_CURRY2({999});

bool operator==(T_curried_plus const &left, T_curried_plus const &right)
{
    return egg::adapted_to<base_plus>(left).id == egg::adapted_to<base_plus>(right).id;
}

void egg_example()
{
    BOOST_CHECK( curried_plus777(4)(9) == 4+9 );
    BOOST_CHECK( egg::adapted_to<base_plus>(curried_plus777).id == 777);
    BOOST_CHECK( curried_plus777 == curried_plus777 );
    BOOST_CHECK(!(curried_plus777 == curried_plus999) );
}

See also

always

Description

always returns a Function Object always returning the same object.

Header

<boost/egg/always.hpp>

Model of

Static Function Object

Notation

bound(x) is __fwd_arg_list([x], by_value).

Valid expressions

Valid expression	Semantics
`always(x)`	A Major Function Object which holds `bound(x)`.
`always(x)(a1,...,aN)`	`__fwd_arg_list(bound(x), by_cref)`
`always_ref`	Static Function Object
`always_ref(y)`	A Major Function Object
`always_ref(y)(a1,...,aN)`	`__fwd_arg_list([y], by_perfect)`

Preconditions

0 <= N && N <= BOOST_EGG_MAX_LINEAR_ARITY.
__typeof(x) is Copy Constructible.

Example

void egg_example()
{
    int ten = 10;
    BOOST_CHECK( always(ten)(1,2,3,4,5) == 10 );
    BOOST_CHECK( &(always_ref(ten)(1,2,3,4,5)) == &ten );
}

See also

identity

identity

Description

identity returns argument unchanged.

Header

<boost/egg/identity.hpp>

Model of

Static Function Object

Valid expressions

Valid expression	Semantics
`X_identity<__Stg = __ud>`	A Major Function Object type
`X_identity<__Stg>()(a1)`	`__fwd_arg_list([a1], __Stg)`
`identity`	`X_identity<>()`

Example

void egg_example()
{
    BOOST_CHECK( identity(10) == 10 );
}

See also

always

apply

Description

apply calls a function with trailing arguments.

Header

<boost/egg/apply.hpp>

Model of

Static Function Object

Valid expressions

Valid expression	Semantics
`X_apply<__Stg = __ud>`	A Major Function Object type
`X_apply<__Stg>()(__pfo,a1,...,aN)`	`__const(__pfo)(__fwd_arg_list(a, __Stg))`
`apply`	`X_apply<>()`

Preconditions

__fwd_arg_list([__pfo,a1,...,aN], __Stg) is a valid expression.

Example

void egg_example()
{
    std::negate<int> my_negate;
    BOOST_CHECK(
        apply(my_negate, apply(my_negate, apply(my_negate, 10))) == -10 );
}

bll_N

Description

bll_N is a Lexically Typed Object which represents boost::lambda::_N.

	Caution
	`bll_N` is neither statically initialized, Default Constructible nor Copy Assignable.

Header

<boost/egg/bll/placeholders.hpp>

Model of

Valid expressions

Valid expression	Semantics
`bll_%%N`	Polymorphic `boost::lambda::_%%N`

Invariants

boost::remove_reference<__decltype(bll_%%N)>::type is T_bll_%%N const.
boost::result_of<T_bll_%%N const(a1,...,aK)>::type is the same as boost::result_of<T_bll_%%N(a1,...,aK)>::type.

Example

void egg_example()
{
    std::plus<int> plus;

    result_of_<
        result_of_<T_lazy(std::plus<int> &)>::type(T_bll_1 const &, int)
    >::type
        f = lazy(plus)(bll_1, 10);

    int i2 = 2;
    BOOST_CHECK( f(i2) == 12 );
}

See also

Boost.Lambda

construct

Description

X_construct is constructor as Function Object.

Header

<boost/egg/construct.hpp>

Valid expressions

Valid expression	Semantics
`X_construct<T, __Stg = __ud>`	A Major Function Object type
`X_construct<T, __Stg>()(a1,...,aN)`	`T(__fwd_arg_list(a, __Stg))`
`X_construct<>`	`X_construct<__mpl::_1, __mpl::_2>`

Example

void egg_example()
{
    BOOST_CHECK( apply(X_construct<int>(), 3) == 3 );
}

See also

generator

construct_braced1

Description

X_construct_braced1 constructs object using braced-initializer.

Header

<boost/egg/construct_braced1.hpp>

Valid expressions

Valid expression	Semantics
`X_construct_braced1<T, __Stg = __ud>`	A Major Function Object type
`X_construct_braced1<T, __Stg>()(a1,...,aN)`	`T t = {__fwd_arg_list(a, __Stg)}; return t;`
`X_construct_braced1<>`	`X_construct_braced1<__mpl::_1, __mpl::_2>`

See also

generator

construct_braced2

Description

X_construct_braced2 constructs object using braced-initializer.

Header

<boost/egg/construct_braced2.hpp>

Valid expressions

Valid expression	Semantics
`X_construct_braced2<T, __Stg = __ud>`	A Major Function Object type
`X_construct_braced2<T, __Stg>()(a1,...,aN)`	`T t = {{__fwd_arg_list(a, __Stg)}}; return t;`
`X_construct_braced2<>`	`X_construct_braced2<__mpl::_1, __mpl::_2>`

See also

generator

construct_variadic1

Description

X_construct_variadic1 constructs variadic object using braced-initializer.

Header

<boost/egg/construct_variadic1.hpp>

Valid expressions

Valid expression	Semantics
`X_construct_variadic1<T, __Stg = __ud>`	A Major Function Object type
`X_construct_variadic1<T, __Stg>()(a1,...,aN)`	`T t = BOOST_EGG_VARIADIC_L {__fwd_arg_list(a, __Stg)} BOOST_EGG_VARIADIC_R; return t;`
`X_construct_variadic1<>`	`X_construct_variadic1<__mpl::_1, __mpl::_2>`

See also

get

Description

get returns the N-th element from the beginning of the Fusion Forward Sequence.

Header

<boost/egg/get.hpp>

Notation

fusion is boost::fusion.

Valid expressions

Valid expression	Semantics
`X_get<N>`	A Major Function Object type
`X_get<N>()(t)`	`fusion::deref(fusion::advance<N>(fusion::begin(t)))`
`X_get_c<n>`	`X_get< __mpl::int_<n> >`
`egg::get<N>(t)`	`X_get<N>()(t)`
`egg::get_c<n>(t)`	`X_get_c<n>()(t)`

Example

void egg_example()
{
    typedef std::map<std::string, int> map_t;
    
    map_t m;
    m["one"] = 1;
    m["two"] = 2;

    int a[] = {1,2};
    boost::transform_iterator<X_get_c<1>, map_t::iterator, int &> it(m.begin(), X_get_c<1>());
    BOOST_CHECK(std::equal(a, a+2, it));
}

See also

pack

Description

pack makes boost::tuple<...> for argument forwarding.

Header

<boost/egg/pack.hpp>

Model of

Static Function Object

Notation

dts_(c1,...,cK) is __decltype(c1),...,__decltype(cK).

Valid expressions

Valid expression	Semantics
`X_pack<__Stg = __ud>`	A Major Function Object type
`X_pack<__Stg>()(a1,...,aN)`	`boost::tuple<dts_(__fwd_arg_list(a, __Stg))>(__fwd_arg_list(a, __Stg))`
`pack`	`X_pack<>()`

Preconditions

If __bytag_at(__Stg, N, I) is by_value, __typeof(aI) is Copy Constructible.

Example

void egg_example()
{
    BOOST_CHECK(
        boost::get<1>(pack(1,'1',std::string("one"))) == '1' );
}

See also