31

使用 SFINAE,我可以检测给定类是否具有某个成员函数。但是如果我想测试继承的成员函数怎么办?

以下在 VC8 和 GCC4 中不起作用(即检测到A有一个成员函数foo(),但不是B继承一个):

#include <iostream>

template<typename T, typename Sig>                                 
struct has_foo {                     
    template <typename U, U> struct type_check;
    template <typename V> static char (& chk(type_check<Sig, &V::foo>*))[1];
    template <typename  > static char (& chk(...))[2]; 
    static bool const value = (sizeof(chk<T>(0)) == 1);
};

struct A {
    void foo();
};

struct B : A {};

int main()
{
    using namespace std;
    cout << boolalpha << has_foo<A, void (A::*)()>::value << endl; // true
    cout << boolalpha << has_foo<B, void (B::*)()>::value << endl; // false
}

那么,有没有办法测试继承的成员函数?

4

4 回答 4

30

看看这个线程:

http://lists.boost.org/boost-users/2009/01/44538.php

源自该讨论中链接到的代码:

#include <iostream>

template <typename Type> 
class has_foo
{ 
   class yes { char m;}; 
   class no { yes m[2];}; 
   struct BaseMixin 
   { 
     void foo(){} 
   }; 
   struct Base : public Type, public BaseMixin {}; 
   template <typename T, T t>  class Helper{}; 
   template <typename U> 
   static no deduce(U*, Helper<void (BaseMixin::*)(), &U::foo>* = 0); 
   static yes deduce(...); 
public: 
   static const bool result = sizeof(yes) == sizeof(deduce((Base*)(0))); 
}; 

struct A {
    void foo();
};

struct B : A {};

struct C {};

int main()
{
    using namespace std;
    cout << boolalpha << has_foo<A>::result << endl;
    cout << boolalpha << has_foo<B>::result << endl;
    cout << boolalpha << has_foo<C>::result;
}

结果:

true
true
false
于 2009-12-27T22:10:59.520 回答
23

joshperry 的答案非常聪明和优雅,但是(正如帖子下面所述)它没有正确检查 foo() 的签名并且不适用于基本类型(如 int):它会导致编译器错误。我将提出一种正确处理继承成员并检查成员函数签名的技术。我不会详细介绍,而是给你两个例子,希望代码能说明问题。

示例 1:

我们正在检查具有以下签名的成员: 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));
};

请注意,它甚至会检查方法的常量性,并且也适用于原始类型。(我的意思has_const_begin<int>::value是错误的,不会导致编译时错误。)

示例 2

现在我们正在寻找签名: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));
};

请注意 MyClass 不必是默认可构造的或满足任何特殊概念。该技术也适用于模板成员。

我急切地等待对此的意见。

于 2012-01-06T03:22:24.617 回答
17

以下是一些用法片段: *所有这些的胆量都在更远的地方

检查x给定类中的成员。可以是 var、func、class、union 或 enum:

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

检查成员函数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;

检查成员变量x

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

检查成员类x

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

检查会员工会x

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

检查成员枚举x

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

x无论签名如何,检查任何成员函数:

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;

或者

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;

细节和核心:

/*
    - 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;
};

宏(暗黑破坏神!):

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)
于 2011-06-12T21:58:27.640 回答
5

由于所有答案对我来说都太复杂了,我想介绍我自己的解决方案,使用std::declvaland 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 {};

注意:不是对签名进行精确检查,而是对具有可转换返回类型的可调用函数进行检查。(编辑:从更改is_sameis_convertible

测试

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;
}

输出

一个有 int get()
两个有 int get()
Not 没有 int get()
一个有 int add(int, int)
两个有 int add(int, int)
Not 没有 int add(int, int)
int 没有 int get()

更新:我的跳棋

/// 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 {};

测试代码

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()");
}

输出

一个有 typedef 类型
两个有typedef类型
Not 没有 typedef 类型
int 没有 typedef 类型

一个具有可转换为 long 的 typedef 类型
两个有 typedef 类型可转换为 long
Not 没有可转换为 long 的 typedef 类型
int 没有可转换为 long 的 typedef 类型

一个有 typedef type = int
两个有 typedef type = int
Not 没有 typedef type = int
int 没有 typedef type = int

一个没有 typedef type = long
二没有typedef type = long
Not 没有 typedef type = long
int 没有 typedef type = long
-------------------------------------------------- ----------
一个有 var x
两个有 var x
Not 没有 var x
int 没有 var x

一个具有可转换为 long 类型的 var x
两个具有可转换为 long 类型的 var x
Not 没有可转换为 long 类型的 var x
int 没有可转换为 long 类型的 var x

一个具有 int 类型的 var x
两个具有 int 类型的 var x
Not 没有 int 类型的 var x
int 没有 int 类型的 var x

一个没有 long 类型的 var x
两个没有 long 类型的 var x
Not 没有 long 类型的 var x
int 没有 long 类型的 var x
-------------------------------------------------- ----------
一个 var v 的值等于 true
两个具有 var v 的值等于 true
Not 没有值等于 true 的 var v
int 没有值等于 true 的 var v

一个没有值等于 true 的 var z
两个没有值等于 true 的 var z
Not 没有值等于 true 的 var z
int 没有值等于 true 的 var z

一个没有 var v 的值等于 false
两个没有 var v 的值等于 false
Not 没有值等于 false 的 var v
int 没有值等于 false 的 var v

一个具有 var v 的值等于 1
两个具有 var v 的值等于 1
Not 没有值等于 1 的 var v
int 没有值等于 1 的 var v

一个没有 var v 的值等于 1 的 int 类型
两个没有 var v 值等于 1 的 int 类型
Not 没有值等于 1 的 int 类型的 var v
int 没有值等于 1 的 int 类型的 var v
-------------------------------------------------- ----------
一个有 get()
二有get()
Not 没有 get()
int 没有 get()

一个具有 get() 的返回类型可转换为 long
两个具有 get() 的返回类型可转换为 long
Not 没有返回类型可转换为 long 的 get()
int 没有返回类型可转换为 long 的 get()

一个 add() 接受两个整数并返回 ~ long
两个 add() 接受两个整数并返回 ~ long
Not 没有 add() 接受两个整数并返回 ~ long
int 没有 add() 接受两个整数并返回 ~ long

一个有 int get()
两个有 int get()
Not 没有 int get()
int 没有 int get()

一不长 get()
二不长get()
not 没有长 get()
int 没有长 get()
于 2014-08-22T13:18:39.193 回答