错误C2678：二进制“==”：没有操作员发现这需要类型的左边的操作数（或没有可接受的转化率）

Question

我试图编译下面的代码：

#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++编译器返回以下编译错误：

...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++编译器返回以下编译错误：

/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; }

错误消失，如果我重载==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

之所以失败就是operator ==为std::pair使用==到对成员，并且是使用argument-dependent lookup (ADL)找到合适operator ==比较为他们。但是您提供了错误名称空间中的重载，因为Point实际上是::boost::geometry::model::d2中的某个东西的typedef，而不是::。

如果移动运营商到正确的命名空间（这是一个很好的想法），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