SFINAE do sprawdzania funkcji dziedzicznych członków

Odpowiedź Joshperry ' ego jest bardzo mądra i elegancka, ale (jak zostało to opisane poniżej) nie sprawdza poprawnie podpisu foo() i nie działa z podstawowymi typami( jak int): powoduje błąd kompilatora. Zaproponuję technikę, która poprawnie obsługuje odziedziczone członków, a także sprawdza podpis funkcji członka. Zamiast wdawać się w szczegóły podam dwa przykłady i mam nadzieję, że kod będzie mówił sam za siebie.

Przykład1:

Sprawdzamy członek z następującym podpisem: T::const_iterator begin() const

template<class T> struct has_const_begin
{
    typedef char (&Yes)[1];
    typedef char (&No)[2];

    template<class U> 
    static Yes test(U const * data, 
                    typename std::enable_if<std::is_same<
                             typename U::const_iterator, 
                             decltype(data->begin())
                    >::value>::type * = 0);
    static No test(...);
    static const bool value = sizeof(Yes) == sizeof(has_const_begin::test((typename std::remove_reference<T>::type*)0));
};

Proszę zauważyć, że sprawdza ona nawet stałość metody i działa również z typami prymitywnymi. (Znaczy has_const_begin<int>::value jest false i nie powoduje błędu w czasie kompilacji.)

Przykład 2

Teraz szukamy podpisu: void foo(MyClass&, unsigned)

template<class T> struct has_foo
{
    typedef char (&Yes)[1];
    typedef char (&No)[2];

    template<class U>
    static Yes test(U * data, MyClass* arg1 = 0,
                    typename std::enable_if<std::is_void<
                             decltype(data->foo(*arg1, 1u))
                    >::value>::type * = 0);
    static No test(...);
    static const bool value = sizeof(Yes) == sizeof(has_foo::test((typename std::remove_reference<T>::type*)0));
};

Proszę zauważyć, że MyClass nie musi być domyślną konstrukcją ani spełniać żadnej specjalnej koncepcji. Technika ta działa również z szablonami.

Jestem niecierpliwie czekam na opinie na ten temat.

Oto kilka fragmentów użycia: / Align = "center" bgcolor = "# e0ffe0 " / cesarz Chin / / align = center / ]}

Sprawdź czy członek x w danej klasie. Może to być var, func, class, union lub enum:

CREATE_MEMBER_CHECK(x);
bool has_x = has_member_x<class_to_check_for_x>::value;

Sprawdzanie funkcji członka void x():

//Func signature MUST have T as template variable here... simpler this way :\
CREATE_MEMBER_FUNC_SIG_CHECK(x, void (T::*)(), void__x);
bool has_func_sig_void__x = has_member_func_void__x<class_to_check_for_x>::value;

Sprawdzanie zmiennej członkowskiej x:

CREATE_MEMBER_VAR_CHECK(x);
bool has_var_x = has_member_var_x<class_to_check_for_x>::value;

Sprawdź klasę członkax:

CREATE_MEMBER_CLASS_CHECK(x);
bool has_class_x = has_member_class_x<class_to_check_for_x>::value;

Sprawdź związek członkowskix:

CREATE_MEMBER_UNION_CHECK(x);
bool has_union_x = has_member_union_x<class_to_check_for_x>::value;

Sprawdź, czy członek enum x:

CREATE_MEMBER_ENUM_CHECK(x);
bool has_enum_x = has_member_enum_x<class_to_check_for_x>::value;

Sprawdź, czy nie ma żadnej funkcji członka x niezależnie od podpisu:

CREATE_MEMBER_CHECK(x);
CREATE_MEMBER_VAR_CHECK(x);
CREATE_MEMBER_CLASS_CHECK(x);
CREATE_MEMBER_UNION_CHECK(x);
CREATE_MEMBER_ENUM_CHECK(x);
CREATE_MEMBER_FUNC_CHECK(x);
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;

Lub

CREATE_MEMBER_CHECKS(x);  //Just stamps out the same macro calls as above.
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;

Szczegóły i rdzeń:

/*
    - Multiple inheritance forces ambiguity of member names.
    - SFINAE is used to make aliases to member names.
    - Expression SFINAE is used in just one generic has_member that can accept
      any alias we pass it.
*/

//Variadic to force ambiguity of class members.  C++11 and up.
template <typename... Args> struct ambiguate : public Args... {};

//Non-variadic version of the line above.
//template <typename A, typename B> struct ambiguate : public A, public B {};

template<typename A, typename = void>
struct got_type : std::false_type {};

template<typename A>
struct got_type<A> : std::true_type {
    typedef A type;
};

template<typename T, T>
struct sig_check : std::true_type {};

template<typename Alias, typename AmbiguitySeed>
struct has_member {
    template<typename C> static char ((&f(decltype(&C::value))))[1];
    template<typename C> static char ((&f(...)))[2];

    //Make sure the member name is consistently spelled the same.
    static_assert(
        (sizeof(f<AmbiguitySeed>(0)) == 1)
        , "Member name specified in AmbiguitySeed is different from member name specified in Alias, or wrong Alias/AmbiguitySeed has been specified."
    );

    static bool const value = sizeof(f<Alias>(0)) == 2;
};

Makra (El Diablo!):

CREATE_MEMBER_CHECK:

//Check for any member with given name, whether var, func, class, union, enum.
#define CREATE_MEMBER_CHECK(member)                                         \
                                                                            \
template<typename T, typename = std::true_type>                             \
struct Alias_##member;                                                      \
                                                                            \
template<typename T>                                                        \
struct Alias_##member <                                                     \
    T, std::integral_constant<bool, got_type<decltype(&T::member)>::value>  \
> { static const decltype(&T::member) value; };                             \
                                                                            \
struct AmbiguitySeed_##member { char member; };                             \
                                                                            \
template<typename T>                                                        \
struct has_member_##member {                                                \
    static const bool value                                                 \
        = has_member<                                                       \
            Alias_##member<ambiguate<T, AmbiguitySeed_##member>>            \
            , Alias_##member<AmbiguitySeed_##member>                        \
        >::value                                                            \
    ;                                                                       \
}

CREATE_MEMBER_VAR_CHECK:

//Check for member variable with given name.
#define CREATE_MEMBER_VAR_CHECK(var_name)                                   \
                                                                            \
template<typename T, typename = std::true_type>                             \
struct has_member_var_##var_name : std::false_type {};                      \
                                                                            \
template<typename T>                                                        \
struct has_member_var_##var_name<                                           \
    T                                                                       \
    , std::integral_constant<                                               \
        bool                                                                \
        , !std::is_member_function_pointer<decltype(&T::var_name)>::value   \
    >                                                                       \
> : std::true_type {}

CREATE_MEMBER_FUNC_SIG_CHECK:

//Check for member function with given name AND signature.
#define CREATE_MEMBER_FUNC_SIG_CHECK(func_name, func_sig, templ_postfix)    \
                                                                            \
template<typename T, typename = std::true_type>                             \
struct has_member_func_##templ_postfix : std::false_type {};                \
                                                                            \
template<typename T>                                                        \
struct has_member_func_##templ_postfix<                                     \
    T, std::integral_constant<                                              \
        bool                                                                \
        , sig_check<func_sig, &T::func_name>::value                         \
    >                                                                       \
> : std::true_type {}

CREATE_MEMBER_CLASS_CHECK:

//Check for member class with given name.
#define CREATE_MEMBER_CLASS_CHECK(class_name)               \
                                                            \
template<typename T, typename = std::true_type>             \
struct has_member_class_##class_name : std::false_type {};  \
                                                            \
template<typename T>                                        \
struct has_member_class_##class_name<                       \
    T                                                       \
    , std::integral_constant<                               \
        bool                                                \
        , std::is_class<                                    \
            typename got_type<typename T::class_name>::type \
        >::value                                            \
    >                                                       \
> : std::true_type {}

CREATE_MEMBER_UNION_CHECK:

//Check for member union with given name.
#define CREATE_MEMBER_UNION_CHECK(union_name)               \
                                                            \
template<typename T, typename = std::true_type>             \
struct has_member_union_##union_name : std::false_type {};  \
                                                            \
template<typename T>                                        \
struct has_member_union_##union_name<                       \
    T                                                       \
    , std::integral_constant<                               \
        bool                                                \
        , std::is_union<                                    \
            typename got_type<typename T::union_name>::type \
        >::value                                            \
    >                                                       \
> : std::true_type {}

CREATE_MEMBER_ENUM_CHECK:

//Check for member enum with given name.
#define CREATE_MEMBER_ENUM_CHECK(enum_name)                 \
                                                            \
template<typename T, typename = std::true_type>             \
struct has_member_enum_##enum_name : std::false_type {};    \
                                                            \
template<typename T>                                        \
struct has_member_enum_##enum_name<                         \
    T                                                       \
    , std::integral_constant<                               \
        bool                                                \
        , std::is_enum<                                     \
            typename got_type<typename T::enum_name>::type  \
        >::value                                            \
    >                                                       \
> : std::true_type {}

CREATE_MEMBER_FUNC_CHECK:

//Check for function with given name, any signature.
#define CREATE_MEMBER_FUNC_CHECK(func)          \
template<typename T>                            \
struct has_member_func_##func {                 \
    static const bool value                     \
        = has_member_##func<T>::value           \
        && !has_member_var_##func<T>::value     \
        && !has_member_class_##func<T>::value   \
        && !has_member_union_##func<T>::value   \
        && !has_member_enum_##func<T>::value    \
    ;                                           \
}

CREATE_MEMBER_CHECKS:

//Create all the checks for one member.  Does NOT include func sig checks.
#define CREATE_MEMBER_CHECKS(member)    \
CREATE_MEMBER_CHECK(member);            \
CREATE_MEMBER_VAR_CHECK(member);        \
CREATE_MEMBER_CLASS_CHECK(member);      \
CREATE_MEMBER_UNION_CHECK(member);      \
CREATE_MEMBER_ENUM_CHECK(member);       \
CREATE_MEMBER_FUNC_CHECK(member)

Ponieważ wszystkie odpowiedzi wydają mi się zbyt skomplikowane, chciałbym przedstawić własne rozwiązanie za pomocą std::declval i std::enable_if (GCC 4.8.3)

#define MEMBER_FUNC_CHECKER(name, fn, ret, args) \
template<class C, typename=void> struct name : std::false_type {}; \
template<class C> struct name<C, typename std::enable_if< \
  std::is_convertible<decltype(std::declval<C>().fn args), ret \
  >::value>::type> : std::true_type {};

uwaga: nie jest to dokładne sprawdzenie dla podpisu, ale dla funkcji wywołanej z wymiennym typem powrotu. (edit: zmieniony z is_same na is_convertible)

Test

struct One {
    int get() { return 0; }
    int add(int x, int y) { return x+y; }
};
struct Two: One {};
struct Not {};

MEMBER_FUNC_CHECKER(has_get, get, int, ())
MEMBER_FUNC_CHECKER(has_add, add, int, (1,2))

int main() {
    cout << "One " << (has_get<One>() ? "has" : "does not have")
        << " int get()" << endl;
    cout << "Two " << (has_get<Two>() ? "has" : "does not have")
        << " int get()" << endl;
    cout << "Not " << (has_get<Not>() ? "has" : "does not have")
        << " int get()" << endl;
    cout << "One " << (has_add<One>() ? "has" : "does not have")
        << " int add(int, int)" << endl;
    cout << "Two " << (has_add<Two>() ? "has" : "does not have")
        << " int add(int, int)" << endl;
    cout << "Not " << (has_add<Not>() ? "has" : "does not have")
        << " int add(int, int)" << endl;
    cout << "int " << (has_get<int>() ? "has" : "does not have")
        << " int get()" << endl;
}

Wyjście

One has int get()
Two has int get()
Not does not have int get()
One has int add(int, int)
Two has int add(int, int)
Not does not have int add(int, int)
int does not have int get()

UPDATE: moje Warcaby

/// Checker for typedef with given name and convertible type
#define TYPEDEF_CHECKER(checker, name) \
template<class C, typename T, typename = void> struct checker : std::false_type {}; \
template<class C, typename T> struct checker<C, T, typename std::enable_if< \
  std::is_convertible<typename C::name, T>::value>::type> : std::true_type {}
/// Checker for typedef with given name and exact type
#define TYPEDEF_CHECKER_STRICT(checker, name) \
template<class C, typename T, typename = void> struct checker : std::false_type {}; \
template<class C, typename T> struct checker<C, T, typename std::enable_if< \
  std::is_same<typename C::name, T>::value>::type> : std::true_type {}
/// Checker for typedef with given name and any type
#define TYPEDEF_CHECKER_ANY(checker, name) \
template<class C, typename = void> struct checker : std::false_type {}; \
template<class C> struct checker<C, typename std::enable_if< \
  !std::is_same<typename C::name*, void>::value>::type> : std::true_type {}

/// Checker for member with given name and convertible type
#define MTYPE_CHECKER(checker, name) \
template<class C, typename T, typename = void> struct checker : std::false_type {}; \
template<class C, typename T> struct checker<C, T, typename std::enable_if< \
  std::is_convertible<decltype(C::name), T>::value>::type> : std::true_type {}
/// Checker for member with given name and exact type
#define MTYPE_CHECKER_STRICT(checker, name) \
template<class C, typename T, typename = void> struct checker : std::false_type {}; \
template<class C, typename T> struct checker<C, T, typename std::enable_if< \
  std::is_same<decltype(C::name), T>::value>::type> : std::true_type {}
/// Checker for member with given name and any type
#define MTYPE_CHECKER_ANY(checker, name) \
template<class C, typename = void> struct checker : std::false_type {}; \
template<class C> struct checker<C, typename std::enable_if< \
  !std::is_same<decltype(C::name)*, void>::value>::type> : std::true_type {}

/// Checker for static const variable with given name and value
#define MVALUE_CHECKER(checker, name, val) \
template<class C, typename = void> struct checker : std::false_type {}; \
template<class C> struct checker<C, typename std::enable_if< \
  std::is_convertible<decltype(C::name), const decltype(val)>::value && C::name == val>::type> : std::true_type {}
/// Checker for static const variable with given name, value and type
#define MVALUE_CHECKER_STRICT(checker, name, val) \
template<class C, typename = void> struct checker : std::false_type {}; \
template<class C> struct checker<C, typename std::enable_if< \
  std::is_same<decltype(C::name), const decltype(val)>::value && C::name == val>::type> : std::true_type {}

/// Checker for member function with convertible return type and accepting given arguments
#define METHOD_CHECKER(checker, name, ret, args) \
template<class C, typename=void> struct checker : std::false_type {}; \
template<class C> struct checker<C, typename std::enable_if< \
  std::is_convertible<decltype(std::declval<C>().name args), ret>::value>::type> : std::true_type {};
/// Checker for member function with exact retutn type and accepting given arguments
#define METHOD_CHECKER_STRICT_RET(name, fn, ret, args) \
template<class C, typename=void> struct name : std::false_type {}; \
template<class C> struct name<C, typename std::enable_if< \
  std::is_same<decltype(std::declval<C>().fn args), ret>::value>::type> : std::true_type {};
/// Checker for member function accepting given arguments
#define METHOD_CHECKER_ANY(name, fn, args) \
template<class C, typename=void> struct name : std::false_type {}; \
template<class C> struct name<C, typename std::enable_if< \
  !std::is_same<decltype(std::declval<C>().fn args)*, void>::value>::type> : std::true_type {};

Kod Badania

struct One {
    typedef int type;
    static constexpr bool v = true;
    type x;
    One(type x = 0): x(x) {}
    ~One() {}
    type get() { return x; }
    type add(type x, type y) { return x+y; }
};
struct Two: One {};
struct Not {};

TYPEDEF_CHECKER(has_type, type);
TYPEDEF_CHECKER_ANY(any_type, type);
TYPEDEF_CHECKER_STRICT(exact_type, type);
MTYPE_CHECKER(has_x, x);
MTYPE_CHECKER_ANY(any_x, x);
MTYPE_CHECKER_STRICT(exact_x, x);
MVALUE_CHECKER(true_v, v, true);
MVALUE_CHECKER(true_z, z, true);
MVALUE_CHECKER(false_v, v, false);
MVALUE_CHECKER(one_v, v, 1);
MVALUE_CHECKER_STRICT(exact_v, v, 1);
METHOD_CHECKER(has_get, get, long, ());
METHOD_CHECKER(has_add, add, long, (1,2))
METHOD_CHECKER_ANY(any_get, get, ());
METHOD_CHECKER_STRICT_RET(int_get, get, int, ())
METHOD_CHECKER_STRICT_RET(long_get, get, long, ())

int main() {
#define CHECK(name, desc, ...) cout << endl; \
    cout << "One " << (name<One, ##__VA_ARGS__>() ? "has " : "does not have ") << desc << endl; \
    cout << "Two " << (name<Two, ##__VA_ARGS__>() ? "has " : "does not have ") << desc << endl; \
    cout << "Not " << (name<Not, ##__VA_ARGS__>() ? "has " : "does not have ") << desc << endl; \
    cout << "int " << (name<int, ##__VA_ARGS__>() ? "has " : "does not have ") << desc << endl
    string sep = string(60, '-');
    cout << sep;
    CHECK(any_type, "typedef type");
    CHECK(has_type, "typedef type convertible to long", long);
    CHECK(exact_type, "typedef type = int", int);
    CHECK(exact_type, "typedef type = long", long);

    cout << sep;
    CHECK(any_x, "var x");
    CHECK(has_x, "var x of type convertible to long", long);
    CHECK(exact_x, "var x of type int", int);
    CHECK(exact_x, "var x of type long", long);

    cout << sep;
    CHECK(true_v, "var v with value equal to true");
    CHECK(true_z, "var z with value equal to true");
    CHECK(false_v, "var v with value equal to false");
    CHECK(one_v, "var v with value equal to 1");
    CHECK(exact_v, "var v with value equal to 1 of type int");

    cout << sep;
    CHECK(has_get, "get()");
    CHECK(has_get, "get() with return type covertible to long");
    CHECK(has_add, "add() accepting two ints and returning ~ long");
    CHECK(int_get, "int get()");
    CHECK(long_get, "long get()");
}

Wyjście

One has typedef type
Two has typedef type
Not does not have typedef type
int does not have typedef type

One has typedef type convertible to long
Two has typedef type convertible to long
Not does not have typedef type convertible to long
int does not have typedef type convertible to long

One has typedef type = int
Two has typedef type = int
Not does not have typedef type = int
int does not have typedef type = int

One does not have typedef type = long
Two does not have typedef type = long
Not does not have typedef type = long
int does not have typedef type = long
------------------------------------------------------------
One has var x
Two has var x
Not does not have var x
int does not have var x

One has var x of type convertible to long
Two has var x of type convertible to long
Not does not have var x of type convertible to long
int does not have var x of type convertible to long

One has var x of type int
Two has var x of type int
Not does not have var x of type int
int does not have var x of type int

One does not have var x of type long
Two does not have var x of type long
Not does not have var x of type long
int does not have var x of type long
------------------------------------------------------------
One has var v with value equal to true
Two has var v with value equal to true
Not does not have var v with value equal to true
int does not have var v with value equal to true

One does not have var z with value equal to true
Two does not have var z with value equal to true
Not does not have var z with value equal to true
int does not have var z with value equal to true

One does not have var v with value equal to false
Two does not have var v with value equal to false
Not does not have var v with value equal to false
int does not have var v with value equal to false

One has var v with value equal to 1
Two has var v with value equal to 1
Not does not have var v with value equal to 1
int does not have var v with value equal to 1

One does not have var v with value equal to 1 of type int
Two does not have var v with value equal to 1 of type int
Not does not have var v with value equal to 1 of type int
int does not have var v with value equal to 1 of type int
------------------------------------------------------------
One has get()
Two has get()
Not does not have get()
int does not have get()

One has get() with return type covertible to long
Two has get() with return type covertible to long
Not does not have get() with return type covertible to long
int does not have get() with return type covertible to long

One has add() accepting two ints and returning ~ long
Two has add() accepting two ints and returning ~ long
Not does not have add() accepting two ints and returning ~ long
int does not have add() accepting two ints and returning ~ long

One has int get()
Two has int get()
Not does not have int get()
int does not have int get()

One does not have long get()
Two does not have long get()
Not does not have long get()
int does not have long get()