是否可以使用 SFINAE/模板来检查运算符是否存在？

Question

我正在尝试在编译时检查运算符是否存在，如果不存在，我只想忽略它，有什么办法吗？

示例运算符：

 template <typename T>
 QDataStream& operator<<(QDataStream& s, const QList<T>& l);

Answer 1

我最终使用了一个后备命名空间：

namespace operators_fallback {
template <typename T>
inline QDataStream& operator<<(QDataStream& s, const T &) { return s; }

template <typename T>
inline QDataStream& operator>>(QDataStream& s, T &) { return s; }

template <typename T>
inline QDebug operator<<(QDebug d, const T &) { return d; }
};

...
inline void load(QDataStream & s) {
    using namespace operator_fallback;
    s >> item;
}

还找到了在编译时检查运算符的正确方法（尽管我将使用后备命名空间）。

或多或少基于这个：

namespace private_impl {
    typedef char yes;
typedef char (&no)[2];

struct anyx { template <class T> anyx(const T &); };

no operator << (const anyx &, const anyx &);
no operator >> (const anyx &, const anyx &);


template <class T> yes check(T const&);
no check(no);

template <typename StreamType, typename T>
struct has_loading_support {
    static StreamType & stream;
    static T & x;
    static const bool value = sizeof(check(stream >> x)) == sizeof(yes);
};

template <typename StreamType, typename T>
struct has_saving_support {
    static StreamType & stream;
    static T & x;
    static const bool value = sizeof(check(stream << x)) == sizeof(yes);
};

template <typename StreamType, typename T>
struct has_stream_operators {
    static const bool can_load = has_loading_support<StreamType, T>::value;
    static const bool can_save = has_saving_support<StreamType, T>::value;
    static const bool value = can_load && can_save;
};
}
template<typename T>
struct supports_qdatastream : private_impl::has_stream_operators<QDataStream, T> {};

template<typename T>
struct can_load : private_impl::has_loading_support<QDataStream, T> {};

template<typename T>
struct can_save : private_impl::has_saving_support<QDataStream, T> {};

template<typename T>
struct can_debug : private_impl::has_saving_support<QDebug, T> {};

//edit 稍微改变了 has_stream_operators。

//edit删除了链接，显然该网站有一些攻击javascript。

Answer 2

这是一个老问题，但值得注意的是，Boost 刚刚为几乎所有运营商添加了此功能，并使用了最新的Operator Type Traits。 OP询问的具体算子是用boost:has_left_shift测试的。

Answer 3

这不太容易，在 C++03 中通常是不可能的。例如，如果将int* 和int* 用于op<<，则会在编译时出现硬错误。所以对于非类类型，你需要过滤掉标准禁止的类型。

对于op+，我曾经写过这样的东西。请注意，我使用的是 C 标头，因为我还需要使用 clang 编译器测试代码，当时它不支持我的 C++ 标头：

#include <stddef.h>
#include <stdio.h>

namespace detail {
struct any { 
  template<typename T> any(T const&); 
};
struct tag { char c[2]; };

int operator,(detail::tag, detail::tag);
template<typename T> void operator,(detail::tag, T const&);
char operator,(int, detail::tag);
}

namespace fallback {
  detail::tag operator+(detail::any const&, detail::any const&);
}

namespace detail {
template<typename T>
struct is_class {
  typedef char yes[1];
  typedef char no[2];

  template<typename U>
  static yes &check(int U::*);
  template<typename U>
  static no  &check(...);

  static bool const value = sizeof check<T>(0) == 1;
};

template<typename T>
struct is_pointer { typedef T pointee; static bool const value = false; };
template<typename T>
struct is_pointer<T*> { typedef T pointee; static bool const value = true; };

template<typename T, typename U>
struct is_same {
  static bool const value = false;
};

template<typename T>
struct is_same<T, T> {
  static bool const value = true;
};

template<typename T> 
struct is_incomplete_array {
  static bool const value = false;
};

template<typename T>
struct is_incomplete_array<T[]> {
  static bool const value = true;
};

template<typename T>
struct is_reference {
  typedef T referee;
  static bool const value = false;
};

template<typename T>
struct is_reference<T&> {
  typedef T referee;
  static bool const value = true;
};

// is_fn checks whether T is a function type
template<typename T>
struct is_fn {
  typedef char yes[1];
  typedef char no[2];

  template<typename U>
  static no &check(U(*)[1]);

  template<typename U>
  static yes &check(...);

  // T not void, not class-type, not U[], U& and T[] invalid
  // => T is function type
  static bool const value = 
    !is_same<T const volatile, void>::value && 
    !is_class<T>::value && 
    !is_incomplete_array<T>::value &&
    !is_reference<T>::value &&
    (sizeof check<T>(0) == 1);
};

template<typename T, bool = is_fn<T>::value>
struct mod_ty {
  typedef T type;
};

template<typename T>
struct mod_ty<T, true> {
  typedef T *type;
};

template<typename T>
struct mod_ty<T[], false> {
  typedef T *type;
};

template<typename T, size_t N>
struct mod_ty<T[N], false> {
  typedef T *type;
};

// Standard says about built-in +:
//
// For addition, either both operands shall have arithmetic or enumeration type,
// or one operand shall be a pointer to a completely defined object type and 
// the other shall have integral or enumeration type.

template<typename T> struct Ty; // one particular type
struct P; // pointer
struct Nc; // anything nonclass
struct A; // anything
struct Fn; // function pointer

// matches category to type
template<typename C, typename T, 
         bool = is_pointer<T>::value, 
         bool = !is_class<T>::value,
         bool = is_fn<typename is_pointer<T>::pointee>::value>
struct match {
  static bool const value = false;
};

// one particular type
template<typename T, bool P, bool Nc, bool Fn>
struct match<Ty<T const volatile>, T, P, Nc, Fn> {
  static bool const value = false;
};

// pointer
template<typename T, bool F>
struct match<P, T, true, true, F> { 
  static bool const value = true;
};

// anything nonclass
template<typename T, bool P, bool Fn>
struct match<Nc, T, P, true, Fn> {
  static bool const value = true;
};

// anything
template<typename T, bool P, bool Nc, bool Fn>
struct match<A, T, P, Nc, Fn> {
  static bool const value = true;
};

// function pointer
template<typename T>
struct match<Fn, T, true, true, true> {
  static bool const value = true;
};

// one invalid combination
template<typename A, typename B>
struct inv;

// a list of invalid combinations, terminated by B = void
template<typename A, typename B>
struct invs;

// T[] <=> T[N] => T*
// void() => void(*)() 
// T& => T
// trying to find all invalid combinations
// for built-in op+
typedef 
invs<
  inv<Ty<float const volatile>, P>,
invs<
  inv<Ty<double const volatile>, P>,
invs<
  inv<Ty<long double const volatile>, P>,
invs<
  inv<Ty<void * const volatile>, Nc>,
invs<
  inv<Ty<void const* const volatile>, Nc>,
invs<
  inv<Ty<void volatile* const volatile>, Nc>,
invs<
  inv<Ty<void const volatile* const volatile>, Nc>,
invs<
  inv<Fn, Nc>,
invs<
  inv<Ty<void const volatile>, A>,
invs<
  inv<P, P>,
void
> > > > > > > > > > invalid_list;

// match condition: only when ECond<true> is passed by specialization,
// then it will be selected.
template<bool> struct ECond;

template<typename L, typename T, typename U, typename = ECond<true> >
struct found_impl;

// this one will first modify the input types to be plain pointers
// instead of array or function types. 
template<typename L, typename T, typename U>
struct found : found_impl<L, 
                          typename mod_ty<
                            typename is_reference<T>::referee>::type, 
                          typename mod_ty<
                            typename is_reference<U>::referee>::type> 
{ };

// match was found.
template<typename F, typename B, typename R, typename T, typename U>
struct found_impl<invs<inv<F, B>, R>, T, U, 
                  ECond<(match<F, T>::value && match<B, U>::value) ||
                        (match<B, T>::value && match<F, U>::value)> > {
  static bool const value = true;
};

// recurse (notice this is less specialized than the previous specialization)
template<typename H, typename R, typename T, typename U, typename Ec>
struct found_impl< invs<H, R>, T, U, Ec > : found_impl<R, T, U> {
};

// we hit the end and found nothing
template<typename T, typename U, typename Ec>
struct found_impl< void, T, U, Ec > { 
  static bool const value = false;
};

using namespace fallback;

template<typename T, typename U, 
         bool found_invalid = found<invalid_list, T, U>::value>
struct is_addable {
  static T t;
  static U u;
  static bool const value = sizeof (detail::tag(), (t+u), detail::tag()) != 1;
};

template<typename T, typename U>
struct is_addable<T, U, true> {
  static bool const value = false;
};

}

template<typename T, typename U> struct is_addable {
  static bool const value = detail::is_addable<T, U>::value;
};

当然，事后做测试很重要

// this one can be added
struct test {
  test operator+(test) { return(*this); }
};

// this one cannot be added
struct nono { };

// this fails because of an ambiguity, because there is a comma
// operator taking a variable parameter on its left hand side.     
struct fails { fails operator+(fails); };

template<typename T>
void operator,(T const&, fails);


int main() {
  printf("%d\n", is_addable<test, test>::value);
  printf("%d\n", is_addable<int, float>::value);
  printf("%d\n", is_addable<nono, nono>::value);
  printf("%d\n", is_addable<int*, int>::value);
  printf("%d\n", is_addable<int[1], int>::value);
  printf("%d\n", is_addable<int[1], float[2]>::value);
  printf("%d\n", is_addable<int*, float*>::value);
  printf("%d\n", is_addable<void*, float>::value);
  printf("%d\n", is_addable<void, int>::value);
  printf("%d\n", is_addable<void(), int>::value);
  printf("%d\n", is_addable<int, void(**)()>::value);
  printf("%d\n", is_addable<float*&, int*&>::value);
}