是否可以使用SFINAE /模板来检查操作符是否存在？ [英] Is it possible to use SFINAE/templates to check if an operator exists?

问题描述

我想在编译时检查一个操作符是否存在，如果它不是我想要的忽略，有什么办法吗？

example运算符：

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

解决方案

我最后使用一个备用命名空间：

 命名空间operators_fallback {
 template< typename T> 
 inline QDataStream&运算符<<（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>>项目; 
} 
  

同样找到在编译时检查运算符的正确方法

pre> 命名空间private_impl {
typedef char yes;
typedef char（& no）[2];

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

无操作符<< （const anyx& const，const anyx&）;
无操作符>> （const anyx& const，const anyx&）;


template< class T>是检查（T const&）;
无检查（否）;

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

template< typename StreamType，typename T>
struct has_saving_support {
static StreamType&流;
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> {};


//编辑更改了has_stream_operators。

//编辑删除链接，显然网站有一些攻击javascript。

I'm trying to check if an operator exists at compile time, if it doesn't I just want it ignored, is there any way to do that?

example operator:

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

解决方案

I ended up using a fallback namespace :

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;
}

Also found the proper way to check for operators at compile time (although I'm going with the fallback namespace).

more or less based on this :

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 changed has_stream_operators a bit.

//edit removed the link, apparently the site has some attack javascript.

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