使用SFINAE，i can detect给定的类具有某个成员函数。但是，如果我想测试继承的成员函数呢？
以下内容在VC8和GCC4中不起作用（即检测到A具有成员函数foo()，但B没有继承成员函数}：
#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？
答案 0 :(得分：28)
看一下这个帖子：
http://lists.boost.org/boost-users/2009/01/44538.php
源于该讨论中链接的代码：
#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
答案 1 :(得分：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（）的签名而且不适用于基本类型（像int）：它会导致编译器错误。 我将提出一种正确处理继承成员的技术，并检查成员函数的签名。我会给你两个例子，希望代码能说明一切，而不是详细介绍。
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，并使用原始类型。 （我的意思是has_const_begin<int>::value是错误的，并且不会导致编译时错误。）
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不必是默认构造或满足任何特殊概念。该技术也适用于模板成员。
我急切地等待有关此事的意见。
答案 2 :(得分：17)
以下是一些使用情况摘要： *所有这些的胆量更远
检查给定班级中的成员x。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;
检查会员班级x：
CREATE_MEMBER_CLASS_CHECK(x);
bool has_class_x = has_member_class_x<class_to_check_for_x>::value;
检查成员联盟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;
检查任何成员函数x，无论签名如何：
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;
OR
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;
详细信息和核心：
/*
- 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（El Diablo！）：
<强> 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 \
; \
}
<强> 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 {}
<强> 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 {}
<强> 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 {}
<强> 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 {}
<强> 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 {}
<强> 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 \
; \
}
<强> 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)
答案 3 :(得分：5)
std::declval和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：签名不是精确检查，但是对于具有可转换返回类型的可调用函数。 （编辑：从is_same更改为is_convertible）
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;
}
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()
/// 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 {};
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()");
}
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()