錯誤C2678：二進制「==」：沒有操作員發現這需要類型的左邊的操作數（或沒有可接受的轉化率）

Question

我試圖編譯下面的代碼：

#include <boost/geometry/geometries/point_xy.hpp> 

#include <iostream> 
#include <utility> 

typedef boost::geometry::model::d2::point_xy<long> Point; 
typedef std::pair<Point, Point> Vector; 

bool operator==(const Point& p1, const Point& p2) { 
    return p1.x() == p2.x() && p1.y() == p2.y(); 
} 

int main() { 
    Vector vec1(Point(0,0), Point(1,1)); 
    Vector vec2(Point(0,0), Point(1,2)); 
    std::cout << ((vec1 == vec2) == false) << std::endl; 
    std::cout << ((vec1 == vec1) == true) << std::endl; 
} 

VS2012 C++編譯器返回以下編譯錯誤：

...VC\include\utility(219): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'const Point' (or there is no acceptable conversion)

GCC C++編譯器返回以下編譯錯誤：

/usr/include/c++/4.8/bits/stl_pair.h:

In instantiation of ‘bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) [with _T1 = boost::geometry::model::d2::point_xy; _T2 = boost::geometry::model::d2::point_xy]’:

test.cpp:22:28: required from here /usr/include/c++/4.8/bits/stl_pair.h:215:51: error:

no match for ‘operator==’ (operand types are ‘const boost::geometry::model::d2::point_xy’ and ‘const boost::geometry::model::d2::point_xy’) { return __x.first == __y.first && __x.second == __y.second; }

錯誤消失，如果我重載==操作符的矢量：

bool operator==(const Vector& v1, const Vector& v2) { 
    return v1.first == v2.first && v1.second == v2.second; 
} 

Answer 1

之所以失敗就是operator ==爲std::pair使用==到對成員，並且是使用argument-dependent lookup (ADL)找到合適operator ==比較爲他們。但是您提供了錯誤名稱空間中的重載，因爲Point實際上是::boost::geometry::model::d2中的某個東西的typedef，而不是::。

如果移動運營商到正確的命名空間（這是一個很好的想法），它的工作原理：

#include <boost/geometry/geometries/point_xy.hpp> 

#include <iostream> 
#include <utility> 

typedef boost::geometry::model::d2::point_xy<long> Point; 
typedef std::pair<Point, Point> Vector; 

namespace boost { namespace geometry { namespace model { namespace d2 { 

bool operator==(const Point& p1, const Point& p2) { 
    return p1.x() == p2.x() && p1.y() == p2.y(); 
} 

} } } } 


int main() { 
    Vector vec1(Point(0,0), Point(1,1)); 
    Vector vec2(Point(0,0), Point(1,2)); 
    std::cout << ((vec1 == vec2) == false) << std::endl; 
    std::cout << ((vec1 == vec1) == true) << std::endl; 
} 

Live example