在Visual C ++和clang中使用C ++ 11 unordered_set [英] Using C++11 unordered_set in Visual C++ and clang

问题描述

我想在跨平台C ++应用程序中使用std :: unordered_set。它在Windows下的Visual C ++中编译和工作像一个魅力，但在Mac OS X下的clang上生成一个致命的编译错误。

我想知道为什么会发生，

  > // 
 // Clang build cmdline：
 // $ clang ++ ./set.cpp -Wall -Werror -Wfatal-errors -std = c ++ 11 -stdlib = libc ++ -o set.out 
 // 
 
 #include< iostream> 
 #include< unordered_set> 
 
 struct Point {
 int x，y; 
 Point（int x = 0，int y = 0）{
 this-> x = x; 
 this-> y = y; 
} 
 bool operator ==（Point const& p）const {
 return this-> x == p.x&& this-> y == p.y; 
} 
 operator std :: size_t（）const {
 return std :: hash< int>（）（x）^ std :: hash< int> 
} 
}; 
 
 typedef std :: unordered_set< Point> points_set_t; 
 
 int main（）{
 Point point1（1,5）; 
 Point point2（1，1）; 
 Point point3（1,5）; 
 points_set_t points; 
 points.insert（point1）; 
 points.insert（point2）; 
 points.insert（point3）; 
 for（points_set_t :: const_iterator it = points.begin（）; it！= points.end（）; it ++）{
 std :: cout< it-> x< ：<< it-> y< std :: endl; 
} 
} 
  

Clang输出：

 在./set.cpp:6中包含的文件中：
在包含在/usr/bin/../lib/c++/v1/中的文件中iostream：38：
在/usr/bin/../lib/c++/v1/ios:216中包含的文件中：
在包含在/usr/bin/../lib/c++/中的文件中v1 / __ locale：15：
在/usr/bin/../lib/c++/v1/string:434包含的文件中：
在包含在/usr/bin/../lib/中的文件中c ++ / v1 / algorithm：591：
 /usr/bin/../lib/c++/v1/type_traits:748:38：致命错误：未定义模板的隐式实例'std :: __ 1 :: hash< Point> ;'
：public integral_constant< bool，__is_empty（_Tp）> {}; 
 ^ 
 /usr/bin/../lib/c++/v1/memory:1948:40：注意：在实例化模板类'std :: __ 1 :: is_empty< std :: __ 1： ：hash< Point> >'
请求这里
 bool = is_empty< _T2> :: value 
 ^ 
 /usr/bin/../lib/c++/v1/memory:1970:44 ：注意：在实例化'__libcpp_compressed_pa​​ir_switch< unsigned 
 long，std :: __ 1 :: hash< Point>，false，false>'的默认参数时，
模板< class _T1，class _T2， unsigned = __libcpp_compressed_pa​​ir_switch< _T1，_T2> :: value> 
 ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
 / usr / bin /../ lib / c ++ / v1 / memory：2354：15：注意：在实例化'__libcpp_compressed_pa​​ir_imp< unsigned long，
 
 std :: __ 1 :: hash& ; >'required here 
：private __libcpp_compressed_pa​​ir_imp< _T1，_T2> 
 ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
 / usr / bin /../lib/c++/v1/__hash_table:527:55：注意：在实例化模板类'std :: __ 1 :: __ compressed_pa​​ir< unsigned long，
 std :: __ 1 :: hash< Point> >'requested here 
 __compressed_pa​​ir< size_type，hasher> __p2_; 
 ^ 
 /usr/bin/../lib/c++/v1/unordered_set:330:13：note：在实例化模板类'std :: __ 1 :: __ hash_table< Point，
 std :: __ 1 :: hash< Point>，std :: __ 1 :: equal_to< Point>，std :: __ 1 :: allocator< Point> >'requested here 
 __table __table_; 
 ^ 
 ./set.cpp:28:18：注意：在实例化模板类'std :: __ 1 :: unordered_set< Point，std :: __ 1 :: hash< Point>，
 std :: __ 1 :: equal_to< Point>，std :: __ 1 :: allocator< Point> >'requested here 
 points_set_t points; 
 ^ 
 /usr/bin/../lib/c++/v1/memory:3076:29：注意：模板在这里被声明
 template< class _Tp> struct hash; 
 ^ 
生成了1个错误。 
 

UPD 使用@ mfontanini的建议工作实施： https://gist.github.com/vbo/6090142 。

解决方案

为了让 std :: unordered_set 使用 Point 类， std :: hash 专用化：

 命名空间std 
 {
模板<> 
 struct hash< Point> {
 size_t operator（）（const Point& pt）const {
 return std :: hash< int>（pt.x）^ std :: hash< int>（pt。 y）。 
} 
}; 
} 
  

您也可以更改 std :: unordered_set 的第二个模板参数（它默认为 std :: hash< Point> ），表示返回所需哈希的函子类型。 >



看起来你试图通过用户定义的转换提供这个哈希实现到 size_t ，但是不行。它在VC中工作的事实是由它们的实现中的一些错误引起的。

I'am trying to use std::unordered_set in cross-platform C++ application. It compiles and works like a charm in Visual C++ under Windows, but generates a fatal compilation error on clang under Mac OS X.

I want to know why it happens and what is the right way to get this working.

Example code:

// 
// Clang build cmdline:
// $ clang++ ./set.cpp -Wall -Werror -Wfatal-errors -std=c++11 -stdlib=libc++ -o set.out
// 

#include <iostream>
#include <unordered_set>

struct Point {
    int x, y;
    Point(int x = 0, int y = 0) {
        this->x = x;
        this->y = y;
    }
    bool operator==(Point const& p) const {
        return this->x == p.x && this->y == p.y;
    }
    operator std::size_t () const {
        return std::hash<int>()(x) ^ std::hash<int>()(y);
    }
};

typedef std::unordered_set<Point> points_set_t;

int main() {
    Point point1(1, 5);
    Point point2(1, 1);
    Point point3(1, 5);
    points_set_t points;
    points.insert(point1);
    points.insert(point2);
    points.insert(point3);
    for (points_set_t::const_iterator it = points.begin(); it != points.end(); it++) {
        std::cout << it->x << ":" << it->y << std::endl;
    }
}

Clang output:

In file included from ./set.cpp:6:
In file included from /usr/bin/../lib/c++/v1/iostream:38:
In file included from /usr/bin/../lib/c++/v1/ios:216:
In file included from /usr/bin/../lib/c++/v1/__locale:15:
In file included from /usr/bin/../lib/c++/v1/string:434:
In file included from /usr/bin/../lib/c++/v1/algorithm:591:
/usr/bin/../lib/c++/v1/type_traits:748:38: fatal error: implicit instantiation of undefined template 'std::__1::hash<Point>'
    : public integral_constant<bool, __is_empty(_Tp)> {};
                                     ^
/usr/bin/../lib/c++/v1/memory:1948:40: note: in instantiation of template class 'std::__1::is_empty<std::__1::hash<Point> >'
      requested here
                                bool = is_empty<_T2>::value
                                       ^
/usr/bin/../lib/c++/v1/memory:1970:44: note: in instantiation of default argument for '__libcpp_compressed_pair_switch<unsigned
      long, std::__1::hash<Point>, false, false>' required here
template <class _T1, class _T2, unsigned = __libcpp_compressed_pair_switch<_T1, _T2>::value>
                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/c++/v1/memory:2354:15: note: in instantiation of default argument for '__libcpp_compressed_pair_imp<unsigned long,

      std::__1::hash<Point> >' required here
    : private __libcpp_compressed_pair_imp<_T1, _T2>
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/c++/v1/__hash_table:527:55: note: in instantiation of template class 'std::__1::__compressed_pair<unsigned long,
      std::__1::hash<Point> >' requested here
    __compressed_pair<size_type, hasher>              __p2_;
                                                      ^
/usr/bin/../lib/c++/v1/unordered_set:330:13: note: in instantiation of template class 'std::__1::__hash_table<Point,
      std::__1::hash<Point>, std::__1::equal_to<Point>, std::__1::allocator<Point> >' requested here
    __table __table_;
            ^
./set.cpp:28:18: note: in instantiation of template class 'std::__1::unordered_set<Point, std::__1::hash<Point>,
      std::__1::equal_to<Point>, std::__1::allocator<Point> >' requested here
    points_set_t points;
                 ^
/usr/bin/../lib/c++/v1/memory:3076:29: note: template is declared here
template <class _Tp> struct hash;
                            ^
1 error generated.      

UPD Working implementation using @mfontanini's suggestion: https://gist.github.com/vbo/6090142.

解决方案

In order to make std::unordered_set work with your Point class, you can provide a std::hash specialization for it:

namespace std
{
template<>
struct hash<Point> {
    size_t operator()(const Point &pt) const {
        return std::hash<int>()(pt.x) ^ std::hash<int>()(pt.y);
    }
};
}

You could also change std::unordered_set's second template parameter(it defaults to std::hash<Point>), which indicates a functor type that returns the required hash.

It seems like you tried providing this hash implementation via a user defined conversion to size_t, but that won't work. The fact that it works in VC is caused by some bug in their implementation.

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