c++ - 错误 C2678 : binary '==' : no operator found which takes a left-hand operand of type (or there is no acceptable conversion)

Question

我正在尝试编译以下代码:

#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; }

如果我为 Vector 重载 == 运算符，错误就会消失:

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

Answer 1

失败的原因是 std::pair 的 operator == 使用 == 来比较 pair 的成员，依次使用 argument-dependent lookup (ADL)为他们找到合适的 operator ==。但是您在错误的命名空间中提供了重载，因为 Point 实际上是 ::boost::geometry::model::d2 中某些内容的 typedef，而不是在 :: 中。

如果您将运算符移动到正确的命名空间(无论如何这是个好主意)，它会起作用:

#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